Tag Archives: 07 27 00−Air Barriers

Wind Load and Air Barrier Performance Levels

Photo courtesy DuPont Building Knowledge Center

Photo courtesy DuPont Building Knowledge Center

by Maria Spinu, PhD, LEED AP, Ben Meyer, RA, LEED AP, and Andrew Miles

Continuous air barriers have become mandatory for the building envelope, with energy codes recognizing the importance of air leakage control. However, simple inclusion of an air barrier requirement does not guarantee the desired performance in the field. These systems must be properly installed, meet the building envelope structural wind loads, and maintain their function over time.

There are two accepted performance levels for commercial air barrier systems, determined by the structural design parameters for the building envelope:

  • ASTM E1677, Standard Specification for Air Barrier Material or System for Low-rise Framed Building Walls, applicable to envelope design specifications of up to 105-km/h (65-mph) equivalent structural loads; and
  • ASTM 2357, Standard Test Method for Determining Air Leakage of Air Barrier Assemblies, for buildings designed to withstand structural loads beyond that level.

This article describes the air barrier performance requirements for the desired wind load design specifications. The performance level is not determined by the type of air barrier material, but by the installation details. Examples of how these details can impact the performance level for a given air barrier system will be provided, with special emphasis on mechanically fastened air barriers.

Summary comparison between ASTM E1677 and ASTM E2357 wall assembly testing.

Summary comparison between ASTM E1677 and ASTM E2357 wall assembly testing.

Air leakage control and air barrier materials
Air leakage control is achieved through a continuous air barrier. Any material with an air permeance less than 0.02 L/(s • m2) @ 75 Pa pressure differential (0.004 cfm/sf @ 0.3 in. w.c. or 1.56 psf pressure differential), when tested in accordance with ASTM E 2178, Standard Test Method for Air Permeance of Building Materials, qualifies as an air barrier. Even though many common building products are air barrier materials (e.g. metal sheets, glass, oriented strandboard [OSB], and gypsum board), a continuous air barrier requires many compatible components to achieve a plane of airtightness. In practice, most air barrier materials are specifically designed membranes effectively integrated into a continuous air barrier system.

Testing of walls with mechanically fastened air barrier systems. Image courtesy DuPont and ATICommon air barrier materials include mechanically fastened (i.e. building wraps), fluid-applied, and self-adhered membranes. The choice depends on many factors, such as the substrate, desired performance level, installed cost, personal preference, local practices, and regional availability.

For example, in framed construction where air barriers are applied over exterior sheathing, building wraps are the most cost-effective. For masonry or concrete backup walls, fluid-applied membranes are the common choice. Self-adhered membranes can be used with either substrate, but most are vapor-impermeable and their use should be limited to specific climates and wall design options.

In the case of vapor-impermeable air barriers, the membrane plays a dual role: air and vapor barrier. While air barriers could be installed anywhere in the building envelope, vapor barrier location and use is climate- and design-specific. For example, vapor barriers are required only in cold climates, and must be installed at the ‘warm in winter’ side of the envelope. In warm-humid climates, a vapor barrier could still be acceptable to the outside of the envelope (where the air barrier is generally installed) when design options for drying pathways are available—such as when vapor-permeable materials must be used at least in one direction (in this example, everything to the inboard from vapor barrier must be vapor-permeable). Exulation wall design (i.e. exterior insulation only, no insulation in the stud cavity) can also use vapor-impermeable air barriers. Building physics must always be considered when an unintended vapor barrier is used in a wall assembly.

There are four essential performance requirements for air barriers:

  • air infiltration resistance;
  • continuity;
  • structural integrity; and
  • durability.1

Another critical property is vapor permeability, which could impact moisture management in wall assemblies. However, the codes do not specify the air barriers’ vapor permeance—the decision is left to the building envelope designer.2

CS_July_2014.inddAir infiltration resistance is an inherent material property for air barrier materials. Other requirements depend not only on material properties, but also on the performance of the installed system determined by the integration of air barrier components into a continuous system, as well as the durability under use conditions. In addition to the primary air barrier membrane, an air barrier system includes installation and continuity accessories, such as primers, mechanical fasteners, seam tapes, flashing, adhesives, and sealants.

This article mainly focuses on structural integrity requirement, which is the ability of an air barrier system to withstand wind loads experienced during the building’s use after construction is complete. There are two accepted performance levels based on building envelope design parameters with regard to wind loads and wind-driven rain. To establish the performance level of an installed air barrier system, air barrier wall assemblies must be tested in accordance with the respective ASTM standards.

Installed air barrier performance and wall assembly testing
Testing is essential for demonstrating performance of installed air barrier assemblies. This process is critical for developing robust installation guidelines for achieving air barrier performance levels consistent with structural design specifications.

As mentioned, ASTM E1677-11 applies to air barrier performance levels for building envelope design requiring up to 105-km/h (65-mph) equivalent structural loads, and up to 24-km/h (15-mph) equivalent wind-driven rain water infiltration resistance. This level is generally adequate for buildings of up to four or five stories, but higher performance is typically required on some low-rise structures like medical facilities and military buildings. ASTM E2357-11, on the other hand, applies to air barrier performance levels for building envelope design structural loads beyond this—such a performance level is generally necessary for buildings taller than five stories.

CS_July_2014.inddBoth test methods are performed on 2.4 x 2.4-m (8 x 8-ft) wall assemblies. ASTM E1677 requires testing of a single, opaque wall assembly (i.e. no penetrations except for the fasteners), while ASTM E2357 involves two specimen—an opaque wall and a penetrated wall that includes standard penetrations such as window openings, external junction boxes, and galvanized duct.

Both test methods require pressurization and depressurization testing, but use different pressure loads and schedules. The major differences between the two test methods are summarized in Figure 1, and consist of the pressure loads, schedule, and requirement for water infiltration resistance testing.

As shown, ASTM E1677 requires five test pressures:

  • ± 75-Pa pressure differential (1.56 psf, 25 mph);
  • two pressures below 75 Pa; and
  • two pressures above 75 Pa.
CS_July_2014.indd

Examples of window flashing for ASTM E2357 performance level.

The pressure loading schedule includes sustained loads of up to ±500 Pa (10.4 psf, 65 mph). This standard requires testing for water infiltration resistance per ASTM E331, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference. Air barriers or air retarders (as they are referred to in ASTM E 331) are classified as either Type I or Type II. Type I air barriers, which can also perform as water-resistive barriers (WRBs), must exhibit no water penetration when tested at 27 Pa (11 in. water pressure difference)—equivalent wind speed of approximately 24 km/h (15 mph)—during a 15-minute test period. Type II air barriers are not required to be tested in accordance with ASTM E 331.

ASTM E2357 requires a minimum of seven test pressures, from ±25 Pa (0.56 psf, 15 mph) to ±300 Pa (6.24 psf, 50 mph). The pressure loading schedule includes sustained, cyclic, and gust winds up to ±160-km/h (100-mph) equivalent wind speed. This standard does not require ASTM E331 testing for water infiltration resistance, which is a significant limitation since many air barriers are commonly required to also perform the WRB function and are exposed to pressure loads above 105-km/h (65-mph) wind.3

A building wrap air and water barrier system is installed over the exterior sheathing, prior to the installation of metal panels. Proper installation is critical for meeting the building envelope structural wind loads and maintaining the air barrier continuity over time.

A building wrap air and water barrier system is installed over the exterior sheathing, prior to the installation of metal panels. Proper installation is critical for meeting the building envelope structural wind loads and maintaining the air barrier continuity over time.

Air leakage results are reported at 75 Pa for both methods. Current codes require the average air leakage rate for air barrier assemblies must not exceed 0.2 L/(s•m2) @ 75 Pa pressure differential (0.04 cfm/sf under a pressure differential of 0.3 in. w.g. or 1.57 psf) when tested in accordance with ASTM E2357 or ASTM E1677.

Since a continuous air barrier experiences both positive and negative pressures during its use, it is important assemblies be tested under both positive and negative pressures. The negative load (under suction) is typically the most severe, as it tries to pull the air barrier off the wall. Different air barrier types have different susceptibility to negative pressure loads.4

For fluid-applied air barriers, wind loads are transferred to the substrate underneath. When the substrate is masonry or concrete, a fully adhered fluid-applied air barrier has excellent structural performance under suction, as the pressure it typically takes to separate it from the substrate far exceeds the actual pressure it must withstand.

However, for framed wall construction, the structural performance of fully adhered fluid-applied air barriers under negative wind loads depends on how well the sheathing is fastened to the building structure. When the exterior sheathing is not installed to withstand the design wind loads, this could reduce the air barrier system’s structural performance. In this case, the typical mode of failure for fluid-applied air barrier is the sheathing pulling over the screws.

CS_July_2014.inddIn comparison, when building wraps are installed over exterior sheathing, the air barrier membrane is supporting the entire load. Consequently, this type of air barrier is more susceptible to wind. The suction forces are transferred through the air barrier membrane to the mechanical fasteners, and then back to the structural supports (i.e. steel or wood studs). As a result, for a mechanically fastened air barrier, the wind load performance is determined by the type of fasteners and the fastener schedule.

The photos in Figure 2 show an example of high-pressure performance testing of commercial building wraps and exemplify the extreme forces experienced by the air barrier wall assemblies under negative pressure loads. The steel studs actually buckle under the pressure differentials used for high performance testing of building wraps (left), but a properly fastened building wrap withstands this pressure and maintains the system’s structural integrity (right).5

These pictures demonstrate the importance of proper fastening of building wraps to withstand high suction loads and maintain the air barrier structural integrity during use. A common mistake with building wraps installation is use of staples for fastening the building wrap into the exterior sheathing (a practice often employed for WRBs in residential construction), rather than employing recommended screws with washers to fasten the membrane into the structural members (wood or steel studs).

Building wrap manufacturers usually provide guidelines on the type of fasteners and the fastening schedule recommended for meeting the desired performance level. Figure 3 provides an example of fastening type and schedule guidelines and the maximum wind loads allowable.

Alternate fasteners are also allowed, when applicable. Examples include standard brick tie base plates and metal plates, metal channels, horizontal z-girts, and wood furring strips mounted vertically. They can be used in conjunction with the manufacturer-recommended fasteners to meet and/or satisfy the desired design performance.

In addition to fastener selection and spacing, other installation details are critical when designing for a specific performance level. Some building wrap manufacturers provide different installation details for ASTM E1677 and ASTM E2357. These include details on sealing of penetrations, transitions, and interfaces. For example, no additional fastener sealing is necessary for building envelope design requiring up to 105-km/h (65-mph) equivalent structural loads (i.e. ASTM E1677), when recommended fasteners and schedules are used. However, if higher air infiltration resistance is desired (i.e. ASTM E2357), self-adhered flashing must be used under the fasteners.

Figure 4 shows examples of alternate fasteners, as well as the use of self-adhered flashing under the fasteners for ASTM E2357 performance level. The same recommendations are also for fluid-applied membrane fasteners.

A mechanically-fastened air and water barrier system is installed over the exterior sheathing. Fluid applied air barrier was also used for concrete masonry unit (CMU) portions of this project (not visible in the picture). Proper integration between the two air barriers used for CMU and gypsum-covered metal stud walls was critical for continuity and structural integrity.

A mechanically-fastened air and water barrier system is installed over the exterior sheathing. Fluid applied air barrier was also used for  concrete masonry unit (CMU) portions of this
project (not visible in the picture). Proper integration between the two air barriers used for
CMU and gypsum-covered metal stud walls was critical for continuity and structural integrity.

Among the most critical details determining the air barrier structural performance level are windows and doors integration into the continuous system. Most manufacturers provide step-by-step window installation guidelines. Changes in the provider’s detailing and sequencing could change the performance level (i.e. ASTM E1677 or ASTM E2357). Figures 5 and 6 show examples of specific details for achieving the desired wind load design specifications.

The detail in Figure 5 shows how the window rough openings are treated with self-adhered flashing, for the high-performance level required by ASTM E2357. For example, when the building has non-flanged, storefront, and/or curtain wall windows, the air barrier membrane is typically cut flush with the edge or the rough opening. Then, the self-adhered flashing is installed to protect the rough opening and provide a positive termination of the air barrier membrane. The pictures on the right show examples of high-performance flashing for non-flanged and/or curtain wall windows that may be bumped out from the wall plane.

Figure 6 shows an example of window flashing for ASTM E1677 performance level. The picture captures the alternate head detail, which is generally allowed for building structures with building envelope design requirements not exceeding ASTM E1677. After the air and water barrier is wrapped into the window rough opening, a top hat is created with sealant to divert water away from the window opening (if the air barrier is also intended to serve as the WRB). WRB cut pieces are then installed (I) by wrapping in and around the studs at the jamb and the head, and stapling to inside framing to secure (A). The next steps include (II): (A) apply a continuous sealant bead along jambs and head, (B) install flanged window, (C) install jamb flashing, and (D) install head flashing.

A properly installed building wrap air and water barrier system is the most cost-effective option for this building with sheathing substrate and multi-story curtain wall consisting of brick and solid-surface cladding panels.

A properly installed building wrap air and water barrier system is the most cost-effective
option for this building with sheathing substrate and multi-story curtain wall consisting
of brick and solid-surface cladding panels.

The recommended installation guidelines are based on many wall assembly tests, and changing the installation details in the field could affect the performance level for the installed air barrier assembly. Engaging the air barrier manufacturers in early design stages is critical to understanding the installation details requirement and the optimal installation sequence to achieve the desired performance level. Additionally, it helps avoid unnecessary delays during the construction phase.

The difference between the two performance levels for air barriers is not always understood by industry professionals and installers, and not clearly stated by codes. For example, American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1-2013, Energy Standard for Buildings Except Low-rise Residential Buildings, Section 5.4.3.1.3.b, defines code-compliant air barrier assemblies as:

Assemblies of materials and components (sealants, tapes, etc.) that have an average air leakage not to exceed 0.04 cfm/sf under a pressure differential of 0.3 in. w.g. (1.57 psf) when tested in accordance with ASTME 2357, ASTM E1677.

As evident from this article, performance levels for ASTM E1677 and ASTM E2357 are not equivalent—nevertheless, ASHRAE 90.1-2013 provides the two options as equals. It is little wonder there is confusion in the industry, and the potential impact of changes to the manufacturer’s installation guidelines is not always appreciated.

A building wrap air and water barrier system is installed over the exterior sheathing, exterior insulation is installed over the air barrier, and exterior cladding (brick, metal panels, and solid surfacing) are installed to the outside.

A building wrap air and water barrier system is installed over the exterior sheathing, exterior insulation is installed over the air barrier, and exterior cladding (brick, metal panels, and solid surfacing) are installed to the outside.

Fortunately, Air Barrier Association of America (ABAA) recently introduced an evaluation process for air barriers, in order to apply consistent standards across the industry. On its website, the group lists the air barriers that have been evaluated and demonstrated to meet ASTM E2357 performance level. (For more, see “ABAA-evaluated Air Barrier Assemblies.”)

Current limitations of air barrier standards
Code requirements on air leakage control have led to a large increase in the number of materials claiming to perform as air barriers. The challenge with some airtight materials is in achieving a continuous and durable air barrier system. For example, many materials designed to perform other functions (e.g. thermal insulation or exterior sheathing) that are also resistant to air infiltration have been promoted as air barriers. While such products are adequate air barrier ‘materials,’ the long term continuity and durability of these air barriers as ‘systems’ is still an open question.

Some such materials have passed the current air infiltration resistance requirements for ‘as-installed’ air barrier assemblies per ASTM E2357 and are listed at the ABAA website. However, these systems may fall short of long-term durability under the use conditions. Additional testing, such as thermal cycling and water resistance, would be necessary to assess the long-term durability of these systems. This discussion is beyond the scope of this paper, but it should be of concern to the industry.

Conclusion
Building energy codes mandate a continuous air barrier for leakage control. The air barrier system must withstand the conditions a building is exposed to during its use. There are two acceptable performance levels for air barrier wall assemblies—ASTM E1677 and ASTM E2357—that are determined by the structural design parameters for the building envelope. Some air barrier manufacturers have developed two-tier installation guidelines for the desired level, and altering the guidelines could change the performance.

A fl uid-applied air and water barrier system is ideal for this building, which consists of multiple substrates and exterior claddings. The air barrier was installed over CMUs and gypsum board sheathing substrates; the multi-story curtain wall consisted of cut limestone on the fi rst fl oor, brick veneer on the upper fl oors, cast stone trim work, and perforated metal panels.

A fluid-applied air and water barrier system is ideal for this building, which consists of multiple substrates and exterior claddings. The air barrier was installed over CMUs and gypsum board sheathing substrates; the multi-story curtain wall consisted of cut limestone on the first floor, brick veneer on the upper floors, cast stone trim work, and perforated metal panels.

A major limitation of ASTM E2357 is the lack of water infiltration resistance requirement. Very few manufacturers integrate testing for ASTM E2357 air infiltration resistance with ASTM E331 water infiltration resistance of installed wall assemblies.

Current test methods are effective in measuring performance of newly installed air barrier assemblies under pressure differentials experienced by above-grade exterior walls and represent a huge step forward from relying solely on materials properties. However, current standards do not provide information about the long-term performance under field use conditions experienced by the buildings, which include seasonal and daily temperature variations.

The air barrier system performance is only as good as the weakest link, and differential expansion and contraction of multicomponent air barrier systems can compromise its continuity. Integration of rigorous structural integrity testing of air barrier wall assemblies with thermal cycling and water infiltration resistance will provide valuable information on the long-term durability of these systems.

Notes
1 These requirements have been described in various articles, including 2006’s “Air Barriers: Walls Meet Roofs,” by Wagdy Anis and William Waterston (www.shepleybulfinch.com/pdf/Air_Barriers_wall_meets_roof_final.pdf) and 2004’s “Air Barriers, Research Report,” by Joseph Lstiburek (www.buildingscience.com/documents/reports/rr-0403-air-barriers). Additional references can be found at the Air Barrier Association of America (ABAA) web site at www.airbarrier.org. (back to top)
2 The impact of air barriers’ vapor permeance on moisture management has been discussed by co-author Spinu in two other articles that were published in The Construction Specifier: April 2007’s “To Be or Not to Be Vapor-Permeable,” and November 2012’s “Designing without Compromise: Balancing Durability and Energy Efficiency in Buildings.” (back to top)
3 The wind loads and schedule considered in these tests have been developed by the ASTM standard committee. While the authors are not part of this committee, it is possible one of the reasons for developing multiple pressure loads is to extrapolate the data at low pressures through linear regression. At low pressure loads, the errors are larger than at high pressures, so it is important to have multiple data points. (back to top)
4 The standards assume the air barrier plane will take the full wind loads (even though this would only be true for pressure-equalized façades). (back to top)
5 The air barrier structural loading is based on the assumption the air barrier plane: (1) takes the full wind loads (even though this only occurs for pressure-equalized façade systems), (2) experiences thousands of cycles of high positive and negative pressure loads during its service life, and (3) experiences two severe storms in the first 15 years of service. The steel studs shown buckling in the picture are at the very high end of the pressure loads. The point is when proper fasteners and spacing are used, air barriers can perform under wind gust conditions. These tests are quite stringent, but air barriers must perform for the life of the building envelope and such conditions could be occasionally experienced. (back to top)

Maria Spinu, PhD, LEED AP, is a building scientist with DuPont Building Knowledge Center, where she has led global building science and sustainability initiatives for the commercial market for the past decade. She is the author of 16 patents and has been a speaker at many regional, national, and international conferences on building science and sustainability topics. Spinu can be contacted via e-mail at maria.spinu-1@dupont.com.

Benjamin Meyer, RA, LEED AP, is a building science architect with DuPont Building Knowledge Center, where he works with customers and industry associations to answer questions on commercial building envelope design. Meyer is on the board of the Air Barrier Association of America (ABAA), a member of the Materials and Resources Technical Advisory Group of LEED, and also a consultant of the ASHRAE 90.1 Envelope Subcommittee. He can be reached benjamin.meyer@dupont.com.

Andrew Miles is a forensics engineer, providing technical support as part of the DuPont Tyvek Specialist network. His responsibilities include mock wall testing and field investigations related to use, performance, and customer concerns. Miles can be e-mailed at andrew.s.miles@dupont.com.

Making Sense of Sprayed Polyurethane Foam

All photos courtesy Spray Foam Coalition

All photos courtesy Spray Foam Coalition

 

by Peter Davis

For decades, the U.S. design and construction industry has turned to sprayed polyurethane foam (SPF) to insulate and air seal buildings. SPF can help provide temperature control in various climates, reduce sounds transmitted through the air, and lower construction costs.

When employed as a roofing material, SPF’s monolithic nature allows for a seamless, self-flashing application that can keep out water. It can also improve energy efficiency through its superior insulating and air barrier qualities, helping building owners and general contractors comply with energy codes and meet performance requirements for green building programs and certifications.

As the use of SPF grows, the industry is working to provide answers so architects, engineers, and construction professionals can be confident when specifying SPF insulation or roofing to achieve energy-saving or sound-dampening.

Types of SPF
SPF insulation can be categorized into three main types:

  • low-density, open-cell;
  • medium-density, closed-cell; and
  • high-density, closed-cell.

The molecular structure of the polyurethane cells in the foam produced determines whether SPF is classified as open- or closed-cell. Each type has certain characteristics determining the applications for which it is most appropriate.

Open-cell SPF
Also known as 1/2-pound SPF, which refers to the density of one cubic foot of the product, open-cell SPF is best suited for applications such as ceilings, interior walls, floors, and the underside of roof decks. As a low-density product, this type uses water as the blowing agent. When the foam forms, the water reacts with other chemicals to produce carbon dioxide (CO2), which expands the cells to form semi-rigid porous polymer foam. The CO2 leaves the cells and is replaced with air, hardening the foam.

Spray polyurethane foam (SPF) is a spray-applied material widely used to insulate buildings.

Spray polyurethane foam (SPF) is widely used to insulate buildings.

Closed-cell SPF
Closed-cell SPF, also known as 2-pound foam, is formed by using a blowing agent instead of water. The agent is retained in the closed cells, making the foam rigid and providing exceptional compressive strength. Closed-cell SPF can be further classified into two types: medium- and high-density. The former can be used to insulate:

  • exterior and interior walls;
  • ceilings;
  • floors;
  • slabs and foundation; and
  • the underside of roof decks.

High-density foam is used primarily in flat or low-slope roofing applications, since its density and rigidity lends itself best to this purpose.

Quality installation
One of the most important considerations for architects and builders is selecting a professional contractor to install SPF. Each manufacturer has its own model specification to help architects and specifiers choose the proper product. A contractor should be able to educate architects and builders about the product, its applications, and installation process, including any mechanical ventilation needs during the installation and afterwards.

Qualified contractors can also explain best safety practices, such as the type of protective equipment workers wear and how they keep others out of the space during installation and curing. The latter is especially important, because other trades and building occupants should not be in the area when SPF is being applied and curing. Re-entry time can vary depending on air temperature, humidity level, and the type of SPF applied. Once the product cures, it is considered to be essentially inert, according to the U.S. Environmental Protection Agency (EPA), meaning the chemicals have stopped reacting. (The SPF contractor can advise when it is safe to re-enter the space.)

General contractors and specifiers should consider using an SPF company that employs individuals who have completed the Center for the Polyurethane Industry’s (CPI’s) SPF Chemical Health and Safety Training, and who have been certified by the Spray Polyurethane Foam Alliance’s (SPFA’s) new Professional Certification Program for SPF applicators. The comprehensive certification program, developed in compliance with American National Standards Institute/International Organization for Standardization (ANSI/ISO) 17024, Accreditation Program for Personnel Certification Bodies, focuses on safety, quality installation, and professionalism.

Air, sound, and vapor barrier

SPF’s monolithic installation allows it to be used around irregular shapes and penetrations.

SPF’s monolithic installation allows it to be used around irregular shapes and penetrations.

A reliable air barrier and a continuous seal are essential elements in creating an energy-efficient, comfortable space. Both types of SPF meet the requirements of an air barrier material at a typically installed thickness of 25 mm (1 in.). When installed with other materials in a building assembly, SPF can provide an effective continuous air barrier.

By acting as both insulation and an air barrier, it could even help lower construction costs, because less air sealing materials would be required to meet local and state building energy codes for air leakage mandates.

Since SPF adheres to the substrate, it allows for easy monolithic installation around irregular shapes and penetrations. The material is applied as a liquid and then expands into foam in any nook and cranny in the enclosure to provide a seal. This offers energy performance and occupant comfort.

Open-cell SPF, typically associated with residential applications, is commonly used to fill cavities in interior spaces or to insulate unvented attics. This type is moisture vapor-permeable, and usually requires a properly designed and installed vapor retarder. Generally, open-cell foam has an R-value between R-3 and R-4 per 25 mm (1 in.) of thickness.

Open-cell SPF has also been used on the underside of roof decks in multiple climate zones for years. As with the usage of all building products, the building science of the structure needs to be understood. Potentially, a vapor barrier may be needed with open-cell SPF. Open-cell is vapor permeable, so depending on the structure, design, and climate zone, a determination of whether a vapor barrier needs to be added should be made. If a roof leaks when open-cell SPF is used on the underside of the roof deck, the water will likely gradually move its way through the open-cell SPF. Since it is an open-cellular matrix, the water, in a relatively short period of time if in sufficient quantity, will pass through the foam, and the leak can be identified and then repaired.

Closed-cell is the dominant SPF material for commercial construction, especially when used as an air barrier and thermal insulation system applied on the building’s exterior, or as foundation and slab insulation. This type of SPF has a higher R-value than open-cell—typically between R-6 and R-7 per 25 mm of thickness. Its relatively low moisture permeability means it rarely requires an additional vapor retarder. An exception may apply in areas, such as bathrooms, with high relative humidity (RH).

Regardless of the project type, understanding SPF and its influence on a building’s energy performance is critical. During the design process, architects and general contractors need to take these impacts into account so they can take advantage of SPF’s energy-saving properties. For example, buildings using SPF as the insulation of choice typically require the use of smaller HVAC systems because less air escapes the building, reducing the heating and cooling loads.

SPF insulation seals gaps to reduce air leaks.

SPF insulation seals gaps to reduce air leaks.

SPF benefits
While SPF is most often associated with energy-saving properties, it has numerous other benefits, including soundproofing. In commercial and residential buildings, open-cell foam is typically used in interior partitions for sound control. Since SPF seals the cracks and crevices in a building, and adds another layer between the interior and exterior, it helps dampen noises that travel through the air, such as the sound of an airplane overhead or a phone conversation in the adjoining office.

Given SPF’s ability to air seal, it is necessary to design proper air distribution systems to control moisture and air flow within the finished building. While a continuous seal is desired, interior spaces require a certain amount of outside ventilation to maintain air quality. Similarly, moisture created by cooking and bathing must be able to dissipate safely within the building.

Structural integrity
Ultimately, all construction projects are judged on their integrity—how long they can withstand the tests of the elements and time. SPF, especially closed-cell foam, enhances a building’s strength and stability because of its rigid structure.

Many of the properties making SPF effective as a stabilizer also make it attractive for flat roofing applications. SPF roofing, a high-density closed-cell foam, can form a continuous insulation (ci) barrier on the top of a roof deck. Since SPF roofing has no seams or joints and is rigid, it forms an impermeable surface. Since it is fully adhered to the substrate, the rigid foam provides exceptional uplift resistance during severe storms producing high winds.

About 10 months after Hurricane Katrina, the National Institute of Standards and Technology (NIST) issued, “Performance of Physical Structures in Hurricane Katrina and Hurricane Rita: A Reconnaissance Report”1 on damage to buildings in the Pascagoula, Mississippi area. It found all but one of the buildings with SPF roofs made it through the storm “extremely well without blow-off of the SPF or damage to flashings.” For the building that was the lone exception—just one percent of its roof area had failed.

An SPF roof properly maintained with regular recoats of the exterior membrane can last for decades. According to SPFA, some SPF roofs have lasted for more than 30 years. Closed-cell SPF also enhances a structure’s resistance to water damage. By acting as a barrier to water and condensation in the building envelope, SPF can help a building resist the growth of mold and mildew. Its ability to adhere to and around surfaces ensures every nook and cranny is filled, so there are no spots for these to grow. Its water-proofing abilities extend to increased floodwater protection as well.

Closed-cell SPF is a material that meets Federal Emergency Management Agency (FEMA) requirements for a Class 5 flood-resistant material—the highest class of materials that can resist damage from floods, according to a FEMA technical bulletin, “Flood Damage-resistant Materials Requirements for Buildings Located in Special Flood Hazard Areas in accordance with the National Flood Insurance Program.” This class of material can submerged for 72 hours, and can easily be dried and cleaned following a flood.

SPF’s monolithic nature allows for a seamless, self-flashing roofing application to protect against moisture.

SPF’s monolithic nature allows for a seamless, self-flashing roofing application to protect against moisture.

Green building benefits
As green building practice and techniques become the norm, many building owners, designers, and general contractors want to reduce the environmental impact of buildings. Due to its superior insulating qualities, SPF allows the building community to achieve a balance between energy efficiency, building durability, and comfort. It can also help them meet the requirements of programs such as EnergyStar and the Leadership in Energy and Environmental Design (LEED) rating program. Additionally, a study by SPFA, “Life Cycle Assessment of Spray Polyurethane Foam Insulation for Residential & Commercial Building Applications,” found energy and environmental benefits of using SPF for retrofits of non-residential roofs and residential applications outweigh the amount of energy and environmental impacts associated across the product’s lifecycle.2

Conclusion
With several types of SPF available and numerous application possibilities, it is worthwhile for architects, specifiers, and builders to gain a deeper understanding of this product. SPF allows for more creative design, filling in cavities and covering surfaces that could otherwise pose challenges. It helps reduce air infiltration, eliminating intrusions from dust and pollen and making buildings more comfortable. As a roofing material and exterior insulator, SPF can strengthen a structure by increasing its water resistance and durability.

Notes
1 To read this report, visit www.nist.gov/customcf/get_pdf.cfm?pub_id=908281. (back to top)
2 The “Life Cycle Assessment of Spray Polyurethane Foam Insulation for Residential & Commercial Building Applications” report can be viewed at www.sprayfoam.org/files/docs/SPFA%20LCA%20Long%20Summary%20New.pdf. (back to top)

Peter Davis is chairman and CEO of Gaco Western, chairman of the Spray Foam Coalition at the Center for the Polyurethanes Industry, and serves on the executive committee of the Spray Polyurethane Foam Alliance (SPFA). He can be reached via e-mail at pdavis@gaco.com.

Avoiding Problems in Aquatics Facilities: Atypical design for atypical buildings

Photo © BigStockPhoto/Nikita Sobolkov

Photo © BigStockPhoto/Nikita Sobolkov

by Jason S. Der Ananian, PE, and Sean M. O’Brien, PE, LEED AP

If an office building in a cold climate is designed to run at a slight positive pressure while omitting air barrier details at the building enclosure, the likely consequences include higher energy costs and potentially isolated condensation events or freezing pipes during very cold weather, with problems developing in five to 10 years. If this was a natatorium, however, the resulting damage may include human-sized icicles at roof eaves, concealed corrosion of metal framing components, widespread efflorescence on exterior concrete and masonry, and premature failure of the building enclosure components—often within months.

Few, if any, building types present the risks and challenges found in indoor swimming pool facilities. With far higher interior moisture loads than typical buildings and a potentially corrosive interior environment, natatoriums put structural and enclosure systems to the test, especially in cold or even mixed climates.

The authors’ firm has investigated dozens of natatoriums throughout the country and witnessed firsthand the swift and severe nature of failures that can result from improper design and construction. In some cases, the design included the primary components necessary for moisture control, but lacked transition details or did not adequately define the system’s continuity. Others were well-designed but poorly constructed, or poorly designed but built exactly as shown on the drawings. Still others may have functioned well from an enclosure standpoint, if not for significant shortcomings in the mechanical systems’ design or operation.

The watercube—Beijing's National Aquatic Center—may be an extreme example, but even the most basic of natatoriums can present challenges for design professionals when it comes to thermal and moisture control. Photo © BigStockPhoto/Liang Zhang

The watercube—Beijing’s National Aquatic Center—may be an extreme example, but even the most basic of natatoriums can present challenges for design professionals when it comes to thermal and moisture control. Photo © BigStockPhoto/Liang Zhang

Problems are often found in natatoriums that are part of a larger athletic complex where designers fail to make the distinction between ‘typical’ enclosure systems and more specialized systems required for pools, or fail to prevent moisture migration between the pool and adjacent spaces.

Moisture loads and controls in natatoriums
The air in a natatorium often contains nearly three times the moisture per unit volume as a typical, non-humidified building. This greatly increases the importance of controlling moisture transport through the building enclosure. Typically, three forms of moisture transportation can contribute to problems:

  • water leakage;
  • water vapor diffusion; and
  • airflow.

Water leakage, which occurs when water finds a path into the building, is controlled through water management and waterproofing systems that are beyond this article’s scope. Water vapor diffusion, or the movement of water vapor driven by vapor pressure differentials, is typically a slow process that can result in long-term moisture accumulation or condensation within the building enclosure cavities; it is controlled by a vapor retarder.

Airflow is the main contributor to water vapor and heat transport in the enclosures of most buildings. Unless it is controlled by an effective air barrier system, air will flow through the building enclosure cavities where it can condense on cooler surfaces as it travels toward the exterior.

Although critical for moisture control in natatoriums, non-humidified buildings can often provide moderate (although not necessarily optimal) performance and avoid condensation without air barriers in the enclosure, and vapor retarders may only be necessary in extremely cold climates. This is reflected in most building codes, which, until recently, made no mention of continuous air barriers and often do not require vapor barriers in warm or moderate climates (Climate Zones 1 through 4, as defined in American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings.

However, it is important to realize the building code is intended for non-humidified/‘general-use’ buildings, and does not specifically cover special buildings such as natatoriums and museums—both of which require atypical interior conditions. The authors’ firm has investigated many natatoriums that complied with building enclosure requirements outlined in the applicable building code, yet still suffered from significant moisture problems. Relying solely on the building code for design guidance is unlikely to result in a durable, functional natatorium.

CS_DECEMBER_2013.indd

Air and vapor flow through natatorium enclosures
High moisture levels in natatorium environments greatly increase the risk of interior surface condensation on cold components, such as windows, doors, roof penetrations, and drains. These conditions also provide an ideal environment for condensation to collect within walls and roofs due to air leakage and vapor diffusion.

Humid air from natatorium spaces that migrates into the exterior walls condense once reaching a surface below the air’s dewpoint temperature. Condensation concealed within wall or roof assemblies may go unnoticed by the building owner until these assemblies are severely degraded. For a natatorium in cold climates, the risk of condensation exists for most of the year, often eight to 10 months. Moisture levels can be significantly reduced by covering the pool when swimmers are not present, but problems may still occur when the natatorium is actively being used.

A properly designed natatorium building should include both a vapor retarder and, more importantly, a continuous air barrier system in the walls and roofs to minimize moisture migration through the enclosure (they may or may not be formed from the same material). Although the terms ‘vapor barrier’ and ‘air barrier’ are often used interchangeably, the two systems have differences in the way they control moisture and the construction necessary for them to be effective. Figure 1 highlights these differences.

Controlling air and vapor flow
Vapor retarders minimize water vapor’s flow through materials by diffusion. Water vapor can flow through the internal pore structure of apparently solid materials such as wood, concrete, and gypsum board. If the air within a building has higher moisture content than the exterior environment, the vapor drive is toward the exterior, tending to ‘push’ water vapor from the inside to the outside.

In cold and mixed climates, this water vapor may condense within the wall or roof as the temperature drops. In this case, a vapor retarder on the interior—or ‘warm’—side of the insulation helps prevent water vapor from reaching colder temperatures, minimizing the risk of condensation in the wall. Conversely, a vapor retarder on the wrong side of the wall, outboard of the primary insulation where it experiences low temperatures, can act as a collection point for condensation and lead to more significant problems than if no vapor retarder was installed. Since the driving force behind water vapor diffusion is relatively minimal, vapor barriers can contain small holes, such as fastener penetrations, and do not require sealed laps to be effective. Even if larger discontinuities exist, damage due to moisture migration in these areas will tend to be localized.

Tracer smoke exfiltration at roof eave. Images courtesy SGH

Tracer smoke exfiltration at roof eave. Images courtesy SGH

Moving air carries both heat and moisture. The magnitude of moisture migration via airflow can be 50 to 100 times that associated with water vapor diffusion alone. Air flows from high to low pressure regions. In buildings, such differentials can result from mechanical system operation, wind, stack effect, or a combination thereof. These pressure differences can exert significant force on air barrier systems, making it necessary for air barriers to have continuous structural support. Even small holes or discontinuities in the air barrier can allow significant air leakage and greatly reduce the system’s effectiveness, especially in buildings with high moisture levels. Complicating matters, a small hole on the interior can lead to air leakage into many other locations, as opposed to creating localized damage as with discontinuous vapor retarders.

The air barrier in a building is more than just a single material. It comprises interconnected components including airtight materials in walls, roofs, doors, windows, curtain walls, and other enclosure elements. To maintain the system’s continuity, airtight transitions are needed between all components, including interior partitions separating the natatorium from adjacent spaces.

The goal of a well-designed and properly installed air barrier is to eliminate uncontrolled airflow through the building enclosure. Uncontrolled airflow results in increased heating and cooling loads; it can also transport moisture or chlorinated air to areas where moisture or odor exposure is undesirable. Critical transitions, such as the roof-to-wall intersection, must be carefully detailed, as poor transitions can greatly reduce air barrier performance and mechanical system effectiveness in controlling building pressure.

The air barrier’s construction is just as important as its design. Success requires coordination between multiple trades at multiple points in the schedule, such as roof-to-wall intersections and window/curtain wall perimeters. Designers must take into account the potential for sequencing conflicts during construction.

Interior partitions that separate interior high-humidity zones from adjacent, non-humidified or even unconditioned interior zones are an oft-overlooked component of the natatorium air barrier system. Natatoriums are commonly part of a larger complex of buildings. Since spaces like offices, storage rooms, and gymnasiums are not usually designed to function under high-humidity conditions, moisture-laden airflow into those spaces through unsealed interior partition walls may lead to significant damage to interior components. Additionally, leakage into adjacent spaces may cause damage to exterior components, if those spaces are located near exterior walls not designed to tolerate high humidity.

Even in the absence of condensation, airflow to and from adjacent spaces can also minimize the mechanical system’s ability to control air pressure within the natatorium (discussed in more detail later in this article). Unless a space is specifically designed to tolerate high humidity, it must be completely air-sealed and isolated from any adjacent spaces that may function as moisture sources.

Natatorium investigation reports reviewed by the authors’ firm almost always cite “improper design/construction of the vapor retarder” as a primary cause of moisture problems. However, in natatoriums, even the absence of a vapor retarder rarely produces the same level of damage as an improperly designed/built air barrier system. This common confusion between air barriers and vapor retarders often results in poorly designed natatoriums and short-term failure of building enclosures.

Controlling interior surface condensation
Interior moisture levels in natatoriums are high, with dewpoint temperatures ranging between 12.8 and 18.3 C (55 and 65 F). Natatoriums in cold (and even mild) climates are susceptible to condensation on interior surfaces that drop below the ambient dewpoint during the winter. As such, interior surfaces in natatoriums must be kept warm, often as high as 18.3 C, to prevent condensation.

This infrared image of an exterior natatorium wall was taken during the winter in a heating climate. The orange/yellow regions indicate higher apparent surface temperatures and locations of air barrier breaches.

This infrared image of an exterior natatorium wall was taken during the winter in a heating climate. The orange/yellow regions indicate higher apparent surface temperatures and locations of air barrier breaches.

Opaque walls and roofs can typically be designed with continuous insulation and high R-values to meet this criterion. Windows, doors, and curtain walls must be high-performance thermally broken systems designed for high-humidity applications, although even the best fenestration likely experiences surface condensation or frost during the coldest times of the year.

Surface condensation can degrade adjacent construction materials and cause ‘fogging’ on glass surfaces. Several design strategies are effective at reducing the risk of interior surface condensation, including:

  • using high-performance fenestration systems;
  • aligning fenestration systems with the insulation;
  • avoiding installing highly conductive materials against fenestration; and
  • providing air curtains or directed flows of warm air over components or using electric heat-trace cables to deliver supplemental heat directly to glazing and framing systems (often the only way to completely eliminate condensation in cold-climate natatoriums).

Unlike ‘passive’ condensation control systems, such as thermally broken window frames, mechanical and electrical systems require regular maintenance to remain operational.

Natatoriums often include skylights to provide occupants with natural light. The increased condensation risk at skylights is due in part to their orientation; mounted in low-slope roofs or near-horizontal applications, skylights tend to lose more heat through radiation to the sky compared to similarly sized windows and doors (especially at night).

Supplemental heating systems can be difficult to install due to the high visibility of skylight systems. A compromise would be to design a high-performance skylight and install a system of gutters around the perimeter, ensuring any condensation is collected and drained (rather than dropping from the ceiling onto occupants). For gutters to be effective, skylights should be fairly steep in slope so condensing moisture flows down the glass surface to the sill gutters, rather than dripping into the space as can occur in low-slope skylights. The maintenance and cleaning of active condensation control or weep systems can be difficult, particularly on skylights located directly above the pool(s).

The photo illustrates the blower door test setup for quantitative air leakage testing with calibrated fans. These blower doors are also used for imposing pressure differential for qualitative testing.

The photo illustrates the blower door test setup for quantitative air leakage testing with calibrated fans. These blower doors are also used for imposing pressure differential for qualitative testing.

Natatorium air pressure control
It is impractical to achieve a perfectly airtight enclosure, and even small amounts of air leakage can result in condensation. Therefore, natatoriums must be maintained at a negative pressure relative to adjacent interior spaces and the exterior to minimize odor migration and potential for airflow-induced condensation in building enclosure cavities during winter. The 2011 ASHRAE Handbook–HVAC Applications recommends maintaining a negative pressure of between 12 and 37 Pa (0.25 and 0.77 psf) in the natatorium to minimize moisture and odor migration.

Interior partition wall openings, including doors, require gaskets to allow the HVAC to more efficiently control building pressures. Maintaining a negative pressure in the natatorium commonly involves balancing the mechanical system to return more air than is supplied into the building while maintaining the minimum required outdoor airflow rate per ASHRAE Standard 62.1-2010, Ventilation for Acceptable Indoor Air Quality.

It is important to understand simply providing more exhaust air than outside air is not sufficient to maintain negative pressure. Even with no outside air and significant exhaust flow, if the quantity of air supplied to the natatorium exceeds the amount returned (due to improperly sized or restricted ductwork, the operation of mechanical systems in adjacent spaces, etc.) the natatorium pressure will still be positive.

The HVAC system’s ability to control building pressure relies heavily on the airtightness of the natatorium enclosure. Controlling building pressures in a ‘leaky’ natatorium is difficult compared to a relatively airtight one. For these reasons, testing and balancing of the natatorium HVAC system should occur following the air barrier system’s completion.

Depending on the climate zone in which the natatorium is located, supplemental exhaust fans may also be necessary to maintain pressure control during the coldest times of year, particularly for facilities with high ceilings or in retrofit applications where the existing mechanical system cannot be practically or appropriately modified.

In cold climates, even with a properly balanced natatorium mechanical system, positive pressure may occur near the ceiling due to stack pressure (i.e. buoyancy of air). The authors often find natatoriums running ‘under negative pressure,’ where the slight differential at the pool deck level is insufficient to overcome stack pressure, allowing for high pressures and significant air exfiltration at the roof level where critical details such as roof-wall intersections often occur.

The supplemental exhaust fan (or even the primary mechanical system fan[s]) is best controlled by a pressure sensor located near the pool ceiling that time-averages the pressure differential between the interior and exterior; a control system speeds up or slows down the exhaust fan accordingly to maintain a zero pressure difference between inside and out. Similarly, pressure should be measured near the high point of the natatorium during testing and balancing of the mechanical systems—preferably during cold weather—to

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obtain a more accurate measure of pressure within the space.

Construction mockups of air barrier systems
Construction mockups are typically required for projects to verify both aesthetic and technical aspects of the design. As part of this project phase, mockups of the air barrier should include typical transitions (e.g. wall-to-fenestration, wall-to-roof, etc.) for review by the architect or third-party inspector. This work shall be used to:

  • test installation methods;
  • determine construction defects (if any);
  • establish the technical and aesthetic standard of care for the project; and
  • refine, if necessary, installation methods in accordance with the design intent before construction proceeds.

Mockups are also helpful for coordinating between trades.

The mockups should be performed as many times as necessary for approval by the architect and/or third-party inspector. Inspection of the air barrier should not be pushed to the punch list phase—testing of air barrier mockups should be performed before cladding installation to ease identifying and repairing of breaches.

Field testing of air barrier continuity
Performing whole building air infiltration testing can help verify the performance of air barrier installations as well as locate system defects in new and existing buildings. Several agencies and state building codes even require whole building air infiltration testing for new buildings. It is prudent to perform testing before the air barrier system is concealed by cladding materials or interior finishes so defects can be identified and repaired more easily. Removing cladding after construction is complete to locate air barrier discontinuities is often costly and disruptive to building occupants.

This poorly painted steel diving platform stair experienced heavy corrosion due to direct wetting.

This poorly painted steel diving platform stair experienced heavy corrosion due to direct wetting.

Field testing requires the building be positively or negatively pressurized using blower door fans or manipulating the HVAC system to force air to leak through any air barrier discontinuities in the building enclosure. Various quantitative and qualitative techniques are available to identify leakage paths, including tracer smoke and infrared (IR) thermography. The architect should write the field testing requirements of air barriers into the project specifications.

Qualitative air leakage testing
ASTM E1186, Standard Practices for Air Leakage Site Detection in Building Envelopes and Air Barrier Systems, describes various qualitative methods to locate air barrier discontinuities. One such practice is to pressurize or depressurize the building or individual spaces by using fans or by manipulating the HVAC system, and then using a tracer smoke source over the interior or exterior surfaces of the building enclosure.

Placing the tracer smoke source at the building interior and pressurizing the building or space to locate air exfiltration sites reduces the influence of wind or stack effect. In this case, tracer smoke will be ‘pushed’ from the building through any breaches in the air barrier and be identifiable at the building exterior (Figure 2).

Although it is possible for some projects to depressurize the building and locate the source of tracer smoke on the exterior, this method may be difficult because of the influence of wind and the risk tracer smoke rapidly dissipates before it is drawn into the interior through the air leakage site.

IR thermography (per ASTM E1186) is another useful and efficient qualitative method to locate discontinuities in the air barrier. The purpose of the IR scans is to identify locations of elevated heat loss through the building enclosure. Air infiltration or exfiltration through the building enclosure affects the temperature of wall or roof components in the region of air leakage pathways, given the interior and exterior temperature difference; IR scanning equipment can be used to detect local surface temperature differences (Figure 3).

The conditions most conducive to accurate IR scans are low winds with a large temperature difference (at least 16 C [30 F]) between the interior and exterior air temperatures. Using fans or the HVAC system to pressurize or depressurize the building during the IR scans can exacerbate air leakage through discontinuities in the air barrier, making it easier to identify air barrier breaches on an IR image. Since thermal bridges or insulation discontinuities in the enclosure can also result in surface temperature differentials, it is typically necessary to perform multiple scans—from both the interior and exterior, and with the building under positive and negative pressure—to isolate the contribution of thermal bridges and more accurately identify air leakage sites.

Quantitative air leakage testing
Blower door testing per ASTM E779, Standard Test Method for Determining Air Leakage Rate by Fan Pressurization, is intended to characterize the airtightness of the building enclosure. The test results can be used to compare the subject’s airtightness to similar buildings or against criteria set by industry standards and governing building codes; it can also be used to determine how readily the HVAC system can be adjusted to control pressure and reduce leakage. The tests are conducted using calibrated fans (Figure 4) to pressurize or depressurize the building under controlled conditions.

The ASTM E779 test procedure typically requires a range of induced pressure difference (pressurization and depressurization) from 10 to 60 Pa (0.2 to 1.25 psf). The measured air leakage flow rates (cubic feet per minute) are typically normalized using the above-grade building surface area (walls and roof) and calculated as an air leakage rate at 75 Pa (0.3 in. water column) pressure difference.

While quantitative testing is useful from an overall performance standpoint, a qualitative leak assessment should always be done in natatoriums due to the potential for condensation problems even at small leakage sites. On the same note, when the building is extremely airtight, but experiences all the leakage at one large air barrier breach in the enclosure, identifying the location of that breach is critical.

compounds.

Corrosion on painted structural steel can be caused due to exposure to airborne chlorine compounds.

Corrosion issues
Although some pools have begun to employ alternative technologies to treat pool water, such as ozone or ultraviolet sterilization, the majority of swimming pools still use chlorine-based chemicals for pool water treatment. This is primarily due to the higher initial cost of alternative treatment systems, but also the entrenched nature of chlorination as a means of water treatment, which has been in use for more than a century. While an effective disinfectant, chlorine has the unfortunate side effect of being highly corrosive to typical steels and even some stainless steel alloys.

The most common form of corrosion in natatoriums is visible surface corrosion, which affects bare steel or steel with insufficient corrosion protection. This can affect both materials which are directly exposed to pool water (Figure 5) and those which have no direct wetting and are affected by the chlorine compounds in the air only (Figure 6). In both cases, corrosion can be greatly exacerbated by improper maintenance of the pool water chemistry, which can result in higher levels of chlorine compounds in both the water and air.

Stainless steels are often used in swimming pools to combat corrosion, but even these specialty metals have their limitations—especially in chlorinated environments. Common alloys such as Types 304 and 316 can work well in areas where the components are frequently cleaned or wetted/splashed, as this tends to prevent chloride compound buildup on surfaces.

Infrequently cleaned surfaces may corrode quickly once a film of chlorides builds up on their surfaces (Figure 7). This is counterintuitive to many designers, since a stainless steel component that never comes into contact with pool water would, at first glance, appear to have little risk of corrosion. Stainless steel ductwork (often specified due to its perceived superior corrosion resistance) is one of the most commonly corroded items, but stainless steel light fixtures or hangers are also at risk.

Some stainless steel components are also susceptible to the more dangerous stress corrosion cracking (SCC). This type of corrosion often produces little to no outward evidence; rather, it affects the steel’s structure, leading to sudden, brittle failure. For SCC to occur, a susceptible grade of stainless steel (most standard chromium-nickel stainless steels, such as Types 304 and 316, fall into this category) must be placed in a corrosive environment and subjected to a tensile load. Hanger rods for overhead components are the most commonly affected, although other formed metal components, which can contain residual tensile stresses from forming operations, can also fail due to SCC.

There have been several examples of structural failures due to SCC in natatoriums, including ceiling collapses in:

An example of the heavy corrosion of stainless steel ductwork that can occur in a 10-year old natatorium.

An example of the heavy corrosion of stainless steel ductwork that can occur in a 10-year old natatorium.

  • Switzerland (1985), due to failure of stainless steel hangers (resulting in 12 reported casualties);
  • Netherlands (2001), due to failure of stainless fasteners; and
  • Finland (2003), due to failed stainless steel hangers.

This risk means using stainless steel in overhead or safety-critical components must be carefully evaluated. In these cases, specialty alloys such as those containing higher levels of nickel and molybdenum (e.g. Types 904L and 254 SMO), which are more resistant to SCC, are likely necessary.1

Although stainless steels tend to be more expensive than painted steel or aluminum, the higher initial cost is typically justified by reduced long-term costs associated with maintenance, repair, or replacement. This is especially true for components at or near the pool deck which are routinely wetted. Most other metals, even those painted or galvanized, will have greatly reduced service lives in these applications.

For larger structural applications, stainless steel is not practical in terms of cost and availability. In these applications, high-performance paints/coatings—often combined with galvanization—are required for long-term performance. As shown in Figure 7, breakdown of applied paints can result in rapid corrosion of the base metal. Even with high-performance coatings, owners should expect some maintenance and eventual recoating of steel components.

Summary of design strategies
Based on the discussion in this article and personal experience in the design, construction, and investigation of natatoriums, the authors present the following summary of design guidelines for indoor swimming pools. (This is not an exhaustive list of all design concerns, but focuses on primary issues with the building enclosure and interior environment.)

  1. Design a continuous air barrier system for the exterior enclosure, including exterior and interior components that separate the natatorium from adjacent spaces.
  2. Design continuous insulation for the enclosure and minimize the incidence of thermal bridges and structural penetrations through the insulation.
  3. Design an appropriate vapor retarder for the enclosure. This can be the same material as the air barrier, depending on the insulation’s location.
  4. Use high-performance fenestration systems aligned with the thermal insulation, combined with active systems such as warm air washes, to minimize the incidence of interior condensation.
  5. Balance mechanical systems to provide negative air pressure within the natatorium for the full height of the space, not just at the pool deck level. Negative pressure levels must be sufficient (or adjustable) to overcome stack pressure down to the local exterior design temperature. Ductwork should be carefully designed and systems balanced/confirmed before filling of the pool. Confirm, through a testing and balancing report, that the airflow into the pool space is less than the airflow back to the mechanical system. Confirm space pressures by direct measurement in addition to measuring supply/return quantities.
  6. Avoid using stainless steel in applications that are deemed ‘safety-critical’ or where components will not be frequently wetted or cleaned. When stainless steel is employed for safety-critical or overhead applications, a specialty alloy is likely necessary.
  7. Avoid specifying stainless steel for ductwork. Non-corrodible fabric ductwork, painted aluminum, or painted galvanized steel are typically better options.
  8. Write tight specifications for air barrier systems, including provisions for field testing of mockups, installed assemblies, and the whole building enclosure. Testing should include both quantitative and (concurrent) qualitative methods to identify overall leakage rates as well as localized breaches in the air barrier system.

Notes
1 For more information, visit www.nickelinstitute.org/NickelUseInSociety/MaterialsSelectionAndUse/~/media/Files/NickelUseInSociety/Architecture/Successful_Stainless_Swimming_Pool_Design.ashx. (back to top)

Sean M. O’Brien, PE, LEED AP, is an associate principal at the national engineering firm Simpson Gumpertz & Heger (SGH), specializing in building science and building enclosure design and analysis. He is involved in both investigation/forensic and new design projects. O’Brien is a member of the American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE), co-chair of the New York City Building Enclosure Council (BEC-NY), and a frequent speaker and author on topics ranging from building enclosure design to energy efficiency. He can be reached at smobrien@sgh.com.

Jason S. Der Ananian, PE, is a senior staff engineer at SGH, specializing in building enclosure design and building science. Der Ananian has more than a decade of experience investigating and designing repairs for art storage facilities, natatoriums, museums, and university facilities. A member of ASHRAE, he has published papers on topics including window flashing, whole-building energy simulation tools, and moisture migration in asphalt shingle roofs, along with quality control of air barriers during construction. Der Ananian can be contacted via e-mail at jsderananian@sgh.com.