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Draining the Rain: Advancements in engineered rainscreen walls


Images courtesy Advanced Building Products

by Keith Lolley, CSI
Over the years, there has been a tremendous amount of innovation in the building industry—still, the ability to achieve a waterproof wall system still eludes design/construction professionals. If the wall is not properly designed, this moisture will remain trapped causing numerous issues, such
as the corrosion of structural reinforcing, and the proliferation of rot and mold.

Trapped moisture will decrease the effectiveness of certain insulations and decrease the overall life span of the building. Anecdotally speaking, roughly 90 percent of all wall failures are the result of moisture-related issues. Moisture intrusion is a concern architects and contractors need to give strong consideration to during the designing stage and especially the building process of projects.

Cavity wall systems
The cavity wall system is designed to properly ‘drain the rain.’ These systems are typically designed with a backup wall, airspace, and outer veneer. Moisture management components—such as through-wall flashings, mortar deflections, and drainage devices—at flashing locations are used to divert moisture entering the wall back to the outside. Figure 1 is a typical cavity wall detail.

There are a few key factors to a successful cavity wall. First, the through-wall flashing specified must last the life of the building. It costs roughly $275/sf to replace failed flashing, so the economic value to doing it right the first time is obvious.

Figure 1

Figure 1: Typical cavity wall design.

Another important aspect to a successful cavity wall is a clear cavity. Without a clear air space, moisture will not drain effectively. A 50-mm (2-in.) cavity is the industry standard for commercial cavity wall construction. However, new energy standards and codes—such as American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, and the 2009 International Energy Conservation Code (IECC)—mean a call for increased insulation. This increase in wall insulation is causing the cavities to get larger and actually making the air space smaller. This air space reduction is causing wall performance concerns. What if there was a way to minimize the cavity space without jeopardizing the overall effectiveness of the wall?

Finding a better way
Engineered rainscreen wall systems have been around for quite some time, but there remains confusion as to the difference between a pressure-equalized rainscreen wall and a cavity wall. First, one must modify the terminology slightly. Pressure equalization is a lofty goal and one that is difficult to truly achieve. In reality, the goal is to create a pressure-moderated wall system. These systems are known as ventilated façades or modified rainscreen walls.

Differential air pressures between the inner wall and outside environment will draw moisture into the wall system’s inner structure. A cavity wall system does nothing to prevent this from happening; a pressure-moderated rainscreen wall, on the other hand, will cut down on the differential air pressure that draws moisture into the building by allowing air into the wall system to partially neutralize the air pressure behind the cladding to the air pressure outside of the wall system (Figure 2).

As air is introduced into the cavity, it works its way up the wall and out through vents installed at the top of the wall. The presence of the air/moisture/vapor (AMV) barrier allows the introduced air to circulate in a convective fashion. This convective airflow removes excess moisture vapor while drying any residual moisture within the cavity at the same time (Figure 3).


Figure 2: These figures demonstrate pressure neutralization. Images courtesy International Masonry Institute

Figure 2A

Figure 3: The correct path of drainage and ventilation can be made possible by utilizing an engineered rainscreen drainage mat.

Pressure-moderated wall systems consist of:

  • backer wall;
  • through-wall flashing;
  • AMV barrier;
  • outboard rigid foam insulation;
  • clear, vented airspace with ventilation devices at the top and bottom of the walls; and
  • tough exterior cladding (Figure 4).
Figure 3

Figure 4: Proper components of an engineered rainscreen wall for cavity walls.

Drainage and ventilation mats are crucial when design parameters only allow a narrow cavity width and reduced air space. By incorporating an all-wall drainage mat in these rainscreen wall applications, the airspace can be reduced from 38- or 50-mm (1 ½- or 2-in.) down to 25 mm (1 in.) without compromising any of its functionality.

Using an all-wall drainage mat could also reduce the cost of the overall wall system. For example, a narrower air space reduces the width of the through-wall flashing needed. This narrower cavity will also reduce the size of the veneer anchoring system, along with the concrete costs. The industry standard for an air space is 50 mm; however, the code minimum is 25 mm, despite this dimension not being recommended by most industry organizations and experts. By building with an all-wall drainage mat and code-minimum air space, the wall will effectively drain and ventilate.

All-wall drainage mats are typically made from either a corrugated sheet, dimpled mat, or a random-entangled nylon or polypropylene net material (Figure 5). These drainage mats are favorable for these wall applications for a number of reasons.

  • they are mold- and mildew-resistant;
  • they allow multi-directional drainage and ventilation;
  • resistant to most known chemicals;
  • manufactured from recycled materials;
  • Class A fire rating (ASTM E84, Standard Test Method for Surface Burning Characteristics of Building Materials); and
  • compatible with freeze-thaw conditions.

When specifying an engineered rainscreen material for masonry applications, it is important the drainage mat have a filter fabric bonded on one side to act as a mortar deflection. Moisture can get through, but the mortar will not. The drainage mat keeps a uniform airspace for proper ventilation and drainage. Without a filter fabric, the scratch coat will clog the drainage medium, reducing the wall’s ability to drain and ventilate (Figure 6).

Figure 4

Figure 5: Entangled matrix bonded atop heat-bonded filter fabric.

Figure 5

Figure 6: All-wall mortar deflection with bonded filter fabric.


Drainage mats should not be the same width as the air space. There needs to be enough space between the back of the brick and the drainage mat for the masons to put their fingers, making it easier to lay the brick, concrete masonry unit (CMU), or stone. As noted, the code minimum is 25 mm. Even when there is a slight mortar buildup, the filter fabric allows the moisture to drain through and down the wall to its exit point.

In commercial applications, it is recommended the drainage plane be 10 mm (3/8 in.) or greater. In Section of the 2010 National Building Code of Canada (NBC), it clearly states there needs to be:

a drained and vented air space not less than 10 mm deep (3/8″) behind the cladding, over the full height of the wall.

Industry involvement and further considerations
The importance of all-wall drainage mat technology is increasingly being seen in the industry. Organizations, such as the Building Enclosure Moisture Management Institute (BEMMI) have worked with stakeholders to establish minimum requirements for engineered rainscreen materials. BEMMI was recently awarded ASTM E2925-14, Manufactured Polymeric Drainage and Ventilation Materials Used to Provide a Rainscreen Function.

Other organizations are acknowledging the need for all-wall drainage mats as well because these mats are not just beneficial in cavity wall applications. Stucco and manufactured stone applications are seeing the value of a drained cavity created by these all-wall drainage mats. In Section 2510.6 of the 2015 International Building Code (IBC), it speaks of designing stucco applications with a non-water-absorbing layer or designed drainage space (Figure 7).

A few other points of consideration for specifying and using all-wall drainage mats include:

  • rainfall totals and frequency;
  • wetting and drying cycles;
  • wind and storm conditions;
  • freeze-thaw conditions;
  • temperature; and
  • humidity.

Many building professionals recommend use of all-wall drainage mats in geographic locations receiving 508 mm (20 in.) of rainfall or more a year. There is a strong push to make the use of all-wall drainage mats code in areas exceeding 1524 mm (60 in.) of annual rainfall. The industry is also seeing drainage mats become code on the state level in areas like Oregon (Figure 8).

Figure 7

Figure 7: Engineered rainscreen drainage and ventilation mat used with manufactured stone applications. It is important to note the entangled matrix is facing the weather-resistant barrier while the bonded filter fabric is facing the scratch coat protecting the channel from being clogged with debris.

Figure 8

Figure 8: Any geographical area shaded in blue should incorporate the proper capillary break within the wall system for drainage and ventilation.











Areas prone to high wind content are prime candidates for rainscreen drainage mats as well. For example, a 80-km/h (50-mph) wind exerts 41.3 kPa (6 psi) of pressure on a wall’s surface. This is enough pressure to force moisture into cracks of any size.

Building with rainscreen technology will allow vapor and liquid water to drain and ventilate properly by reducing the amount of moisture allowed to linger in a wall. Building owners should see a significant decline in efflorescence, staining, structural decay, and poor indoor air quality (IAQ), along with an increase in the lifecycle of the building. All-wall drainage mats have proven to be an effective way to improve the durability and performance of a building.

Keith Lolley, CSI, is a graduate of Southern New Hampshire University with a bachelor’s degree in business management. He has been involved in the construction industry for 16 years and is the vice president of Advanced Building Products, while currently holding a seat on the board of directors for the Building Enclosure Moisture Management Institute (BEMMI). Lolley can be reached at klolley@abp-1.com.

Continuing Education on Continuous Insulation

continuous - Tom's article 2015 - 5th-&-Alton Shopping-Center

All images courtesy Sto Corp.

by Tom Remmele, CSI
Continuous insulation (ci) has been a component of exterior wall assemblies for more than 40 years in North America and even longer in Europe. It has always been the smart way to design wall assemblies from the standpoint of energy conservation and water management. By minimizing energy loss caused by thermal bridging and the risk of condensation caused by water vapor diffusion, exterior ci can improve building durability and benefit the environment.

Standards-writing and regulatory bodies, government agencies, and the building science community are in alignment in viewing exterior ci as a sensible strategy to conserve energy in buildings. The American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE) has steadily driven energy conservation standards—and hence, the International Energy Conservation Code (IECC)—to ci prescriptive R-value requirements (in combination with stud cavity insulation) as a pathway to greater energy conservation in buildings.

In sponsoring the 2012 IECC code changes, the Department of Energy (DOE) helped achieve the “largest one-step energy efficiency increase in the history of our energy code.” Building Science Corporation identifies the ‘perfect wall’ (i.e. one working in any climate zone), as having ci outbound of the structure. Thus, for the foreseeable future ci is likely to be a fixture in most exterior wall assemblies.

Types of foam plastic ci
Figure 1 summarizes properties of rigid cellular polystyrene and polyisocyanurate (polyiso) thermal insulations as published in ASTM C578-14, Standard Specification for Rigid, Cellular Polystyrene Insulation, and ASTM C1289-14, Standard Specification for Faced, Rigid Cellular Polyisocyanurate Thermal Insulation Board, respectively. Commonly used insulating materials conforming to these property requirements are Type I expanded polystyrene (EPS), Type IV extruded polystyrene (XPS), and Type I, Class 1 or 2 polyiso.


Common types of rigid foam plastic continuous insulation (ci) used in exterior wall assemblies.

Each insulating material has its own benefits and limitations influencing which one should be used for a given project. For example, EPS is the insulation commonly used in exterior insulation and finish systems (EIFS) because of its dimensional stability, water vapor permeability, and adhesion compatibility with EIFS adhesives and base coats. Due to their higher R-values, XPS and polyiso boards are more commonly used behind brick veneer to allow for thinner wall sections. This becomes important when considering the total thickness of a brick veneer cavity wall with ci and the implications on size and thermal bridging of supporting shelf angles and lintels.

Water vapor permeability of the insulating material can be an advantage or disadvantage. For example, in hot, humid climate zones where vapor drive is predominantly inward, exterior polyiso or XPS insulation can retard inward vapor drive and reduce the potential for condensation on the relatively cold conditioned surface of interior drywall over metal studs. In mixed climates, where vapor drive is both inward and outward for long periods during the course of a year, EPS ci is advantageous because its higher vapor permeability allows water vapor to diffuse, which aids in drying of the wall assembly in the event of condensation.

A wall analysis during design is a valuable tool for selecting the best type of ci material in this regard. Dynamic computer models are a good approach, since they characterize wall assembly hygric performance through seasonal change, but even a simplified steady-state analysis for worst case winter and summer months can be a helpful tool to assist in making material choices for a given wall assembly.

Other things to consider beyond physical properties are jobsite handling, storage, and compatibility with other materials, as well as the construction Type, whether Types I−IV or Type V, and design wind pressure requirements. EPS and XPS have limited ultraviolet (UV) resistance and should not be left exposed to sun for extended periods as the surface will degrade (chalk).

While this degradation has no significant effect on R-value, it can interfere with adhesion of joint treatments, tapes, EIFS base coats, and membrane materials that rely on adhesion to the surface, unless the surface is rasped or sanded to remove the chalked material. Chalking is not an issue when the insulation is ‘faced’ with glass mat facing or aluminum foil facing as with most polyiso boards, although adhesion to the facing materials still has to be evaluated.

Equally important to consider (if not more so) on jobsites is the combustibility of foam plastics. They should be protected from sparks, flame, or any other source of ignition. All foam plastic insulation boards are produced with flame retardant, but they behave differently in the presence of flame; while EPS and XPS melt, polyiso chars.

Despite their combustibility, all these foam plastic insulating materials can be used on buildings required to be of noncombustible construction (Types I−IV) with proper material and ‘end use’ testing to support the proposed assembly. Likewise, they can all be used in wind-resistant assemblies provided they are constrained (i.e. sandwiched) in the negative and positive direction by another material (e.g. sheathing/cladding) capable of resisting design wind pressures. Alternatively, appropriate tests can be performed to determine wind load resistance of the insulation relative to project and/or building code requirements.

Fire safety considerations
Fire safety in the design of foam plastic-based wall assemblies is an important factor when considering their use. Since such materials are combustible, building codes strictly regulate the use of foam plastics. Chapter 26 of the 2015 International Building Code (IBC) has seven requirements that must be met for foam plastics to be approved for use in walls (Figure 2).

CS Feb FIgure 2 (2)

Summary of the 2015 International Building Code (IBC) Chapter 26 requirements for use of foam plastic insulation in exterior wall assemblies.

For the design professional, listing and labeling of the insulation by an approved independent third party is the first step in verifying code compliance. Most insulation board manufacturers hold International Code Council Evaluation Service (ICC-ES) evaluation reports (ESRs), or Underwriters Laboratory (UL) or other listings, simplifying verification.

These listings also demonstrate other aspects of code compliance, for example, compliance with flame spread and smoke development criteria to qualify as a Class A building material, or special uses such as below-grade or attic insulation. Other code compliance requirements are more difficult to verify because they involve wall assembly tests that may exist with the insulation board manufacturer, the cladding manufacturer, or, in some cases, with the air barrier/water-resistive barrier (WRB) manufacturer.

Potential heat, a measure of the foam plastic’s stored heat energy, is a function of the type of foam plastic insulation, its thickness, and density. IBC effectively limits potential heat for construction Types I−IV to the insulation thickness and density successfully tested in the National Fire Protection Association (NFPA) 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-load-bearing Wall Assemblies Containing Combustible Components.

NFPA 285 is a qualifying wall assembly test for the use of foam plastic in wall assemblies of Types I, II, II, or IV construction. As a ‘worst-case’ surrogate for exterior wall fires, the test addresses the effects of a simulated fire in an interior room and vertical flame propagation from floor-to-floor and room-to-room vertically and laterally. An example of an assembly that meets NFPA 285 acceptance criteria is shown in Figure 3, and the actual test is depicted in Figure 4.


Figure 3: Exterior brick veneer wall assembly with ci that complies with National Fire Protection Association (NFPA) 285 acceptance criteria (refer to International Code Council Evaluation Service Reports (ICC-ESRs) 1233 [6] 2141[7]).

continuous_NFPA 285 Test

Figure 4: As shown above, NFPA 285 test exposes the wall assembly to fire from an interior compartment and evaluates vertical and lateral flame propagation.
















While the test enables approval of the assembly in Types I−IV construction, it also establishes limits—maximum allowable thickness and density of insulation and detailing around the opening that must conform to (or be more conservative, from a fire protection standpoint) what was tested. Test results can sometimes be extended to other claddings or backup wall construction when evaluated by a qualified fire protection engineer.

For example, in Figure 3, the results of the fire tests with masonry veneer over steel stud wall construction were extended to backup wall construction of concrete or concrete masonry unit (CMU) in lieu of steel stud with gypsum sheathing. Once again, ICC-ES evaluation reports can be a valuable resource for the design professional to know what assemblies have been tested and meet acceptance criteria, or where results of tests have been extended, evaluated, and recognized by ICC-ES.

NFPA 285 is not the only assembly test to be considered. ASTM E119-12a, Standard Test Methods for Fire Tests of Building Construction and Materials, is necessary when walls are required to have an hourly fire-resistance rating—a common requirement for commercial office, institutional, and some retail and multi-family type construction. The test evaluates the ability of the assembly to resist temperature rise, collapse, flaming, or ignition on the unexposed side of the assembly.

If the assembly is asymmetrical, it must be tested from both sides—in other words, with the fire originating from the interior or exterior. Further, the effects of a hose stream (used to provide additional structural evaluation) are evaluated to ensure the unexposed side remains intact and there is no breach or collapse of the assembly as a result of the hose stream.

Engineering analysis or modeling by a qualified fire protection engineer can be done to qualify substitute materials or to make minor revisions to what was tested to provide the design professional with a wider range of material options for the wall assembly. For example, if an hourly rating is achieved with a frame wall assembly with gypsum sheathing on the exterior and gypsum wall board on the interior, it is readily assumed a ‘mass wall’ fire-resistive wall construction (e.g. 152-mm [6-in.] cast-in-place concrete or 203-mm [8-in.] CMU) would provide equal or better resistance than the frame wall with the same exterior ci and cladding assembly. ICC-ES evaluation reports, UL listings, and Gypsum Association’s (GA’s) Fire Resistance Design Manual are valuable resources for the design professional to identify tested fire-resistance rated wall assemblies.

The last of the assembly tests is NFPA 268, Standard Test Method for Determining Ignitability of Exterior Wall Assemblies Using a Radiant Heat Energy Source. The test evaluates a wall assembly’s susceptibility to ignite from the radiant heat produced by a fire in an adjacent building. It is an important test for EIFS and other foam plastic-based wall assemblies that do not conform to one of the six wall covering exceptions listed in Section 2603.5.7 of the 2015 IBC:

minimum 15-minute thermal barrier;
minimum 25-mm (1-in.) of concrete or masonry;
at least 9.5-mm (38-in.) glass-fiber-reinforced concrete (GFRC);
metal-faced panels meeting the prescribed composition and thickness;
minimum 22.2-mm (78-in.) stucco; and
minimum 6-mm (14-in.) fiber cement lap, panel, or shingle siding.

A final requirement of the code for all types of construction is separation of the combustible foam plastic insulation from interior space with a 15-minute thermal barrier, typically 13-mm (12-in.) interior drywall or exterior gypsum sheathing. Commercial attic space or the interior wall area above suspended ceilings must have this 15-minute thermal barrier in place on the interior if it does not exist on the exterior side of the wall to separate the foam plastic insulation (with some exceptions permitted in the 2015 IBC’s Section 2603.4.1). Between-the-stud fiberglass batt insulation does not count as a thermal barrier since it is discontinuous.

Thus, building codes strictly regulate the use of foam plastics in wall assemblies. Manufacturers of wall assembly components—cladding, ci, air barrier, and sheathing—must demonstrate compliance with these requirements. ICC ESRs are an excellent resource to facilitate verification of wall assembly compliance.

Moisture-related durability considerations
One of the ways exterior ci can help in managing water is by changing the location of the dewpoint in cold climate zones so water vapor diffusion condensation potential is minimized or eliminated. Continuous insulation can also aid in controlling moisture in hot humid climate zones as demonstrated in recent research conducted by the US DOE and the EIFS Industry Members Association (EIMA). The research compared the hygrothermal performance of various wall assemblies—EIFS, stucco, brick, and fiber cement siding (15 assemblies in total)—installed on a test hut (Figure 5) exposed to natural weather in Hollywood, South Carolina (Climate Zone 3A).


Figure 5: The image to the left shows an EIFS Industry Members Association/Department of Energy (EIMA/DOE) test hut in Hollywood, South Carolina with wall panels monitored for a two-year period for hygrothermal performance. On the right, the test hut rainwater collection device ‘delivers’ rain into the panel at the plane of the water-resistive barrier (WRB) during the second year of exposure. The device was intended to simulate a flaw (breach) in the panel exterior wallcovering.

Temperature, heat flux, relative humidity (RH), and moisture content measurements were taken 24 hours a day with sensors placed in the wall panels. After a little more than a year of exposure, a flaw (i.e. opening) was created in some of the wall panels to introduce rainwater onto the plane of the WRB behind the cladding. The cladding with ci performed the best from the standpoint of temperature and moisture control as measured by the heat flux sensor on the inside face of interior gypsum wallboard and the relative humidity sensor on the face of the wall sheathing directly behind the WRB (Figure 6).

continuous_EIMA Test Hut Data Profiles

Figure 6: Assembly with exterior ci shows improved moisture control in comparison to assemblies without ci as indicated by relative humidity (RH) measurements at the exterior face of the sheathing. The assemblies were installed over nominal 2×4 wood framing. In order, they are (a) 102-mm (4-in.) brick veneer cavity wall over paper water-resistive barrier (WRB) on 11-mm (7/16-in.) oriented strandboard (OSB) with unfaced R-11 batt insulation, (b) 102-mm exterior insulation finish system (EIFS) with fluid-applied air barrier/WRB on 13-mm (1/2-in.) plywood with no batt insulation, and (c) 22-mm (7/8-in.) portland cement stucco over two layers of paper WRB on 11-mm OSB with unfaced R-11 batt insulation.

The closer the heat flux sensor stayed to the zero base line (which would represent constant interior temperature), the better the assembly’s thermal performance. An average monthly relative humidity of below 80 percent was considered acceptable based on ASHRAE STP 160, Criteria for Moisture-control Design Analysis in Buildings. The ci assembly proved not only to be best from a thermal standpoint, but also kept wall components dry, even when rain was deliberately directed into the assembly during the second year of exposure. This is important not only from the standpoint of durability, but also because insulation, if it stays moist, loses some of its insulating value.

Key factors in the moisture control success of the ci assembly were:

exterior ci kept wall sheathing above the dewpoint during winter;
combination of low water absorption exterior finish materials and relatively low water vapor permeability of the insulation prevented high exterior RH in summer from significantly increasing the sheathing’s relative humidity;
seamless fluid-applied air barrier/WRB behind the cladding was effective in resisting air leakage (and condensation potential) and was unaffected by the rain introduced into the wall during the second year of exposure (the other claddings had paper WRBs); and
drainage feature of the ci assembly prevented excess amounts of rain from accumulating in the assembly.

As building codes have evolved to the point where ci is now mandatory for many wall assemblies, rigid foam plastic ci wall assemblies have become more prevalent than in the past. They have special design considerations that need to be addressed at the design stage with an awareness of what the building code requires in relation to the use of foam plastics and their effects on the physics of the wall construction, as well as design details.

While this feature looked at the basic types of materials available, and focused on fire safety and moisture-related durability, this author is also developing another technical article that explores the added complexity of design details, along with structural considerations, environmental impacts, and cost control for a future issue of The Construction Specifier.

Tom Remmele, CSI, is the director technical services/R&D for Sto Corp., a manufacturer of air barriers, coatings, exterior insulation and finish systems (EIFS), and stucco products. He has held technical management positions in the construction industry for more than 25 years. Remmele is a past Technical Committee chair of the EIFS Industry Members Association (EIMA). He can be reached at tremmele@stocorp.com.

Much to Think About with Cavity Walls

slaton patterson sutterlinFAILURES
Deborah Slaton, David S. Patterson, AIA, and Jeffrey N. Sutterlin, PE

In response to greater focus on building envelope energy performance, insulation use in the exterior wall cavity has increased. For all U.S. climate zones, the 2012 International Energy Conservation Code (IECC) requires continuous insulation (ci), which is defined by the American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) as “insulation that is continuous across all structural members without thermal bridges other than fasteners and service openings.” In cavity wall construction, this is typically accomplished with a continuous plane of rigid or semi-rigid insulation outboard the water (or weather)-resistive barrier/air-vapor barrier (WRB/AVB).

Foam plastics (e.g. extruded polystyrene [XPS]) and semi-rigid mineral wool insulation have been the most commonly used in exterior wall cavities for this purpose. Each has certain advantages and disadvantages. For example, XPS has a slightly higher R-value (nominally 5.0 per inch) as compared to mineral wool (nominally 4.2 per inch), but is considered combustible while mineral wool is not. Use of foam plastic insulation within the exterior wall cavity of Type I to IV construction triggers the need for testing per National Fire Protection Association (NFPA) 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-load-bearing Wall Assemblies Containing Combustible Components.

In the 2012 International Building Code (IBC), Section 1403.5 also requires combustible WRBs in the exterior wall assembly of buildings greater than 12 m (40 ft) in height comply with NFPA 285 testing of the assembly. The 2015 IBC appears to have recognized the burden this requirement has placed on the construction industry; NFPA 285 testing is no longer required when the WRBs are the only combustible material present, and are covered with non-combustible claddings like brick, terra cotta, concrete, or metal.

High-density closed-cell foam plastic insulations can function as air barriers and Class 2 vapor retarders. However, when improperly detailed or installed, they can retard the drying of moisture that enters the wall assembly and collects against the WRB/AVB. Thus, care must be taken to detail and install the insulation to minimize the passing of bulk water inboard of its exterior face.

Mineral wool insulation, while typically free-draining, can retain moisture and wet the WRB/AVB until the moisture drains through or evaporates. Some mineral wool insulation products are manufactured with enhanced water-resistance, making them more suitable for use in an exterior wall cavity or rainscreen application. No matter which insulation is used, the wall cavity should be designed with sufficient ventilation provisions to allow materials within to dry out.

WRB/AVBs used inboard of the insulation have evolved to include fluid-applied products, which have different properties than traditional sheet barriers. Recognizing the potential for moisture or bulk water that enters the wall cavity to be held against the WRB/AVB by the insulation, the designer must understand the limitations of all products involved in the installation to avoid the failure shown in the photo below.

Moisture collecting on the horizontal surface likely contributed to the failure of this fluid-applied water-resistive barrier/air-vapor barrier (WRB/AVB). Photo courtesy Jeffrey N. Sutterlin

Moisture collecting on the horizontal surface likely contributed to the failure of this fluid-applied water-resistive barrier/air-vapor barrier (WRB/AVB). Photo courtesy Jeffrey N. Sutterlin

The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect those of the CSI or The Construction Specifier.

Deborah Slaton is an architectural conservator and principal with Wiss, Janney, Elstner Associates, Inc. (WJE) in Northbrook, Illinois, specializing in historic preservation and materials conservation. She can be reached at dslaton@wje.com.
David S. Patterson, AIA, is an architect and senior principal with WJE’s Princeton, New Jersey, office, specializing in investigation and repair of the building envelope. He can be e-mailed at dpatterson@wje.com.
Jeffrey N. Sutterlin is an architectural engineer and senior associate with WJE’s Princeton office, specializing in investigation and repair of the building envelope. He can be contacted via e-mail at jsutterlin@wje.com.

Using Temperature to Control Condensation in Cold Climates

Photo © BigStockPhoto/Pavel Losevsky

Photo © BigStockPhoto/Pavel Losevsky

by Daniel Tempas

Designers have been concerned about condensation in walls for decades. Since the mid-1970s, the greater amounts of insulation specified in the building envelope has increased the likelihood for condensation somewhere in the assembly. Many articles have been written over the years describing the physics of the problem and, for the vast majority of the time, there has been a laser-like focus on one solution.

Initially, water vapor diffusion was seen as the likely culprit for condensation problems and designers and consultants spent hours running and analyzing wall assemblies using the ‘profile’ (or ‘dewpoint’) method (Figure 1). With such analyses came the concept the wall system should be tuned for maximum condensation resistance by altering or selecting the appropriate permeability of the wall components.

The rule of thumb became to place low-permeability materials/retarders on the wall’s warm side, and higher permeability materials on the cold side (Figure 2). In this fashion, the designer strove to make it difficult for water vapor to enter the wall (lessening water’s ability to condense in the wall) and easy for water vapor to leave the wall (drying out any water that still managed to get inside). Manufacturers began to introduce high-permeability air barriers, water barriers, and sheathings along with ‘smart’ vapor retarders for the warm side of the wall.

This low-perm/high-perm strategy reveals two goals in wall design: the efforts to decrease condensation potential and increase drying potential. Reducing condensation potential is fairly well-understood but increasing drying potential is a less commonly sought after goal. Both are important for robust wall design.









Problems with permeability
While all this sounds good, it was not necessarily preventing condensation problems. There are some basic facts about permeability designers need to understand to get a better grasp on not only controlling condensation, but general wall design.

Fact 1: If a material’s temperature gets low enough, water vapor will condense on or in it, regardless of how high its permeability.
This is something to keep in mind in cold climates. This author has seen both fiberglass batts and high-perm air barriers with ice encrusted on their surfaces. When a material gets cold, its effective permeability dramatically drops. High permeability is useless at low temperatures. In other words, condensation is a temperature-related phenomenon.

Fact 2: Cold water dries slower than warm water, no matter how permeable the shell surrounding it.
Increasing a wall assembly’s drying potential is an important and valuable goal. However, water at lower temperatures will take a long time to dry because the related evaporation rate is slow. Simply put, robust drying potential cannot be achieved in the layers of a wall assembly that are at low temperatures.

For example, one can consider a puddle on a sidewalk (Figure 3). How long does it take that puddle to dry? If the ambient temperature is 32 C (90 F), it will not take long at all, perhaps only several minutes. However, when the ambient temperature is only 4 C (40 F), the puddle might take hours or even days to evaporate. This is an example of the profound effect temperature has on evaporation rate.

Fact 3: Air movement transports far more water vapor than diffusion.
This is something that has been understood by building scientists for quite some time, and has been filtering into the design community for decades. However, the subtle ramifications of this knowledge are just now finding their way into the world at large. The fact air movement is so dominant in water vapor transport (and subsequent condensation) means any vapor retarder must work either as, or in conjunction with, a near perfect air barrier.

Any installation flaw or penetration in the air/vapor barrier on the higher temperature side will result in an amount of air leakage that will overwhelm any planned benefit from that barrier’s diffusion characteristics. This will result in a much greater potential for condensation in or on any layer that is at a low enough temperature for condensation to occur. Additionally, this means diffusion-based analyses of the wall system are rendered moot.

Fact 4: Water vapor does not move from areas of higher temperature to lower temperature.
Thinking this is the only direction water vapor flows is incorrect. Water vapor moves from areas of high concentration to low concentration, regardless of the direction of heat flow. This is an important concept when it comes to understanding drying verses condensation.


Temperature to the rescue
After considering these four facts regarding water physics, it would seem there is a great deal of confusion and trouble regarding wall design. The manipulation of material water vapor permeabilities in a wall design cannot achieve a truly robust assembly. What can be done?

‘Temperature’ is the common thread running through the facts regarding water vapor condensation in wall assemblies. A wall assembly’s temperature profile plays a critical role in the ability to resist condensation and promote drying. This is not an unknown concept, of course—a quick search of building science literature will yield the occasional article mentioning the importance of the temperature profile. The problem is temperature profile manipulation is far down the list of the wall designer’s methods for creating a more robust wall. It is seen as unimportant when in reality, it is the opposite.

As much of the wall insulation as possible should be placed on the outbound side of the assembly (Figure 3). This is easy to do whether the base wall is metal stud, concrete masonry unit (CMU), or poured concrete. In cold-weather conditions, this will warm the entire interior wall, changing the temperature profile with far-reaching consequences (Figure 4).

For example, designing a wall assembly so more of the components will be in the higher temperature portion of the wall profile significantly reduces the potential for condensation. Not every part of a wall is equally sensitive to exposure to moisture. A standard rainscreen veneer wall assembly (Figure 5) is not sensitive to water, as it must be exposed to the elements on a constant basis. The support elements for the veneer are also not sensitive to water—they are in the drainage space behind the veneer and quite a bit of water reaches that space. As for the insulation layer on which the supports rest, it too must be moisture-resistant for the same reason. If condensation can be forced to happen only around components immune to water, then the wall design is completely robust in its resistance.

Designing a wall assembly so more of the components will be in the higher temperature portion of the wall temperature profile also significantly increases the drying potential for any water that does find its way into the wall. Referring back to the puddle example, higher temperatures means much higher drying rates. Combine the greater drying temperature with the longer drying time and one has a wall with a drying potential increased by an order of magnitude or more.

The importance of temperature modification to improve walls systems can be better understood when considering that both condensation and drying are two-step processes (Figure 6):

  • movement of water vapor to or from the point of condensation or drying; and
  • actual phase change of water from the vapor phase to the liquid phase (condensation), or vice versa (drying).

No matter how rapidly water vapor is transported to a given location in a wall assembly, either by the slow process of diffusion or the rapid process of air infiltration, condensation will not take place if the temperature of that location is high enough. This is also true in the drying process. No matter how easy it is for water vapor to exit a given location in a wall assembly, either by the slow process of diffusion or the rapid process of air infiltration, drying will not take place when the temperature of that location is too low. Again, temperature plays a critical role in the condensation and drying processes in a wall assembly. Altering the temperature profile of a wall assembly through judicious placement of materials is an effective method to control these processes.

The aforementioned Fact 4 about the true nature of the movement of water vapor makes it clear even when the exterior sheathing/insulation is completely impermeable, the drying potential of this wall is much greater than the previous design and the condensation potential is much lower. Since it is at a temperature near to that of the interior, any water in the stud cavity will have a much higher evaporation rate, which means a much higher drying rate. Also, it will easily dry to the building interior.

Proper placement of the right insulation negates the need for a vapor retarder. Why worry about water vapor getting into the wall when most of it is at a temperature far too high for condensation to take place? If the insulation has been well-chosen, any condensation taking place toward the exterior of the building will be minute and meaningless. Besides, the stud cavity needs to dry to the interior, and an interior vapor retarder will only get in the way.

The overall robustness one gains from placing most wall components in the highest temperature part of the temperature profile overwhelms almost every other condensation/drying consideration in the wall design.

Using the temperature profile of a wall as part of the design process leads to a wall that is easier to build. Relying on permeability (to alter water vapor diffusion rates) in the design process for a wall assembly results in a dependency not only on material properties, but also on the quality of installation.

A critical part of any vapor retarder (or air barrier) is its continuity. Any flaw in the installation process of that air/vapor retarder that results in breaches of its continuity heavily compromises its ability to reduce condensation potential. This would include unrepaired construction damage or poorly sealed seams. Even normal penetrations in the wall assembly, like outlets and switches, present opportunities for discontinuity in the air barrier/vapor retarder.

On the other hand, manipulation of the temperature profile of a wall assembly is only about positioning the right amount of insulation in the right location in the wall. A board of insulation is far more robust that film of plastic, making insulation continuity far easier to achieve. Also, the outside of the wall typically has far fewer penetrations, making them far easier to handle.












Designing wall assemblies by adding or altering the permeabilities of the wall components is an artifact of the limited analysis tools relying on investigation of water vapor movement via diffusion. Such walls gain only mild improvements in condensation resistance and, more importantly, drying potential. To create a truly robust wall system with the greatest condensation resistance and drying potential, designers must look at altering the temperature profile of the wall assembly by moving insulation as far as possible to the wall’s exterior.

This does not mean one should no longer think about, or design with, the permeability of materials in mind, of course. Rather, it means the water permeability analysis/profile part of design efforts should be relegated to the proper place in the design consideration hierarchy: behind the wall temperature profile design effort.

Daniel Tempas is a building envelope technical service representative for Dow Building Solutions; he has held technical and engineering positions at the Dow Chemical Company for almost 30 years. Tempas is a (HERS) rater, a Leadership in Energy and Environmental Design (LEED) Green Associate, and a member of the RESNET Training Committee. He has also been a member of ASTM, Exterior Insulation and Finishing Systems Industry Members Association (EIMA), and Building Thermal Envelope Coordinating Council (BTECC). Tempas can be reached atdtemp@dow.com.

Concern regarding long-term insulation data

The December 2013 issue of The Construction Specifier included the article, “Out of Sight, Not Out of Mind,by Ram Mayilvahanan. The feature focused on expanded polystyrene (EPS) and included reference to a particular industry study. In response to the piece, we recently received the following e-mail from John Ferraro, executive director of the Extruded Polystyrene Foam Association (XPSA):

This article included conclusions on the long-term thermal performance of XPS in below-grade applications contrary to more broadly evaluated and accepted industry data. It references a 2009 evaluation published by the EPS Industry Alliance (IA) industry trade organization, then known as EPSMA, and since republished in many forms by EPS-IA members.

In our opinion, the results of this EPS evaluation, which in essence rely on one data point, are not well-supported and are inconsistent with previous significant research conducted in this field. This EPS evaluation also was not independently peer-reviewed within the industry. The data used was reportedly the result of tests conducted by the same test lab and at the same test site, which were apparently employed in two prior studies: Society of the Plastics Industry’s (SPI’s) 1994 report, “Expanded Polystyrene Thermal Insulation Performance in a Below-grade Application” (Twin City Testing Corp.) and AFM Corp.’s 1996 report, “Thermal Transmission and Moisture Content Analyses Conducted on Buried EPS Perform Guard Insulation” (Maxim Technologies/Twin City Testing). There are unanswered questions surrounding the data reliability from these previous analyses that may also carry forward into the EPS evaluation.

The long-term thermal performance of below-grade foundation insulation is an important building design consideration that directly impacts building comfort and energy conservation. We want to draw your attention to a more comprehensive and objective review of the long-term thermal performance of polystyrene foam insulation in below-grade applications that was conducted by the American Society of Civil Engineers (ASCE) 32 Committee during its revisions to ASCE 32-01, Design and Construction of Frost-protected Shallow Foundations.

This committee’s work was documented in the technical paper, “Below-ground Performance of Rigid Polystyrene Foam Insulation: Review of Effective Thermal Resistivity Values Used in ASCE Standard 32-01, Design and Construction of Frost-Protected Shallow Foundations,” which was published in the Journal of Cold Regions Engineering in June 2010.

Based on this critical review of frost-protected shallow foundation designs, the ASCE committee recommends for below-grade vertical orientation (i.e. exterior of walls) using effective in-service design R-value equal to:
● 90 percent of the ASTM C578, Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation, R-value for XPS; or
● 80 percent of the ASTM C578 R-value for EPS because of the potential for water absorption.

The ASCE committee also recommends for below-grade horizontal orientation (i.e. under concrete slabs) using effective in-serve design R-values equal to:
● 80 percent of the ASTM C578 R-value for XPS; or
● 65 to 67 percent of the ASTM C578 R-value for EPS because of the potential for water absorption.

We believe it is very important to provide your readership and the industry with objective and accurate information to support and facilitate informed choices in building design. By reporting data from a single, non-peer-reviewed, narrow-scope study and ignoring the vast amount of research and experience, this article does not serve the best interest of the industry.