Tag Archives: WRB

Getting Along With Stucco: Sometimes it just needs space

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All images courtesy Alta Engineering Co.

by Brett Newkirk, PE
There are two maxims about stucco application over wood-framed structures: first, it will crack, and second, owners will not do much about it. Water intrusion through stucco claddings is so common in Florida, re-skinning buildings here after five or 15 years is commonplace, even though it is not always warranted.

In some cases, re-skinned buildings are needing to be re-skinned again due to moisture intrusion years later. Damage to building structures is frequently related to poor installation of the veneer system and associated flashings.

In some examples, the stucco systems have been installed in general compliance with the building code requirements and product manufacturer instructions, yet damage to the wood substrate still ensues (Figure 1). Why then, does a code-compliant system fail?

This article includes a study of code requirements for stucco veneer systems to help explain the situation for design/construction professionals.

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Figure 1: Damage to an unpenetrated wall through a stucco veneer over Grade D paper and a polymeric water-resistive barrier (WRB).

Bond-break layer
A major change was introduced to the 2003 International Building Code (IBC) and the 2004 Florida Building Code (FBC), which required two layers of an approved water-resistive barrier (WRB), equivalent to two layers of Grade D paper, behind stucco veneers. The purpose of the second (outboard) layer is to create a ‘bond-break’ between the back plane of the stucco rendering and the WRB’s front face.

The bond-break layer (BB) is intended to provide a disruption to the potential capillary movement of moisture from the stucco across the WRB and into contact with the wood wall sheathing. To that end, the bond-break layer is supposed to create a small air space, allowing gravity to draw moisture to the base of the wall, where it presumably will drain out of the wall system before it absorbs across the cross-section of the WRB. While this code change was a huge stride and of sound substance, it was not quite enough.

The problem is the method by which stucco is secured to the walls and through the WRB and BB. Fasteners, typically staples, are installed with a pneumatic tool that draws the lath tight against the BB and WRB. The tightly bound sandwich of lath, BB, WRB, and wood wall sheathing at each connection does not offer sufficient separation to break the capillary path of moisture travel or allow for drying of moisture in the system, even when a BB is present. Thus, the BB’s purpose is negated, and capillary movement across the system is possible.

However, the greater concern is a hole (or two, thanks to a staple) is conveniently created by the fastener at the precise location where the weatherproofing sandwich is tightly squished together (Figure 2). Consequently, permeation through the WRB is not required. Instead, moisture accumulates within the tightly bound assembly, clings to the fastener shank, and travels through the fastener hole in the WRB into contact with the wall sheathing (Figure 3). Equally important is the inability of moisture within the assembly to dry due to the tightly bound, unventilated space between the veneer and the wood substrate. Instead, the chronically damp conditions are favorable for wood decay.

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Figure 2: Typical stucco section at fastener—the two tiny holes are located right where the waterproofing is squished.

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Figure 3: Damage to wood sheathing from state penetration.

 

 

 

 

 

 

 

 

By code, the metal lath is fastened to the wall sheathing with more than one fastener per square foot of wall area. Some rough math reveals there are on the order of 6000 holes through the WRB for a one-story 230-m2 (2500-sf) structure footprint.

Stucco accessories, such as casing beads, weep screeds, and control joints, are typically installed prior to lath. Accessories are also typically secured with pneumatic fasteners, which draw them tight to the BB, eliminating any meaningful drainage space behind the accessory. Consequently, water within the system tends to accumulate along the horizontal edges of the accessories, where it may find weaknesses in the WRB, prompting migration to the interior. Of course, fasteners securing the accessories also serve as conduits for water to migrate through the WRB.

Water-resistive barrier performance
Sheet-good WRBs are tested in a laboratory setting to ensure they perform under a battery of tests established by the International Code Council Evaluation Service (ICC-ES). The most commonly used WRBs are polymeric sheets called ‘house wraps’ or ‘building wraps.’

ICC-ES AC38, Acceptance Criteria For Water-resistive Barriers, evaluates the WRB material’s tensile strength, vapor transmission, air permeance, and resistance to water penetration, to name a few. The polymeric material’s adequacy in resisting water penetration is judged by its ability to prevent water passage for two hours when subjected to a 25 to 550-mm (1 to 24-in.) column of water on one side. Once proving performance, the WRB is considered code compliant. However, the battery of laboratory tests fails to include one little detail of real life: holes in the WRB from veneer fasteners.

A single fastener penetration would result in a ‘failure’ of the AC38 water penetration resistance test. Why should one expect field performance of a product that is installed in a different (and far inferior) manner than that under which it was tested and approved?

To this author, even more perplexing is AC38’s seeming double standard for the use of paper-based WRBs, which are not subjected to the same tests as those required of polymeric WRBs. Grade D paper is used as the backing for paper-backed lath, whose installation is the most common means of creating the BB layer. By definition, Grade D paper allows water to pass after only 10 minutes of exposure with no meaningful hydrostatic pressure, while polymeric WRBs are expected to perform for hours, at pressures of up to 5.36 kPa (112 psf). Why then are two layers of Grade D paper the code-mandated baseline for WRBs behind stucco veneers? Obviously, the very low tolerance of Grade D paper to resist moisture absorption can cause substantial moisture-related distress to wood-framed buildings where it is used.

Mockup testing
Full-scale testing was performed by this author to gage the performance of several ‘code-compliant’ stucco wall assemblies, with various polymeric-based and felt-based sheet-good WRBs. The tested wall assemblies consisted of 1.2 x 1.2-m (4 x 4-ft) specimens constructed with wood framing and oriented strandboard (OSB) sheathing, and then clad with various types of WRBs.

A three-coat stucco system was then applied over paper-backed metal lath secured with 25-mm (1-in.) crown staples, confined within 22-mm (7/8-in.) plastic casing bead accessories that were secured around the wall’s perimeter (Figure 4). A 0.6 x 0.6-m (2 x 2-ft) observation port was created in the center of the OSB substrate, where gypsum wallboard wrapped in kraft paper was substituted for the OSB (Figure 5). This way, the wallboard could be removed and the back side of the WRB could be observed after testing. The kraft paper also served as an indicator of water contact during the test.

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Figure 4: Front side of a typical mockup wall test setup.

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Figure 5: Back side of a typical mockup wall test setup.

 

 

 

 

 

 

 

Testing was performed using a variation of ASTM E2273, Standard Test Method for Determining the Drainage Efficiency of Exterior Insulation and Finish Systems-clad Wall Assemblies (AC38 drainage test), with the additional step of performing observations for moisture ingress during the test. (ASTM E2273 only calls for a comparison of the volume introduced to the specimen to that which escapes at its base. This measurement was not of interest to the author as it related to this evaluation.)

Water was introduced to the drainage cavity at the top of the wall at a rate of 3.38 L/min/m2 (5 gal/sf/hr) for 30 minutes. Perhaps unsurprisingly, the testing revealed water penetration occurred through the fastener penetrations in both polymeric and felt-based WRBs (Figure 6). Water migration also occurred directly through the field of some woven polymeric WRBs and Grade D paper. Intrusion through Grade D paper-based WRB mockups was substantially more severe than that of the polymeric or felt-based WRBs.

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Figure 6A: Water penetration through fastener holes in a spun-bound polymeric WRB following testing. B: Water penetration through fastener holes in a woven polymeric WRB following testing. C: Water droplet below staple shank penetration through WRB during testing. D: Hyrion paper shows water ingress through fastener penetration.

Magnified observation of staple-penetrated WRBs reveals an oblong, torn annular opening is created around the shank (Figure 7). The torn annulus was more significant at woven than spun-bound WRBs. In some cases, an additional indentation and hole through the WRB was noted due to the actuation of the pneumatic tool which impacted its surface. Additionally, use of ‘slap’ or ‘hammer-tacker’ staples caused rupturing of the WRBs at the staple shank hole and the tool’s impact location, which could allow water passage.

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Figure 7: The left image shows a 10x view of a staple shank shows an oblong torn hole in the WRB (and water migration through it). The right image depicts an enlarged, torn hole in the WRB at the staple penetration.

Where to go from here
It probably does not take the foregoing technical discussion to understand holes are paths for water entry or wood that stays wet will rot. So why do we build walls with thousands of holes in them, prevent them from drying out, and expect them to have watertight performance?

If repeating an activity and expecting a different result is the definition of insanity, then why do we tear stucco veneers off of water-damaged building structures and then replace it the same way? Accepting the realities of stucco cracking and inadequate owner maintenance, something else needs to change to give stucco-clad buildings
a longer life.

Based on the author’s testing and experience, there are a few installation practices that can help mitigate the compression of the veneer’s weatherproofing sandwich, which helps prompt drying and greatly reduces the likelihood of moisture migration through fastener penetrations in the WRB.

Separate stucco from WRB with furring or other drainage media
There are many randomly oriented polymeric filament products, typically 6 mm (1/4 in.) in thickness, that provide this function (Figure 8). Often called drainage media, drain screens, furring, or drainage mats, these products should be installed behind all accessories.

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Figure 8: Test wall with drainage media installed in front of the WRB.

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Figure 9: The recommended stucco section at a fastener.

 

 

 

 

 

 

 

 

The resulting air space creates a functional capillary break between the back side of the stucco rendering and the WRB, even at fastener locations (Figure 9). This gap provides an easy, unobstructed path for moisture to travel vertically down the face of the WRB (within the media). In combination with through-wall flashing vents and drainage weeps at the stucco base, the air gap also prompts ventilation within the cavity that prompts drying of moisture.

Control the installation force and depth of fasteners
This can be achieved by reducing the air pressure for pneumatic tools, but is most effectively completed by use of pan-head screws, which have installation depths that can be easily controlled and adjusted (Figure 10). The use of screws also inherently reduces the number of penetrations through the WRB by half, when compared to staples that have two shanks. Further, the cross-sectional diameter of the screw is greater than that of a staple shank, rendering it less vulnerable to corrosion over time. (The drainage system anticipates water to drain across the unprotected fastener shank.)

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Figure 10: Pan-head screws are used to secure lath to the wood frame.

Use fluid-applied weather barriers.
Fluid-applied weather barriers cannot be torn; they create a ‘gasket’ effect around penetrating fasteners. However, these products’ detailing and reinforcing requirements demand a more skilled and conscientious craftsman to properly install. Designers should also recognize fluid-applied barriers are typically vapor-impermeable, which should be a consideration in the envelope design.

Conclusion
The recommendations in this article are an additional step to prevent water intrusion through stucco-veneered walls and to prompt drying beyond current code requirements. Of course, these suggestions are no guarantee against water penetration—after all, one still has thousands of un-gasketed holes through the sheet-good water-resistive barrier. However, with proper WRB installation, appropriate flashings and drains, incorporation of drainage media, and controlled depth fasteners, a stucco veneer has a much better chance at providing long-term performance.

Brett Newkirk, PE, is a practicing structural engineer with Alta Engineering Company in Jacksonville, Florida. He specializes in the diagnosis and repair of moisture-affected structures, and is a recognized author and leader in the building envelope repair industry in the southeastern United States. Newkirk is an associate member of the American Society of Civil Engineers (ASCE) and sits on ASTM committees for wood and gypsum. He can be reached at brett@altaengineeringco.com.

Much to Think About with Cavity Walls

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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.

WRB: Water (or Weather?)-resistive Barrier

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

Using the acronym ‘WRB’ is common, but the intended meaning is often misunderstood, as it can refer to either a ‘water’ or ‘weather’-resistive barrier, with the two having different performance expectations.

The 2012 International Building Code (IBC), Section 1403.2−Weather Protection requires exterior walls “provide a building with a weather-resistant exterior wall envelope…designed and constructed… to prevent the accumulation of water within the wall assembly by providing a water-resistive barrier behind the exterior veneer.” Water-resistive barriers are defined in Section 1404.2 as “a minimum of one layer of No. 15 asphalt felt, complying with ASTM D226, Standard Specification for Asphalt-saturated Organic Felt Used in Roofing and Waterproofing, for Type 1 felt or other approved materials…to provide a continuous water-resistive barrier behind the exterior wall veneer.”

For adhered stone veneers, Section 1405.10.1.1 requires water-resistive barriers to be installed per section 2510.6, which in turn directs the reader back to Section 1404.2, but adds the requirement the water-resistive barrier be vapor-permeable with the equivalent performance of two layers of Grade D paper when applied over wood-based sheathing. (Section 2510.6 of the 2015 IBC has been revised to require a vapor-permeable barrier with performance at least equivalent to two layers of a water-resistive barrier complying with Type I of ASTM E2556, Standard Specification for Vapor Permeable Flexible Sheet Water-resistive Barriers Intended for Mechanical Attachment, which includes polymer-based barriers.) It should be noted IBC’s Chapter 14−Exterior Walls does not reference weather-resistive barriers.

The American Architectural Manufacturers Association (AAMA) defines weather-resistant (not ‘resistive’) barriers as a surface or a wall responsible for preventing air and water infiltration to the building interior. Manufacturers of polymer-based barriers (i.e. building wraps) also distinguish between water-resistive and weather-resistive barriers, with the latter providing the added benefit of also serving as an air barrier for the vertical building enclosure.

While not as potentially destructive as bulk water leakage, moisture transport via air infiltration can contribute to moisture-related problems in the building enclosure. Weather-resistive barriers are often more robust as compared to water-resistive barriers and require taping of all laps and terminations to resist not only water penetration, but also air infiltration.

Code-mandated water-resistive barriers are typically limited to residential and low-rise structures, while weather-resistive barriers are commonly specified for commercial buildings or projects where a higher level of performance is desired of the vertical building enclosure and control of interior environmental conditions is critical.

It is important the design professional and installer understand the intended purpose of the specified WRB—to resist water or to resist air and water—as the installation varies between the two types of barriers. However, regardless of whether the WRB is intended to function as a water- or weather-resistive barrier, the WRB must be properly installed so as to maintain continuity of the barrier. This requires the WRB to be properly integrated with flashings, wall openings, and all adjacent enclosure assemblies, and to be sufficiently overlapped, correctly shingled, and properly sealed or taped at exposed laps (horizontal and/or vertical, depending on its intended purpose), as barrier discontinuities result in potential entry points for water (and air) to migrate into the building.

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.

 

Clarification on wall systems article

The April 2013 issue of The Construction Specifier included a technical feature by J.W. Mollohan, CSI, CCPR, CEP, LEED GA, entitled, “Exterior Wall Assemblies: Are You Getting What You Specified?”  We received the following letter from Cliff Black, a CSI member and a building envelope product manager for Firestone Building Products.

I am writing in regard to the article on exterior wall assemblies. I agree with the author the issue is certainly a challenging one for the design and specifying community. I would like to cite the bracketed statement at the top of page 57, which states, “buildings of two stories or more.” This appears to be taken in the context of the design of 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, addressing multi-story fire propagation.

However, the International Building Code (IBC) 2603.5 states NFPA 285 is required for buildings of any height for Types I through IV construction incorporating combustible plastic insulation in the exterior wall assembly. IBC Chapter 14 (“Exterior Walls”) calls for differing requirements for water-resistant barriers (WRBs) and various combustible claddings, qualified by height.

In this case, I believe the statement should read “buildings of any height,” rather than “buildings of two stories or more.”

 

Mr. Mollohan replied to Mr. Black, and has allowed us to share it with other readers of the magazine:

 

Good catch, Clint! You are absolutely correct that one must be familiar with multiple chapters of the IBC to determine whether an NFPA 285 test is required. My error, and your correction, illustrates the difficulty of this provision. I am attaching an adaptation of a flow chart originally created by Barbara Horwitz-Bennett of DuPont Building Innovations for guidance to interested readers:

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