Specifying building wrap for moisture management and drainage

by arslan_ahmed | March 31, 2023 10:00 am

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By Bijan Mansouri

Among the many performance characteristics expected of today’s building wraps (also referred to as weather-resistive barriers, water-resistive barrier [WRB], or housewrap), draining bulk water is increasingly important. When rainwater infiltrates the space between cladding and sheathing, it can create major problems if it is not managed quickly.

In many areas around the country, building codes are driving the need for better moisture management solutions. At the same time, growing preference for absorptive cladding materials such as fibre cement has made moisture management more important than ever, with some industry manufacturers even going as far as to require drainage gaps behind materials.

Advances in material technology have resulted in innovative solutions for protecting homes and buildings from the elements while allowing them to release water vapour, vapour buildup, and drain bulk water. With a growing number of products hitting the market to address this need, it is important for specifiers to understand how each building wrap performs before selecting it for a project.

Understanding the codes

The 2018 International Building Code (IBC), Section 1402.2, “Weather Protection,” requires exterior walls to “provide the building with a weather-resistant exterior wall envelope…designed and constructed in such a manner as to prevent the accumulation of water within the wall assembly by providing a water-resistive barrier behind the exterior veneer…and a means for draining water that enters the assembly to the exterior.”

This water-resistive barrier, as defined by Section 1403.2, comprises at least “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…attached to the studs or sheathing.”

It is important to note the difference between a weather-resistant barrier and a WRB, as they have distinct purposes but are often confused with one another. The American Architectural Manufacturers Association (AAMA) defines weather-resistant barriers as a surface or a wall responsible for preventing air and water infiltration to the building interior. The differentiating factor is a weather-resistant barrier must also prevent air infiltration, while WRBs are only responsible for stopping water intrusion.

Weather-resistant barriers are commonly specified for commercial buildings or projects, where a higher level of performance is required of the vertical building enclosure, and when it is critical to have greater control of the internal environmental conditions.

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WRBs, on the other hand, are usually limited to residential and low-rise structures. In fact, building codes require the use of such barriers, but some provinces have added even more prescriptive measures to their codes to include the use of drainage planes.

In the U.S. for example, Oregon requires the building envelope to consist of an exterior veneer, a WRB, a minimum 3-mm (0.125-in.) space between the weather-resistant barrier and the exterior veneer, and integrated flashings. The envelope must provide proper integration of flashings with the WRB, the space provided, and the exterior veneer. In lieu of providing the 3-mm (0.125-in.) space between the exterior veneer and the WRB, builders can install the exterior veneer over a barrier that is manufactured to enhance drainage and meets the 75 per cent drainage efficiency requirement of ASTM E2273, Standard Test Method for Determining the Drainage Efficiency of Exterior Insulation and Finish Systems (EIFS) Clad Wall Assemblies, or other recognized international standards.

Cladding and vapour drive

Another factor driving the growing demand for drainable wall assemblies is cladding; specifically trends in materials and potential issues associated with those materials. Reservoir cladding such as stucco, for example, can present major problems if the wall assembly is not constructed in a way to allow adequate drainage.

In fact, it was widespread failures of stucco cladding installed over wood-based sheathing that lead both the IBC and IRC to require “a minimum 4.7-mm (0.1875-in.) space…between the stucco and water-resistive barrier or a space having a drainage efficiency of not less than 90 per cent, as measured in accordance with ASTM E2273 or Annex A2 of ASTM E2925, be added to the exterior side of the water-resistive barrier.”

ASTM E2273 was itself developed in response to thousands of EIFS buildings which experienced failures due to the lack of drainage between adhered expanded polystyrene (EPS) rigid insulation and sheathings, both oriented strand board (OSB) and gypsum board.

Brick and fibre cement siding remain popular with home buyers and, similar to stucco, are reservoir claddings are also presenting specific moisture management challenges. In fact, several fibre cement siding manufacturers are beginning to stipulate that wall assemblies using their products incorporate drainage to qualify for warranty coverage.

This is because moisture absorbed by reservoir claddings such as brick, fibre cement, stucco, and unpainted wood can be driven inward by solar radiation. The heating of the stored water raises its vapour pressure, driving it both inward and outward. This inward vapour drive is even higher when the indoor environment is cooled by air conditioning. Proper drainage strategies ensure the inwardly driven moisture does not infiltrate the wall cavity, where it can create mould issues and other potential health threats.

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Typically, this inwardly driven moisture vapour is managed by separating the cladding from the rest of the assembly with a capillary break, which can be an air gap or a sheathing material that sheds water or does not absorb or pass water.

Impermeable sheathing, such as extruded polystyrene (XPS), is a possibility for halting inward vapour drive. In these assemblies, the inwardly driven moisture condenses on the surface of the XPS sheathing and drains downward.

However, in situations where a reservoir cladding is paired with a highly permeable sheathing, sich as gypsum board (which can be as high as 50 perms), or a moisture-retentive material such as OSB, an air gap may not be enough to slow down inward moisture intrusion. In these applications, an added weather-resistant barrier—commonly referred to as a building or housewrap—is needed to reduce unwanted moisture intrusion.

In an article on the topic, Drain the Rain…On the Plane…the Drainage Plane,¹ building scientist Joseph Lstiburek argues there are two reasonable ways to deal with inward vapour drive—a “vapor throttle” or an air gap. Lstiburek writes, with a small air gap (less than 4.7-mm [0.1-in.]), there is a “sweet spot” for vapour transmission control of the water control layer. That “sweet spot” is somewhere between 10 and 20 perms. If it is any more permeable, the moisture driven out of the back side of the reservoir cladding into the air space will blow through the layer, through the permeable sheathing, and into the wall cavity. Any lower, and the outward drying potential of the cavity is compromised.

Evolution of building wraps

Due to their durability and ease of installation, building wraps made of polyethylene or polypropylene fabric have been a popular method of protecting against moisture intrusion since the 1970s. However, as building assemblies have gotten tighter, building wraps have taken on a new function: helping remove trapped water from the building enclosure. Their unique functionality enables them to block moisture from the outside while allowing walls to “breathe” to prevent vapour buildup. The latest innovations in housewrap technology are taking this moisture removal function one step further to incorporate drainage capabilities as well.

Historically, drainage has been achieved by using furring strips that separate the wrap from the structural sheathing and framing, but today’s advanced building wrap products create integrated drainage gaps through creping, embossing, weaving, or filament spacers. By eliminating the need for furring strips, these wraps can help to reduce material costs and streamline installation.

The drainage efficiency of a building wrap is generally tested in accordance with ASTM E2273. In simple terms, this test involves spraying water onto a wall assembly and measuring its collection over time. However, given the variety of drainable building wraps available, how quickly bulk water is drained can vary significantly. The following is a look at a few methods modern building wraps use to drain water from a wall assembly.

Integrated rainscreens

One method for achieving bulk water drainage is the integration of a rainscreen material onto the building wrap. These products eliminate the cost and time-consuming labour of installing furring strips by creating a gap between the sheathing and the cladding, which facilitates both drainage and continuous airflow. Where wood strapping only vents approximately 85 per cent of the wall, building wraps with an integrated rainscreen provide a continuous vented airspace over the entire surface area of the wall, providing greater drainage and more effective drying. Further, since many rainscreen products use a matrix of plastic material to achieve the gap, they are not subject to saturation and decomposition that could compromise wood furring.

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Rainscreen products are recommended in areas with wind-driven rain, extreme amounts of rainfall (1016 to 1524 ml [40 to 60 in.] annually), or elevated temperature and humidity. Coastal areas and hilltop exposures are prime examples of when this technology would be ideal. In these situations, the importance of creating a drainage plane is heightened when using absorptive cladding material such as wood or fibre cement.

Wraps with spacers or dimples

Another approach gaining popularity is the incorporation of unique spacers, or dimples, onto the building wrap itself. These drainable building wraps can achieve a 1 mm (0.039 in.) drainage gap through a pattern of integrated spacers and can be as much as hundred times more effective than standard building wraps at removing bulk water from the wall. Most can be installed in any direction without affecting performance. These products are recommended behind wood, fibre cement, vinyl, stone, and stucco cladding, or where budgets do not allow the use of a full rainscreen system.

Integrated fibrous layer

The forefront of drainable building wrap technology are products that create a drainage gap through a layer of polypropylene fibres. The drainage gap created by this technology has been shown to achieve 94.8 per cent drainage efficiency, as per ASTM E2273, conducted by a third-party lab, without sacrificing any of the durability and ease of installation benefits that builders and contractors expect from premium building wraps.

Similar to dimpled wraps, these products can also be installed in any direction without affecting performance. They are also vapour permeable, so moisture will not be trapped in the wall assembly and lead to moulding or rotting issues.

Conclusion

A growing preference for reservoir claddings and new developments in building codes have resulted in specifiers taking a closer look at moisture management and drainage strategies. Each project is different, and with myriad of building wraps available, alongside sheathing and cladding materials, managing water intrusion and vapour drive can be a complex equation to solve. However, the specification process is made easier with a greater understanding of how these components can work together to balance drainage and offer breathability to the wall, while potentially reducing material costs and helping to keep projects on-schedule and on-budget.

Notes

¹ Refer to “BSI-112: Drain the Rain…On the Plane…the drainage plane*” by Joseph Lstiburek. Visit buildingscience.com/documents/building-science-insights-newsletters/bsi-112-drain-rainon-planethe-drainage-plane[5].

Author

Bijan Mansouri is the technical manager at TYPAR Construction Products. He has been with Berry Global for nearly 30 years, working in different technical capacities. Mansouri is responsible for building code requirements, the design and development of new construction products, training builders and architects on application of new and existing products, and creating and educating on the proper practices and installation of building envelopes. He can be reached at bijanmansouri@berryglobal.com.

Source URL: https://www.constructioncanada.net/specifying-building-wrap-for-moisture-management-and-drainage/

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