Specifying building wrap for moisture management and drainage

Photos courtesy TYPAR Construction Products.

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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