Moisture management through drainage

Housewrap works in conjunction with other weatherproofing products, such as window flashing, to keep out excess water. When installed as a complete system, some manufacturers offer extended warranties.

New generation of housewrap
Recent advancements in housewrap technologies include new products integrating drainage gaps into the material itself through creping, embossing, weaving, or filament spacers. These new products eliminate the need for furring strips, helping reduce material costs and streamline installation.

These new housewraps are drainable without sacrificing any of the durability and ease of installation benefits builders and contractors have come to expect from premium housewraps, since they essentially handle and install the same. They are also vapour-permeable, so moisture will not become trapped in the wall assembly and lead to mould or rot issues.

Other considerations
With so many options, how does one know what type of weather protection is best for a particular project? Outside of the aforementioned performance metrics, there are numerous other factors to consider.

A key consideration is the type of cladding being used. When installing vinyl siding, which has built-in drainage holes and fits loosely on the wall, an ordinary smooth-faced housewrap should provide adequate drainage. However, tightly fastened cladding such as cedar siding or fibre cement board might allow water trapped between the siding and a smooth housewrap to pool; this water could eventually make its way through the housewrap and into the framing. These are cases where a drainable housewrap would provide significant benefit.

Reservoir claddings such as brick, stucco, and certain types of stone present another set of issues. Since these materials hold water, once they get wet, the stored water can migrate elsewhere and cause problems. In these applications, it is imperative to separate the cladding from the rest of the assembly with a capillary break, which can be an air space or a material that sheds water or does not absorb or pass water.

Additionally, certain cladding materials such as cedar and stucco often contain chemicals called surfactants (or surface active agents) that can affect the water resistance of housewrap. These chemicals, which are also present in solutions used to power-wash siding, can reduce the surface tension of water, easing its ability to pass through microscopic openings in the membrane. This means if the housewrap will be exposed to surfactants, it is important to select a product that has added protection against their harmful effects.

As building assemblies have gotten tighter, housewraps have taken on a new function – helping remove trapped water from the building enclosure.

Geography and climate are important, as well—specifically as they relate to annual rainfall. As a rule, the Building Enclosure Moisture Management Institute (BEMMI) recommends any area receiving more than 500 mm (20 in.) of annual rainfall should incorporate enhanced drainage techniques in the wall system, especially if using an absorptive cladding material. Areas receiving 1016 mm (40 in.) or more should utilize rainscreen design, regardless of cladding material. The geographic orientation of the wall, amount of roof overhang, altitude, and even nearby trees can also have impacts on how much water intrusion can be expected and how likely it is to dry.

In many places, new and evolving building codes are driving the need for better moisture management solutions. Canada’s wetter coastal provinces now require, per the National Building Code of Canada (NBC), wall assemblies be built with capillary breaks between the weather-resistive barrier (WRB) and the exterior veneer.

Proper installation is key
No matter how many benefits it offers, a housewrap must be properly installed in order to do its job. Housewrap should be installed from the foundation upward, making sure to overlap joints with the higher course overlapping the lower. All horizontal seams should overlap a minimum of 50 mm (2 in.), and vertical seams at least 150 mm (6 in.).

One must keep in mind horizontal laps are just as important as vertical laps because windblown rain can travel sideways, or even up and over a properly installed lap. Manufacturer-recommended tape should be used to cover any tears and holes, and galvanized roofing nails or plastic cap nails should be used to attach the housewrap to the structural sheathing and framing.

When installed as a complete system, some housewrap products offer added assurance in the form of extended warranties. For example, a single-source proprietary assembly could consist of a WRB, flashings, and construction tape collectively covered by a lifetime limited warranty for both materials and labour. When in doubt, it is important to check the manufacturer’s website for additional guidance.

Advances in technology and building codes are driving adoption of better moisture management systems. The building industry is also starting to realize the race toward a ‘waterproof’ wall assembly is a fantasy, with design professionals now looking for smarter strategies for managing unwanted moisture and helping walls to dry when they inevitably get wet.

Today’s drainable housewrap products offer a powerful new defense against the elements by adding drainage capabilities to a solid mix of water resistance, durability, and breathability.

Bijan Mansouri is the technical manager at Typar Construction Products. He has been with Berry Plastics for the past 25 years, working in different technical capacities. Mansouri is responsible for building code requirements, design and development of new construction products, and education on proper practice and installation of building envelope. He received his bachelor’s and master’s degrees in mechanical/aerospace engineering, and is a member of the Air Barrier Association of America (ABAA) and ASTM. He can be reached at

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