Moisture management through drainage

Photo © BigstockPhoto

By Bijan Mansouri
There are many products on the construction market aimed at moisture management, from traditional felt paper, rainscreen systems, and caulks to sealants and self-adhered flashing membranes. However, as design/construction professionals race to keep walls dry, they may be inadvertently making it easier for moisture-related issues to fester. After all, no matter how tightly buildings are constructed, water is inevitably going to find its way in. There is no such thing as a waterproof wall, but there are walls built so tightly they are all but guaranteed to get wet and stay that way.

Once accepting the reality water infiltration cannot be completely prevented, the real question becomes how to deal with this fact. The walls lasting the longest are the ones designed to realistically manage moisture and dry out, not those trying to achieve the unachievable. Choices for managing moisture are expanding, too, driven by advances in technology, evolving building codes, and a growing customer concern with mould prevention, indoor air quality (IAQ), and energy efficiency, among many other factors.

Wrap it up
Water can find its way into a wall via numerous paths. High humidity and extreme temperatures can cause vapour diffusion when warm indoor air leads to condensation on colder outside surfaces. Reservoir cladding such as brick and stucco can absorb and store moisture, which can be driven back into the wall assembly when warmed by the sun. Wind-driven rain can be forced into small openings in the exterior cladding at joints, laps, and utility cutouts. Further, wind blowing around the building can create a negative pressure within the wall assembly, siphoning water into the wall.

While exterior cladding is the first line of defense against outside water infiltration, housewraps have become a popular way to block whatever water is able to sneak through. Since the 1970s, plastic housewraps made of polyethylene or polypropylene fabric have been a popular choice among builders and contractors for their durability and ease of installation—especially compared to older methods of using asphalt-impregnated felt paper.

However, as building assemblies have gotten tighter, housewraps have taken on a new function—helping to remove trapped water from the building enclosure. Their unique functionality enables them to block moisture from the outside while also allowing walls to ‘breathe,’ preventing vapour buildup. Recent innovations in product technology are taking this moisture removal function one step further to incorporate drainage capabilities.

Some drainable products feature an additional layer of embedded polypropylene fibres that create a highly efficient gap for shedding excess water.
Images courtesy Typar Construction Products

Performance characteristics
By considering certain performance characteristics for housewrap products, one can determine a valuable starting point for deciding which product is right for a project.

Water resistance
As its most basic function, a housewrap must hold out liquid water. A premium product will be able to pass both a ponding test (which measures resistance to a pond of 25 mm [1 in.] of water over two hours) and a more stringent hydrostatic pressure test where the wrap is subjected to a pressurized column of water for five hours.

Tear resistance and tensile strength are excellent measures of a housewrap’s durability, since it must be able to withstand the handling and application process without losing any of its water resistance. In Canada, ultraviolet (UV) and low-temperature resistance are also important measures of durability because prolonged exposure to the elements can compromise the integrity of the product or cause it to crack.

Vapour permeability
Permeability measures the amount of vapour transmission a housewrap allows over a period. For a product to be considered a housewrap (rather than a vapour retarder), the permeance rating must be higher than 5 perms. However, there are various ways permeability is achieved, and it is important to note a higher perm rating does not always equal a better housewrap.

For example, some wraps may have mechanical microperforations that may allow the passage of more water vapour, but could also be more vulnerable to bulk water leakage. Generally, it is better to go with a higher-quality, nonperforated or microporous product, as this can allow for sufficient vapour mitigation while providing excellent resistance to bulk water.

Drainage is widely accepted as one of the most effective measures for reducing moisture damage due to rain penetration. Drainage is a critical component in allowing a housewrap to do its job, particularly in keeping walls dry. Usually this involves use of furring strips separating the housewrap from the structural sheathing and framing, but new technologies have emerged that are helping to simplify this process.

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