Moisture management through drainage

by Katie Daniel | September 20, 2017 11:43 am

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.

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[5].

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