Moisture Management with Smart Vapour Retarders
By Christopher McLellan
In the Sept. 2015 issue of Home BUILDER we saw Doug Tarry’s solution for insulating basements to prevent moisture problems and the associated call-backs. One innovative component Doug used in his wall assembly is a vapour retarder film that becomes more open to water vapour at higher humidity levels, improving the drying potential of the wall. In this article we will examine these smart vapour retarders in more detail, and examine other assemblies that can benefit from this technology.
How does moisture get into an assembly? It can be there at the time of construction, for example in the concrete of foundation walls at the time of placement, or in the framing materials. After construction it can be transported as liquid water through leaks in the weather barrier, or as airborne water vapour both by air leakage and diffusion. Excessive moisture in assemblies can cause mould and durability issues, as well as decreased thermal performance of insulation. The increased requirements for thermal insulation and more stringent air tightness in energy codes can lead to an increased risk of moisture problems in assemblies.
Building tighter air barriers can prevent moisture from being transported into assemblies, but conversely it can also reduce the potential for letting moisture escape. Adding low vapour permeance layers, such as exterior foam insulation, can limit the potential of walls to dry to the exterior, and similarly impermeable roofing materials on unvented roofs will do the same. Where 6 mil polyethylene sheet is then installed on the warm side as a vapour barrier, often serving as an air barrier as well, the assembly can no longer effectively dry to either the interior or the exterior. With good detailing of the weather barrier to keep water out, and the use of dry materials in the assembly, this may not be a problem. Where this is not always possible, such as in concrete walls that are curing or in many renovation scenarios, a moisture management approach using a smart vapour retarder can be an effective solution to allow assemblies to dry to the interior.
How Smart Vapour Retarders Work
Quite simply, Smart vapour retarders change their vapour permeability depending on the ambient relative humidity (RH). At low RH, these films behave more like a vapour barrier, while at high RH they are more permeable to vapour, in effect behaving more like housewrap— airtight but vapour permeable. Since vapour follows the direction of air movement from high pressure to low pressure, this vapour barrier behaviour is necessary in the winter when the vapour drive is generally from warmer/high pressure areas to cooler/low pressure areas. Therefore, from inside to outside, as it is at this time when the ambient RH is low. In the summer, when ambient RH is high, the film becomes open to the diffusion of vapour, allowing the assembly to dry to the interior. This makes smart vapour retarders well suited to heating climates such as Canada’s.
One product readily available in Canada, MemBrain by CertainTeed, has a permeance that ranges from less than 57 ng/Pa•s•m2 (1 perm) at approximately 40% RH or less, to more than 572 ng/Pa•s•m2 (10 perm) at 70% RH, and increasing to near 1150 ng/Pa•s•m2 (20 perm) at 85% RH. This vapour retarder achieves this variability by increasing pore size as RH increases. In the presence of moisture the nylon film swells and softens, creating pores through which water vapour can diffuse. It is in effect a sieve, enabling a controlled flow of water molecules through it, while keeping out the larger molecules that make up air (primarily nitrogen and oxygen), as well as water molecules bonded to other water molecules in liquid form.
Other materials beyond nylon films also share this behaviour, with smart vapour retarder products being manufactured using a polyethylene copolymer, as well as reinforced paper. Additionally, plywood and OSB sheathing both become more open at higher RH, with 11 mm (7/16”) OSB ranging from 114 ng/Pa•s•m2 (2 perm) at 50% RH to 687 ng/Pa•s•m2 (12 perm) at 85% RH.
In addition to allowing assemblies to dry to the interior, smart vapour retarders can be used as an air barrier (when evaluated as such) when lapped and sealed. Installation is very similar to working with the typical 6 mil polyethylene that trades are familiar with.
To allow smart vapour retarders to function effectively it is important to avoid finishing assemblies with low permeance materials such as vinyl wallpapers or vapour barrier paints. Gypsum board with latex paint is generally considered to be a good choice. Smart vapour retarders are not recommended for use in areas of constant high humidity, but are fine for residential bathrooms due to the buffering effect of finish materials.
Christopher McLellan is the Director of Technical Services at the Canadian Home Builders’ Association, representing the association at the national level in its codes and standards activities and technical research interests. He holds Master’s and Bachelor’s degrees in engineering in addition to being a licensed carpenter.