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As a building envelope engineer working in Manitoba, I see the ubiquitous use of vapor retarders in residential applications because they are the only accepted prescriptive method of vapor control under the Canadian National Building Code (NBC). In this post, I will discuss an alternative method that controls the temperature of a condensing surface for vapor control management.
Note: although they are commonly referred to as vapor “barriers” in the Canadian building codes, here I will refer to them as vapor retarders, as the materials being discussed have varying levels of permeability, which are the vapor retarder classes the American International Residential Code (IRC) users know.
Understanding vapor
Conditioned buildings require vapor control to resist the phase change of water vapor (gas) to liquid within a building enclosure assembly. This phase change is commonly referred to as condensation or dew when it occurs on the surface, and interstitial condensation when it occurs within the building assembly.
Air holds a finite amount of water vapor, directly related to its temperature. In general, condensation occurs when water vapor reaches a relative humidity of 100% (porous surfaces can get condensation at relative humidity levels less than 100% due to capillary condensation). This occurs in buildings during winter-heating months, when warm, moist interior air passes through a building assembly from the inside and condenses on cold exterior surfaces. In summer months, it’s the opposite—warm, moist exterior air comes through the building enclosure into conditioned interiors.
Moisture-sensitive materials, such as wood, steel, paper-faced gypsum, etc., that accumulate moisture but can’t dry out are subject to rot. Hence, the need for vapor control.
Three methods of control
To prevent condensation, there are three methods to control vapor:
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4 Comments
Grant,
Interesting blog!
I may have this wrong but my understanding is:
- For code purposes vapour-barriers (60 ng / pa m2) are measured when closed, so don't "smart" or variable-perm membranes qualify as vapour-barriers along as they meet that less than 1 US perm criteria? I know they are being approved for use on projects here on Vancouver Island.
- While Method 2 "controlling the condensing surface temperature" is not listed as an approved strategy in the NBC, if the continuous exterior insulation qualifies as a vapour-barrier (less than 1 US perm), and is "sufficiently close to the warm side of the assembly to prevent condensation at design conditions" ( BCBC 9.25.4.3.(3) ), isn't that method effectively code compliant?
Hi Malcolm. Thanks.
1. You are correct. The no greater than 60ng/Pa s m2 is according to the desiccant dry cup method. Most smart membranes (but not all) meet this. The ones that don't are the ones that are coming out of Europe and made for vapor diffusion through the roof to act as water control - not designed for interior side vapour control.
2. Thanks for highlighting this. This clause would only apply to insulation that qualifies as a vapor barrier, as you pointed out. Semi-rigid mineral wool, unfaced EPS/GPS (Perm 2-3), and semi-rigid wood fiber board would all be excluded. I personally, along with many others, purposely specify vapor open exterior insulation products to maximize drying to the exterior.
Grant,
It will be fascinating to see if our code evolves in the same direction as the IRC, moving toward the gradations in permeance of various classes of vapour-retarders, rather than the longstanding instance on interior vapour-barriers.
Again - thanks for the blog. Interesting stuff!
Agreed. Our (NBC) codes methodology has to change towards vapor control, not vapor retarders - as well as restrictions to the use of sheet polyethylene vapor barriers. The industry needs a wake up call. With the prevalent use of vapor impermeable acrylic stucco's alongside interior vapor impermeable barriers, we are seeing widespread wall rot. The system simply does not have any drying potential or redundancy.
Thank-you for the comments!
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