Typically, in residential construction the poly vapour barrier also acts as the air barrier. *Although vapour barriers are not critical to be continuous, it is critical for air barriers to be continuous. The fact is, at some point, somewhere, your building assembly will fail - something as simple as failed sealant at a window. Buildings should be constructed to be robust so that when they do suffer a localized failure, they can recover. The problem is that this results in a concentration of humid air flowing through that hole, while the envelope is limited to dry to only the exterior (if a vapour barrier is installed on the inside). This could occur perhaps at joints, but more typically it’s where trades such as electricians have punched a hole through the barrier and was covered up before it was noticed and repaired. While these barriers need to be continuous*, in reality there are always going to be some unintentional holes through the vapour barrier. Second, construction itself is not perfect, and neither is the installation of a vapour barrier. First, the vapour impermeable barrier is on the wrong side during the summer months in air-conditioned buildings - and today most buildings are air conditioned. This is because the use of vapour impermeable barriers has two significant flaws. The location of the vapour retarder prevents the warm and relatively humid air to travel into the wall where it may condense.īuilding scientists agree that the use of vapour impermeable barriers are rarely necessary and in fact can be a poor strategy in all except the coldest climates such as northern Canada and Alaska. states, where heating dominates over cooling, the vapour retarder is installed on the warm side – typically directly behind the drywall. Installing the wrong barrier can result in building failure Lstiburek designated that a vapour barrier is a Class I vapour retarder (Lstiburek, 2011). Joseph Lstiburek, a renowned building scientist and principal of Building Science Corporation, expanded on the Canadian General Standards Board (CSGB) approach by categorizing the water vapour permeance of vapour retarders in Table 1 below where a perm is a unit of measurement usually reserved for water vapour permeance (1 US Perm = 0.66 SI Perm = 57.2 ng/(s m 2 Pa)). Equally important is knowing the permeance of the given material.Ī sneak peak at vapour retardant barriers Understanding the difference between these terms is critical knowledge for building designers and contractors. In contrast, a vapour retarder should be used to describe a barrier that has some level of vapour permeance. The term vapour barrier should be reserved to referencing a barrier that essentially does not allow water vapour to pass, and is designated as vapour impermeable. There appears to be confusion within the building industry between the terms vapour barrier (VB) and vapour retarder (VR). Canadian Contractor is pleased to publish the third in a series of articles by Grant Walkin, a building envelope specialist, focusing on the science behind today’s energy-efficient homes.
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