| The Intergovernmental Panel on Climate Change (IPCC) estimates that buildings account for 40% of the global energy use. The IPCC believes substantial improvements to building efficiency can be implemented easily by improving building enclosures through increased levels of insulation, optimizing glazing areas and minimizing infiltration of outside air.
Building enclosure design encompasses a wide range of parameters but the transport of heat, air and moisture through the enclosure is of primary importance. In predominantly cold Canadian climates, adequate thermal insulation, effective air barriers, and proper moisture control are crucial for energy savings and durability of the structure.
For decades, standard construction practice in Canada dictated a polyethylene sheet behind the interior drywall layer to serve as a vapour barrier for assemblies with traditional fibre-based cavity insulation. If the polyethylene sheet was sealed carefully enough it had the added benefit of reducing air leakage. Unfortunately, vapour barriers place the emphasis on the wrong moisture transport mechanism; air leakage can have 10 times or greater the wetting potential than vapour diffusion. Regardless, code enforcement personnel continued (and continue in some areas) to require vapour barriers in all climates, all assemblies, and all occupancies. To do so, they overrule the provision in Part 5 of The National Building Code of Canada that states vapour barriers are not required if it can be shown that the uncontrolled vapour diffusion will not affect the operation of the building and systems, or the health and safety of the occupants.
Foam plastic insulations perform better than fibre-based insulation in terms of the combined resistance to transmission of heat, air and vapour. This research investigated several types of open cell and closed cell spray polyurethane foam insulation in a variety of assembly configurations both in lab tests and hygrothermal simulations. The simulations were extrapolated to seven Canadian climate categories and three levels of interior relative humidity. The goal was to determine which spray polyurethane foam applications required the addition of a dedicated vapour barrier layer beyond what the foam itself could provide.
The moisture content of the oriented strand board sheathing layer (OSB) in the tested and modelled assemblies was used as the performance evaluation point because during wintertime vapour drives, the wood sheathing is the most likely condensing surface. Prolonged high moisture content (greater than 20%) in wood and wood products in wall assemblies leads to mould growth and decay. By this measure, if the wood sheathing moisture contents stay within the safe range (less than 19%) a vapour barrier is not necessary. The results are presented in Table 7-4.
The performance of assemblies containing closed cell spray foam was excellent for all climates and humidity levels. Their performance was equivalent to traditional wall assemblies incorporating a polyethylene sheet vapour barrier. The performance of assemblies with open cell spray foam was equivalent to traditional wall assemblies containing no vapour barrier. Open cell spray foam and fibreglass batt both require additional vapour control layers with all but the mildest Canadian climates with the lowest interior humidities. However, in those mild climates with low interior humidities, the only vapour control layer required was a medium permeance latex paint with primer. | 