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According to the U.S. Department of Energy, to build effective vapour barriers, a variety of materials are available, including:
- Elastomeric coatings.
- Aluminum foil.
- Paper-backed aluminum.
- Polyethylene plastic sheet.
- Asphalt-coated Kraft paper.
- Metallized film.
- Vapour retarder paints.
- Extruded polystyrene or foil-faced foam board insulation.
- Exterior grade plywood.
- Sheet-type roofing membranes.
- Glass and metal sheets.
A vapour barrier is compared to a raincoat, while an air barrier is compared to a windbreaker. In certain situations, an air barrier would suffice to prevent water vapour from migrating through air currents, rather than a vapour barrier.
Air barriers are used to keep air, as well as the moisture that comes with it, out of a building envelope. A vapour barrier’s sole purpose is to prevent moisture from being transported into a home’s building envelope by vapour diffusion. Since the amount of moisture carried by airflow is 50 to 100 times greater than that carried by vapour diffusion, a high-quality air barrier, is much more important than a vapour barrier.
Any unbroken surface that is impermeable to water vapour can be used to create a vapour barrier. Polyethylene film, with a thickness of 0.002 to 0.008 inches, is a decent choice (0.05 to 0.2 millimetres). Even with a coating of foil, impregnated kraft paper isn’t a good option because there’s no way to close the gap between one sheet and the next, allowing moisture to pass right through.
You must provide a venting path for the water to get through the insulation because no vapour barrier is ideal and some water can still get in. After that, you can staple or use mastic to bind your vapour barrier to your insulation. Staple the tabs on the paper backing of the insulation to the edges of the wall studs. Be sure to overlap several feet of vapour barrier at all joints to prevent vapour from passing through. To prevent air from escaping around the windows, staple the vapour barrier to the window frames. Finally, leave plenty of excess material in all corners to prevent the plastic from being ripped when the wallboard is mounted. After you’ve built your vapour barrier, you can start putting up your interior walls.