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Vapour barriers are one of the most misunderstood features of a modern stud frame house, and from what I've seen the confusion is felt by many home owners and even some professional builders.
The fact the long-term future of your home's wooden structure depends on a good vapour barrier is easily forgotten in the upheaval of most major renovations. Everyone wants to get the mess over with as fast as possible, so it's full speed ahead-especially during the early stages when wall cavities are wide open and enthusiasm is running high. It's easy to forget the seemingly insignificant details surrounding proper vapour barrier installation at times like these, but care taken here is every bit as important as a skillfully wielded saw, hammer or drywall trowel. This is especially true in our Canadian climate with its long, cold heating season.
If you're involved in any kind of renovation that includes the inside surface of exterior walls or ceilings with unheated attics above them, then correct knowledge about vapour barriers is crucial to the success of your job. Whether you do it yourself or hire the work out, there's more than meets the eye when it comes to proper vapour barrier installation.
Why is a Vapour Barrier Important?
A vapour barrier can be any material that stops the passage of vapour-laden air, but in residential applications the term is used more to describe flexible polyethylene sheet plastic that's often clear and has a thickness of at least 6mil, a designation printed on a label or right on the plastic itself. Some brands of fiberglass insulation include a layer of foil on one side that acts as a built-in vapour barrier too.
The difference between success and failure with vapour barrier installation is a subtle one. The bottom line is that a solid, impervious layer of plastic or foil completely surround the warm side of all exterior walls and ceilings; interior air must never be allowed to filter into internal wall or ceiling cavities. Here's why:
During winter the interior temperature of our homes is typically around 20°C, with a relative humidity level fluctuating between 35% and 65%. Outside, however, the air is much colder and as a result, its ability to hold water vapour is greatly reduced. The warmer the air, the more water vapour it can hold. That's why when a cold and warm weather front meet in the atmosphere, lots of rain or snow results.
Imagine pushing a temperature and humidity probe all the way through the exterior walls of your house during winter, from inside to outside. As it traveled through the wall, it would record increasingly colder temperatures and higher relative humidity. It's this gradient of temperature and moisture-
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holding capacity during winter that makes a vapour barrier essential. If the warm air inside your house were allowed to pass through the wall as this imaginary probe did, it too would get colder as it progressed to the outside, and eventually its load of water vapour would become greater than it could hold and it would condense to form liquid water within the wall cavities or attic. Very bad news. Dampness in these locations promotes the growth of nasty microorganisms and in extreme cases will foster rot that will ruin the structure. Painted drywall alone isn't enough the stop the slow, steady passage of warm, moist indoor air into wall and ceiling cavities.
Installing a Vapour Barrier
Polyethylene sheet plastic is the standard choice for vapour barriers in Canada because it's inexpensive, easy to work with and effective. The drawing shows the inside surface of a typical wood frame home with vapour barrier installed. The bulk of this is simply held onto the wall frame with 1/4-inch staples. Big deal, right? Even a trained seal could do this part of the job properly. It's working around the windows, doors, electrical boxes and seams between sheets where craftsmanship and patience is really required.
To be effective a vapour barrier must be continuous. This means snaking behind electrical boxes and around rough door and window openings, and fusing seamlessly with neighboring sheets. This would be impossible if not for the all important, two-stage installation approach. Stage one is installing wide polyethylene strips around openings in the frame during the earliest stages of construction. Special rolls of dark plastic are sold specifically for this purpose. Overlap these strips on the inside and outside surfaces of the wall where they join the flat sheets of vapour barrier. For similar reasons, install 18-inch x 18-inch pieces of site-cut plastic underneath all electrical boxes in walls and ceilings requiring vapour barriers. Make the holes for the wires as small as possible, then seal them up with acoustic caulking.
Before fitting insulation in wall cavities, pipes and wires must be installed. Run these up through floors or within interior walls to avoid trouble sealing exit points when the main vapour barrier sheets are installed.
Stage two involves stapling large sheets of poly against major wall and ceiling surfaces and joining them to the strips of plastic around door/window openings and electrical boxes. Acoustic caulking will ensure a vapour-proof seal. What's more it's nondrying, so it stays soft and squishy long after application, allowing drywall or other finished wall coverings to be applied overtop without trouble even months later.
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