When warm air comes in contact with a cold surface.
When warm air expands, there is more space between its molecules compared to cold air. Expansion allows warm air to hold more water vapor. When this warm air encounters the cooler surfaces of your home, such as exterior walls and ceilings below attics, it cools down and contracts, squeezing out water that collects on the cool surfaces.
This is the same process that:
- produces water droplets on a cold glass of water in the summer
- creates condensation on older windows during the heating season and
- makes cold water pipes "sweat" in the basement during the summer.
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Vapour barrier paint.
Vapour barrier paint is water based, there is no flammability hazard while being applied. The paint is such a good vapour barrier that it should not be used where a breathing coating is required. The low odor paint needs time to cure and should not be applied when humidity levels in the house are high.
Attic insulation without a vapour barrier.
During my years as a building inspector I have seen many attics that did not have a vapour barrier installed underneath the insulation. This was especially the case in older wood lath & plaster constructed homes. It is a bad idea to try to pull back the insulation in the attic and try to install plastic behind it. It is best to leave the attic insulation in place and not to disturb it. Fibres may get airborne which could be a health hazard.
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The best way to add a vapour is by painting the ceilings in the rooms below the attic with a vapour barrier paint. Vapour barrier paint has a smooth, creamy consistency and can be applied by brush, roller or spray equipment. It is designed to prevent the intrusion of water vapor into thermal insulation.
Why is a vapour barrier so important?
It is important to understand how water vapor acts in order to understand the uses of water vapor barrier paint. In a process called convection, warm air always moves toward colder air. Thus, during cold months, the warm air in your house is constantly seeking to escape to the outdoors. Warm air also expands, putting more space between its molecules compared to cold air. Expansion allows warm air to hold more water vapor. When this warm air encounters the cooler surfaces of your home, such as exterior walls and ceilings below attics, it cools down and contracts, squeezing out water that collects on the cool surfaces.
Vapour barrier paint or an oil based paint?
During my 28 year home inspection career I have seen many homes that did not have a vapour barrier in the attic. I always told the prospective home buyers to paint the ceilings below the attic with two coats of an oil-based paint to create an effective vapour barrier. There are many experts who may disagree with me about this method but here is a quote from the Natural Resources Canada website in an article titled VAPOUR BARRIER MATERIALS, "In most older houses, the layers of oil-based primer paint and varnish finishes can function as an adequate vapour barrier for walls and ceilings. Areas that most often require special applications of vapour barriers include interlocking ceiling tiles and new drywall. Pay special attention to areas of high humidity, such as kitchens and bathrooms." I agree with that statement, so if you have trouble finding vapour barrier paint apply two coats of an oil-based paint to create an effective vapour barrier. |
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