a foam insulated airtight attic could be a big energy saver

Aug 9 2009 | modified: Sep 2 2009 6:24am

If you used spray foam insulation to make your attic airtight, would this lead to moisture buildup and mold?

Study finds the answer to this question is surprisingly NO. You can spray foam insulation on the underside of a roof, remove insulation from the attic floor and close off attic vents. This turns the attic into a space that you are heating and cooling, much as if it were the top of a living room with a cathedral ceiling. You'll be heating and cooling a larger space, so in some cases it won't save money. But if you have heating and air conditioning ducts within the attic, the savings could be significant.

Until the 1930s, attic vents weren't common. However, studies done in very cold climates raised concerns about condensation that could occur when warm indoor air leaks into attics in the winter around ceiling light fixtures and other voids. Gradually, building codes came to require vents so excess moisture could evaporate. In recent years, building scientists began to realize that the opposite situation also occurs: When an air conditioner is running in the summer, the attic vents bring in humid, warm air from outdoors. This can make the attic more damp than it would be otherwise, inviting the very mold problem the vents were intended to prevent.

Several years ago, proponents of unvented attics and insulated roof decks succeeded in getting their ideas written into the International Residential Code. Many communities adopt this as their building code. It now allows unventilated attics provided there is no vapor barrier between the top ceiling and the attic, and provided there is insulation that air can't get through on the back of the roof, between the rafters.

For insulating the back of a roof, spray foam works especially well because it creates a continuous barrier, assuming it's installed correctly. Installation is definitely a professional job, not DIY.