The key to insulating your rim joists is to use a product that air seals the area above your foundation and around the top of your sill plate. Not only is insulating your house good for your budget by saving you money, but it also increases the resale value of the home, adds energy efficiency to your house and provides an immediate cost savings to heat the house.
The problem that most homeowners experience when they insulate their rim joists is that they insulate with poor materials that do not air seal their house. If you use fiberglass batts or cellulose you may get a thermal barrier – not an air barrier – and the heat from your basement will filter out through the insulation. It will then discolor and turn black, possibly causing condensation on the inside or outside of the rim joist that may lead to mold or mildew.
Again, the solution is spray foam and not foam board. When you get down to the cost of buying the foam board, cutting the foam board, then buying the air sealing product and applying it, it is really a lot cheaper just to have a spray foam professional come and seal in your rim joists in your basement. Doing it yourself probably isn’t the answer, either, because “do-it-yourself spray foam” is expensive and messy. If it doesn’t mix well when you apply it then you’ll have unmixed sticky foam on your hands – which doesn’t actually insulate.
The right spray foam to use is closed cell spray foam. Because of the closed cell foam properties, spray foam creates not only a vapor barrier but an air barrier, as well. The best part? The vapor barrier automatically switches from hot side to cold side depending on cllimate (heating season or cooling season). Your cost savings should be between 18 and 20% after applying the foam to the rim joists.
Make sure your applicator not only applies foam to the rim joist but to the top of the sill plate all the way out to the edge of the foundation. You want not only the rim joist, but the whole top of the foundation sealed off where air can come up and into your house from inside the cement block, the stone foundation or the poured cement. The thickness of this foam should be at least three inches but no less two. Contact us with any questions!
Dr. Spray Foam (Mark Gugino)