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  • Writer's pictureDarrin Card

Drafty Basements

Most of the clients of my home inspection business know how much money they can save on heating and cooling if they have the proper amount of insulation in their attic and walls and they are correct. What most don't understand is how important it is to properly insulate your rim joist. Uninsulated rim joists will allow air infiltration, which can be a major cause of drafts in the home. Many of the homes I inspect do not have properly insulated rim joists. Even if they are insulated, they may not be insulated according to best practices.

What is the Rim Joist?

The rim joist is a board that runs perpendicular and at the ends of the joists. It helps the joists from twisting and creates the final two sides of the "box" of a floor system. It rests on top of a sill plate on top of the basement wall and the sub-flooring rests on top of it. This creates "pockets" between the joists, under the subfloor, and on top of the sill plate.

Why Insulate the Rim Joist?

The rim joist is an area that is often overlooked when insulating older homes. Todays construction standards call for the area to be insulated and air sealed when built. In one study ( insulating and air sealing the rim joists in a home reduced the air infiltration rate by 11.4%. This can lead to a significant savings in terms of heating and cooling costs.

The other reason to insulate the rim joist is the potential for condensation on the rim joist. As hot air hits the uninsulated side of a rim joist there is chance it will condense if the board has been cooled by outside air or drafts.

How To Insulate Your Rim Joist

There are three main ways to insulate your rim joist. Two of them are recomended, one is widely used but no longer recomended and I will explain why shortly.

Fiberglass Insulation

The fiberglass insulation method is widely used, easy to install, and affordable, but it doesn't provide the air infiltration or moisture protection of the other two methods. Because of this I have stopped recommending it as a viable solution. Too often I have pulled back the insulation to find it soaking or a rotted board.

Spray Foam Insulation

The spray foam is, in my opinion, the most effective method of insulating, however it can get pricey. Foam kits are now available at most home centers, I'm not talking about the little 12oz aerosol cans though. The spray foam, when correctly applied, creates a moisture and air free barrier and offers superior insulating value per inch applied.

Closed Cell Ridgid Foam Board

This is the method I recommend most often. Installing a 2" thick piece of ridgid foam insulation in the pocket and caulking the edges creates a moisture and air resistant barrier. This method is cost effective but a little labor intensive.

So there are two viable ways you can insulate your rim joist and save money every year in heating and cooling costs. If you would like advice on insulating your rim joist or would like an inspection please do not hesitate to give me a call.

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