The wooden framing in most homes rests on top of a solid concrete or concrete block foundation. In homes built before 1980 or so, the lowest section of wood, called the "mud sill," rests directly on top of the concrete. While the connection is secure from a strength standpoint, in terms of eliminating air infiltration, things could be much better. The problem is the rough and variable surface of the top of the foundation wall. While there are many areas where the wood presses down tightly, other areas may leave a gap through which wind can enter.

What you can do:

1. The gaps, which collectively might add up to a hole the size of a basketball in the exterior envelope, can usually be sealed with either caulk or cans of spray foam. This procedure, which can be done either on the inside or outside of the house (depending on which offers the best access) requires that you first brush away the dirt and cobwebs from the concrete and wood so the caulk or foam will stick to both surfaces.
   From that point on it's just a matter of aiming the caulk tube's tip or spray foam applicator tube at the gaps and gunning them full of caulk or foam. It's a job that doesn't have to be neat or precise, just thorough. Once you're finished, you will have stopped up one of the leakiest places in the home.
   
2. In newer homes, the gap between the mud sill and the top of the foundation wall is filled with a thin, compressible length of foam material. The foam creates an airtight seal that does not need remedial caulking or foaming. However, it's worth checking along this area anyway, as occasionally the foam sealer didn't get placed exactly where it should have been. Also, the top of the foundation wall might be too uneven for the foam to fill the gap, someone might have forgotten to put it in place, or it might stop short of the corners. In any of those cases, a shot of caulk or foam can quickly remedy the problem.

Savings:

These steps are important parts of sealing ALL the spaces in your home. If all steps are done you can save up to 30% on your home heating bills. Doing these by themselves might save 2-3% on your energy bill.

Learn more on our Detecting Air Leaks and Caulking and Weatherstripping pages.