Rim Joist and Box Sill Insulation

All buildings are are subject to something called the "stack effect". When a building has openings, cracks, or ventilation at the top of the building, and similar ventilation at the bottom, there can be air movement either up or down, through the building. When the outside air is colder (denser) than the inside air, the flow goes out at the top, and in at the bottom. The reverse is true when the the outside air is warmer than the inside air.


One common area for air to come in is in the basement where your first floor meets the basement foundation walls. When you are in the basement looking up, you can see floor joists running across the basement ceiling. Capping those off around the perimeter of your house are rim joists. The sill plate is what those two pieces sit on. The concrete and wood can't form an airtight seal because wood shrinks over time and concrete is never poured perfectly. This causes gaps to become more substantial over time and allows a constant stream of air into the house. 


These problems can be easily fixed by air sealing and insulating the rim joist and box sills. This is most convenient to perform in unfinished basements/crawl spaces and has the capability of creating a noticeable comfort increase as well as a decrease in utility costs.