Air Seal and Insulate Your Box Sills and Rim Joists in your Basement

Air Seal and Insulate the Box Sills and Rim Joists in your Basement

Basements are often the second largest area of heat loss in a typical Vermont home after the attic (see what to do about that attic hatch). Some of the leakiest parts of the basement are the box sills...the space between your foundation and the first floor.

When you inspect your box sills, you might see that they are already insulated with fiberglass. Unfortunately, fiberglass does not stop air leaks effectively. Here's the basic steps to fix this up:

  1. Pull out any loose fiberglass (and save it to put back in).
  2. Cut a rectangle of two inch rigid foam insulation board and put it in the box, all the way to the back. It doesn't have to be a super tight fit, though the tighter it is, the less expanding foam you need to use to seal it.
  3. Use a can of expanding foam to air seal around the entire rectangle.
  4. Place the fiberglass back in for extra insulation.
  5. Repeat! And as you go, if you see any gaps, like space between the foundation and the board sitting on top of it, or any cracks in the foundation...anything that looks like air might flow through it, use that foam can to seal them up.

Diagram for insulating box sills, showing 2 inch insulation foam board pushed to the back of the box with expandable foam sealing it in.


Here's a video that shows this process and more you could do in your basement. The goal is to stop cold air from coming in through the basement and up into your living space. The tighter your basement, the harder it is for hot air to escape through your attic. If you have any questions, call cusomter support at Efficiency Vermont: 888-921-5990.

Air sealing and insulating rim joists and box sills is a DIY Home Weatherization project that qualifies for a $100 rebate from Efficiency Vermont when you do three – so save your receipts! Check out other DIY options