Insulate Top of Basement Wall

Note: There are two videos on this page.
Also note: Wear long sleeves, protective eye wear, and a dust mask when cutting and installing Fiberglas insulation.

This job needs no experience to do, except time and effort to cut, seal and clean. The results are immediate – warmer, cooler, and quieter. It took me 2 long weekends to get this project done. With 2 people, it could be done better and easier.

Insulate basement rim bond joist and bay window floor with Fiberglas, and caulk for air infiltration.

I found it easier to use the caulk. The foam was best when I had room to work because it was messy.

The worst job, but very important, is cleaning the sill at the top of the foundation wall and sides of the joist for good adhesion. Caulk and foam will not stick to dirt, oils, or crumbling stone. There is stuff from when the place was built that has blown in from the outside, so vacuum, stiff brush and vacuum again, maybe use water, too.

Caulking at the bottom of the bond board seals air infiltration at the top of the foundation wall. And caulking at the top of the bond board prevents air into the upstairs floor and basement. Caulking the sides is minor.

There are ¾“ gaps to fill with caulk, so foam could be better at times.

The newer fiberglass is not like the old stuff; it’s less harmful, does not itch and generally is easy to use.

Doing this method of using only caulk and fiberglass would take 1/3 the time of the following job, and can be done by 1 person.

Insulate basement rim bond joist with foam board, fire stop, and can foam for windows.

Caulk around the rim joists, but use foam board instead of fiberglass, and spray can foam to seal.

Can foam stops air infiltration, foam board insulates to R10 at joists, and using Fire Stop foam protects.

Cut foam board into 14.5” x 7.5” pieces to fit at rim bond and seal with can foam at top and bottom. Go slow and be patient.

Cut a bevel on the board to give the foam more surface area to adhere.

I used the foam, it cost $100, with foam board, caulk and supplies another $100. The job can be done faster if organized.

This winter, it was often below 0 degrees around Detroit, but the basement stayed above 60 degrees. In the fall, it was a constant 66 degrees, and the summer it’s a cool 70 degrees.