Published on May 3rd, 2014 | by Henri


Types of insulation to use in a cinderblock basement

Q.    I know you’ve given advice in the past about basement insulation but I have some questions. I’m working on a ’60s ranch house with a typical cinderblock basement. Lately I’ve read and heard that it’s better to insulate rim-joist pockets with rigid insulation versus batt insulation because of moisture and dampness from the cold. Is that right?

Then my next question, is it better to use rigid or batt insulation between the studs and to complete the work with vapor barrier and drywall?

A.    Rigid insulation at the rim joists is fine as long as it is installed very tightly and caulked around its perimeter to prevent moisture by-pass. R-19 or greater fiberglass insulation with a plastic vapor retarder applied over it and stapled tightly all around also works well.

I do not recommend insulating any foundation, especially cinderblocks, lower than 2-feet below grade unless you are certain that:

  1. You have a functioning foundation drain;
  2. The backfill around the foundation was done with coarse, well-draining material and, most importantly;
  3. The grade slopes away from the foundation for the water to drain away quickly.

It also helps to have a healthy stand of grass to draw moisture from the ground. Insulating deeper without these conditions risks cracking the walls from deep frost penetration.

The choice of which insulation to use on the walls is up to you. Rigid insulation is applied directly to the walls with adhesive, and the studs are set against it. If fiberglass is chosen, the studs should be set 1-inch from the walls, strips of insulation cut to fill these spaces, and unfaced insulation pressed in place between the studs to fill the entire spaces between the walls and the inner faces of the studs. A 6-mil plastic vapor retarder should be stapled to the studs and drywall applied over it.

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