What Is the Best Way to Insulate a Basement?

How to insulate a basement: start with the rim joists

The rim joist is usually the leakiest part of a basement. Sealing it with closed-cell foam stops cold air at the perimeter, which also warms the floor above. See why your first floor is cold for more.

Then the foundation walls

Concrete and block walls feel cold and can carry moisture. Closed-cell spray foam applied directly to the walls adds insulation and acts as a vapor retarder at the proper thickness. It does not trap moisture against the wall the way fiberglass batts with a plastic vapor barrier can.

What to avoid

• Fiberglass batts pressed against bare concrete, which can hold moisture and grow mold
• Insulating over active water problems. Fix grading, gutters, or leaks first.

Finishing the basement?

If you plan to frame and drywall, insulate before the walls go up. Spray foam behind the framing gives you a warm, dry finished space. Most basements must meet code for fire protection over foam, which your contractor and drywall plan should account for.

Further reading: Where to Insulate in a Home (U.S. Department of Energy).