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Below-Grade Insulation

Q: How do I insulate my cold concrete-block basement walls?

illustration of basement wall with XEPS foam insulation
Illustration by Harry Bates

My home has a finished walk-out basement with exposed concrete-block foundation walls. The living areas are very difficult to heat. What's the best insulating strategy?
Ben Kramer, Westerville, Ohio


Tom Silva replies: I'd insulate the inside of the walls, which will isolate all that cold masonry from the rest of the room. And the best way to do that is to cover them with extruded expanded polystyrene (XEPS) foam insulation board, which has an insulating value of about R-5 per inch, a high compressive strength, and low moisture absorption.

I like to adhere a continuous layer of 2-inch-thick foam to the walls, then put 1x3 wood strapping vertically over the foam, 16 inches on center. Just drill pilot holes every 16 inches or so through the strapping and foam and into the block, then drive in 3½-inch Tapcon masonry screws or spring spikes. They both work great. Attach the drywall to the strapping with drywall screws. That should put a good dent in your winter heating bills.


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