Insulating the foundation

Insulating the foundation

Adding below-grade foundation insulation. Right now, the only widely-available materials for this task are XPS foam and EPS foam. Both types of foam are produced from the same polystyrene plastic, but they are manufactured with different processes and using different gasses (called “blowing agents”). There’s an endless debate in the building science community about which type of foam is better for various applications, but in the end I had to choose one and I went with Owens Corning XPS (aka “the pink stuff”). Compared with EPS, XPS has a bit more insulation per inch, is far more resistant to water, and is easier to cut and shape. The downsides are that it uses a less environmentally friendly blowing agent and it costs more.

Most houses in Anchorage use a single two-inch layer of below-grade foundation insulation. For my foundation I used a three inch layer on the bottom with a two inch layer over that, for a total of five inches. This should keep the foundation (and the footings) super cozy throughout the winter. I glued the insulation to the block wall with Loctite PL Premium urethane glue, and all seams between the insulation panels are taped with acrylic-based 3M flashing tape. I checked to make sure the glue was compatible with both the foundation waterproofing and the insulation. Ditto with the tape. It’s important to make sure your building products won’t react with each other and fail over time.

Insulating the foundation Insulating the foundation Insulating the foundation

Photos taken October 13-18, 2019. Posted December 15, 2019.

View on Instagram

Check out the AK House Project Instagram account for feedback and comments on posts.

Subscribe
Notify of
guest
3 Comments
Oldest
Newest Most Voted
Inline Feedbacks
View all comments
Kevin
Kevin
2 months ago

Did you look at blue board at all? I’m resealing my foundation after the membrane failed and am adding insulation. I went to Polar Supplies and they told me that blue board is like the pink stuff but it has borax baked into it and is therefore pest resistant. It’s made locally and it may be cheaper.

Kevin
Kevin
2 months ago
Reply to  Paxson Woelber

You are right, I called again and it was in fact EPS. Thanks for the tip on the rock wool, i’ll do some research and see if that’s a viable solution for me!