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.

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