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.
Photos taken October 13-18, 2019. Posted December 15, 2019.
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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.
Usually “blue board” refers to a type of sheetrock designed for use under plaster, but I’m guessing you’re referring to a brand of EPS foam with borates for insect resistance. The “pink stuff” is Foamular XPS. XPS has significantly higher R-values than EPS and is more resistant to water intrusion (though EPS also drains more quickly). XPS is also generally more expensive, and is currently produced with blowing agents that are not very environmentally friendly. In my opinion XPS is nicer to work with than EPS because EPS releases clouds of styrofoam particles (aka EPS snow) when you cut it, but that’s probably not a huge concern for most folks.
I’m not aware of significant problems with exposed below-grade foam in Alaska, other than frost-jacking if you don’t fasten it well. We’re the only state that currently has no termite risk. In places where there is concern about termites, some people have put a thin coat of cement-based material over the foam. I can’t speak to how well that might work. Below-grade mineral wool insulation is just now starting to hit the market, though when I was at this stage in my project this was a relatively new option and there wasn’t much info about it (and no local availability). I went with XPS, but if I were building again I’d probably look into below-grade mineral wool a little more. I like that it’s inert and doesn’t need flame retardants or insect treatment.
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!
Do you think using the xps on both side of the foundation would have the same r-value as the ICF block? How well has the insulation performed? I’m building in big lake with ether ICF or poured foundation and am trying to figure out the best method vs price.
Provided that the quantities of foam are equivalent, putting XPS on both sides of the foundation wall would result in exactly (or almost exactly) the same R-value as what you’d get with XPS ICF blocks. By adding the foam later instead of using it as a form, you’re basically just reverse-engineering an ICF block. I’m not sure why you’d want to put foam on both sides of a foundation wall, though. Typically if you’re adding insulation later you’d only add it to the outside of the wall. That keeps the concrete fully inside the thermal envelope, makes your interior space as large as possible, and allows the concrete to dry to the interior (though that might be a negligible benefit).