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I'm planning on insulating my currently uninsulated 6-inch concrete foundation wall. This is zone 5, so code says R-15 is required. I can achieve that with 2 layers of 2" EPS foam board (nominal R7.7 per layer). What I'm wondering is if I can (and should) do better by including dead air space (as mentioned in the installation guide, though not for this specific application), or if that will make something bad happen on a crawlspace wall. Water is not a huge problem here in Boise, ID, but it's just semi-arid and we do get rain/snow in the winter and spring.

Here's how the wall looks now, followed by 3 options I can think of. Spacers would be strips left over after cutting boards to size, and they could completely enclose the dead air space. enter image description here

Option 1: no air gaps enter image description here

Option 2: air gap between boards enter image description here

Option 3: air gap between boards, and between board and wall enter image description here

Also, I can't find a consistent answer on how far down the wall the insulation should go. All the way to the footer? Above-grade only?

Vulcan
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