What foam board, EPS or XPS, is best for the foundation walls of an unvented crawl space?
Shakennotstirred
| Posted in Energy Efficiency and Durability on
What foam board, EPS or XPS, is best for the foundation walls of an unvented crawl space?
What is the recommended R-value (i.e. how thick) for this application in climate zone 5?
Thank you.
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Replies
Bob,
Q. "What foam board, EPS or XPS, is best for the foundation walls of an unvented crawl space?"
A. Either one will work, but most green builders prefer EPS over XPS because the blowing agents used to make EPS are more environmentally benign.
Q. "What is the recommended R-value (i.e. how thick) for this application in climate zone 5?"
A. You should install at least as much insulation as required by the 2012 IRC for basement walls, namely R-15 for climate zone 5. If you use XPS, you'll need at least 3 inches. If you use EPS, you'll need about 4 inches.
For more information on insulating crawl space walls, see Building an Unvented Crawl Space.
An alternative R15+ stackup would be 1.5-2" of EPS with a 2x4 studwall insulated with unfaced R15 rock wool or unfaced R13 fiberglass batts. The batts alone would qualify as the code-required ignition barrier for the foam, but you'd still need an interior air-barrier, so half-inch gypsum is still the right stuff. Put an inch or two of foam below the bottom plate of the studwall as a thermal & capillary break. In zone 5 an inch of EPS isn't sufficient exterior-R for few point control on air-permeable cavity insulation, but 1.5" is.
If you have a source for reclaimed roofing foam, all-foam solutions can be cheaper than batts, and ANY type reclaimed foam is greener than virgin-stock. I did my zone-5 basement with 3" of reclaimed roofing iso mounted with furring through-screwed to the concrete, with seams & edges sealed with can foam. The gypsum is screwed to the furring. It takes longer TapCons than box-stores carry to get there, but it's do-able, but you may have to shop online for the fasteners. For 4" foam the fasteners are an even tougher issue, making a foam-trapping studwall a somewhat more practical solution.