Open vs. closed-cell foam in an unvented crawl space
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| Posted in Energy Efficiency and Durability on
After speaking to several spray foam contractors about enclosing the crawlspace on our 1950’s home in Zone 4 (Washington, DC) there seems to be no consensus on whether it is best to use open or closed cell foam on the interior walls. The walls of the crawlspace are of block construction and the floor is dirt. Your thoughts please….
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Replies
Woody,
Closed-cell spray foam is better in all respects than open-cell spray foam.
Its only disadvantages are its higher cost and the fact that it uses blowing agents with a high global warming potential.
For more information, see Building an Unvented Crawl Space.
Thx Martin. I had read the Building an Unvented Crawl Space before I posted. You talked about closed cell being used in stone walls because of their lack of smooth vertical surface I presume but not specifically block wall applications. How do the costs differ between CC and the rigid board you suggest.
If you use EPS or polyisocyanurate rigid foam it costs about 10 cents per square foot per unit-R, but factor in the scrap fraction and installation labor too. As a DIY it'll be cheaper than sprayed closed cell polyurethane, but may be more expensive if you're paying a decent labor rate for a pro to install it.
Closed cell spray polyurethane runs about 17-18 cents per R-foot, installed.
Open cell spray polyurethane runs about 13 cents per R-foot installed.
The cut edges of polyiso should not be in direct contact with a floor slab or soil, since it can wick moisture. The foil facers are very waterproof (and vapor barriers), thus wall-contact isn't a problem for the foam. With EPS it doesn't matter (if it works as dock floats, it's not going to be saturated in your crawl space, ey? ;-) ), but it takes about 50% more thickness to achieve the same R value.
In a DC climate it's worth putting 3-4" of EPS on the walls (R12-16), and 1.5-2" of EPS on the crawlspace floor too, under the ground vapor retarder. (See Table 2, p.10 the row for zone 4 http://www.buildingscience.com/documents/bareports/ba-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones A 1.5-2" non-structural concrete "rat slab" atop the vapor retarder is also recommended, to both protect the foam when you are down in the crawl for servicing ducts/plumbing/electrical/etc, and to keep rodents from gaining access to the crawl space (a problem that can crop up in any neighborhood.) The floor-foam keeps it warmer/drier and less prone to mold issues.
EPS and polyiso are both blown with pentane, which has less then 1% of the global warming potential of HFC245fa the most commonly used blowing agent for closed cell polyurethane.
Thx Dana Helpful as always. Woody