How much rigid insulation for my crawlspace walls?
damunk
| Posted in Energy, Heating & Insulation on
I’m pulling out the r-19 fiberglass insulation in my crawlspace joists and wish to insulate the block walls instead. How thick do I go? I live in Southern NY state. The entire crawl space sits at or above grade (built on ledgerock). The exterior already has fieldstone veneer (4″ (+) (-) and it’s std. 8″ hollow block. Obviously the insulation, at this point, has to go on the interior of the block. Thanks!!!
Replies
2 inches would be minimum/standard, I'd think. If you can afford the 3", I'd do that. You're only going to want to do it once ... so go the max you feel like spending. The material difference will be half again more $, but the install effort/cost will be the same either way.
I found my answer on the Building Science Corp. site. Amazing how thorough they explain the "science". They show a 2" layer of R-10 rigid followed by a layer of 2" fire rated 2" foam w/an R value of 13. Sounds right being that I'm pulling R-19 out of the floor at the crawl. Now to figure the venting...
For venting....don't--at least, not in the traditional way. The IRC (International Residential Code, section R408.3) allows for unvented crawls if you follow certain requirements.
In your case, it seems you could use the method where:
1) The exposed earth is covered with a continuous Class I vapor retarder (like 6 mil Visqueen), with 6" overlap of joint edges, sealed or taped, and extend the vapor retarder 6" up the stem walls, sealed or taped to the wall.
2) Install a small, continously operating, exhaust fan that exhausts just 20 CFM per 1000 sq. ft. of floor space.
3) An air path (small duct or transfer grille) that connects the main living space above with the crawl.
This approach keeps moisture out of the crawl, and produces a slight depressurization of the crawl to keep odors from transferring to the main living space, while also lowering humidity in the crawl.
The downside, to me, is that you have an out-of-sight-out-of-mind exhaust fan that will sometime need maintenance or replacement.
I'm going this exact route, but I thought that rather than the transfer duct from living to crawl I'd simply install (2) or (3) 100cfm blowers blowing out to the exterior and have the depresured crawl draw from the living space through "the cracks" at wall/floor junction and the countless holes from wiring et al. Blocking off the four foundation vents presently there (of course). The space is 1000sq. ft. and 4' high. I also have some Radon present so I was going to do the perf. pipe (in gravel) around the perimeter and stub up through the 6ml poly. I was going to wait on carrying this up through the roof until I see what my Radon amts. are once the foundation blowers are in place. Thanks for thoughts.
200-300CFM is a lot, and probably more than you need if they operate continuously. I would think 20-50 CFM would be adequate. At 20 cfm, you exhaust a volume of air equal to over 7 times the volume of the crawl during a 24 hr period.
This should also carry out any radon that might seep thru the poly vapor barrier.
I agree it's a lot, but without a "source" of air to draw from I thought the 200+ cfm would create an adaquate draw, pulling some "conditioned air" through and out. Thanks for your interest! Regarding the Radon. First going to simply pipe out through the block rather than straight up through the roof. I'll find a spot away from a window. I need to go on the NY state Radon site and see what they have to say.