Conditioned Crawlspace

jjackAugust 27, 2011

I'm about to condition my ~600 sq. ft crawlspace by removing existing batt insulation, spray foaming the rim joists, installing Thermax foam board on the foundation walls, and putting down 8 mil poly (half the floor is dirt, half concrete slab). I have gotten two quotes. One was $9,000 and one was $5,000. The $9k seems insanely high, and even the $5k seems just high.

When I had the house inspected before I bought it, my home inspector guessed it would cost about $2k to do this. (He recommended that I take the above steps.) So, I'm trying to figure out which one of us is smoking crack.. Does anyone even have a ballpark guess for this?


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Dow Thermax is quite expensive. I've read prices up to $80 per 4x8 ft. sheet. You can use cheaper XPS (extruded polystyrene), but it will usually require an "ignition barrier". But some municipalities may allow it installed with no barrier.

Labour costs are tremendously variable. Here, even general labour would be billed at $30+ per hour.

BTW, your crawlspace can't simply be enclosed without attention to other basics, such as radon mitigation and air supply/exhaust. (See Building Science Corp. link below for details.)

If you're not satisfied with the quote, you should seek competitive estimates from licenced contractors who specialize in sealing crawlspaces.

Here is a link that might be useful: Building Science Corp on Conditioned Crawls

    Bookmark   August 27, 2011 at 11:01AM
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Windows on Washington Ltd

Why are you going to put thermax board on the crawlspace walls?

It is expensive and in this application, it could be just as easily insulated with PSK fiberglass insulation.

Don't get me wrong, fiberglass is usually an awful insulation medium, however, in this case it does just fine.

Make sure that your liner is sealed to the crawlspace walls.

Get additional quotes. The 5K number is much closer to reasonable but still more than we would charge based on your square footages and work description.

    Bookmark   August 28, 2011 at 11:44AM
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it could be just as easily insulated with PSK fiberglass insulation.

Easy, yes. Cheaper, yes. But here's why you shouldn't install it.

From the assorted Ph.ds, building engineers, architects and building scientists at Building Science Corp.:

"Air permeable insulation such as fiberglass batts, fiberglass blankets or spray applied cellulose should be avoid when insulating crawl space perimeters and rim joist assemblies. The air permeance characteristics of these types of insulations do not prevent moisture laden air from accessing surfaces that may be cold enough to condense water." p. RR--0401, p. 3

And, since you don't mention your climate, if you're not in a humid hot climate, a vented crawlspace may be more efficient. (See link.)

Here is a link that might be useful: Building an Unvented Crawlspace

    Bookmark   August 28, 2011 at 2:35PM
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Climate: Minnesota. So, brutally cold 8 months of the year, hot and humid 4 months of the year.

About the Thermax, I currently have fiberglass batts in there (between the joists and on wall connecting to garage). The foundation walls are bare. The batts are not working much at all.. it is freezing in there in the winter and damp in the summer. I am going to turn this into a conditioned space.

    Bookmark   August 28, 2011 at 10:17PM
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