Sealed crawl with passive exhaust?
I work with a non-profit building small, affordable houses in climate zone 4A. We have tried a few different methods of sealed/ conditioned crawls, but I am interested in finding the best, inexpensive method that will work long term for our buyers. So it needs to require minimal to zero maintenance or understanding of the science behind it for the people living in the house.
One method that has been suggested by a HERS rater is to keep the crawl separated from the living area, provide a small supply of conditioned air with damper, and a passive exhaust with damper at the other end of the crawl.
This makes sense to me, and I like that it eliminates the issues that buyers have with sharing air with the crawl. But does this provide enough de-humidification in a humid climate? Does this meet code? Are there other issues I may not be thinking about?
Thanks for any info or advice.
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There are 2 options with a crawl space.
1: Crawl space is outside the thermal envelope, that means there is nothing serviceable down there and it is openly vented to the outside, care must be taken to accommodate condensation on the underside of the envelope.
2: Crawl space is like a mini basement, fully air sealed, waterproofed, and conditioned like the rest of the house.
E. Lingerfelt,
Your proposed method of conditioning the crawl space can work, but not with a passive exhaust vent. This approach requires an exhaust fan -- in the words of the IRC, “continuously operated mechanical exhaust ventilation at a rate equal to 1 cfm for each 50 square feet of crawl space floor area.” The makeup air for the exhaust fan has to be conditioned air from the house above, introduced into the crawl space through a floor grille.
Another alternative is to introduce conditioned air into the crawl space via a forced-air register delivering 1 cfm of supply air from the furnace or air handler for each 50 square feet of crawl space area. This method also requires a floor grille connecting the upstairs conditioned space with the crawl space. The disadvantage of this method is that crawl space air is introduced into the home.
For more information, see Building an Unvented Crawl Space.
Depressurizing the crawl with a fan isn't exactly the right thing to do, since it depressurizes the whole house driving parasitic outdoor air infiltration into the house.
It's better to give the crawl space full swaps with conditioned space air. In a sealed conditioned crawlspace this isn't any "worse" than a fully conditioned basement from an indoor air quality point of view, which is just fine. In this case the conditioned basement may only suitable for leprechauns and elves, but as long as they don't bite, sing too loud at night, or stink up the place you're fine. :-)
Thank you for the replies.
My interest in trying to find a method that keeps the crawl space air separate is that it is very difficult for us to control how homeowners care for the crawl space. If they store something they shouldn't there or inadvertently puncture the vapor barrier, it is compromising their indoor air quality.
Is it conceivable that the crawl space can be conditioned separately from the living area? As a separate zone? It doesn't sound like there is a good way to do that, but I'm open to any ideas.