Vented crawlspace – air barrier
I am trying to figure out the construction details for a new home build in Washington state, climate zone 4C (marine). I have been spending many hours trying to educate myself on GBA and with the “Musing of an Energy Nerd” (great book Martin).
Given the unique climate in the Pacific North West, the prevailing design for homes is vented crawlspaces. They appear to work well due to the lack of hot humid air conditions. I also understand that an unvented crawlspace is also an option but brings with it an additional level of concern for venting and radon control.
So the thing I can’t figure out is how to combine a vented crawlspace with the principle of air tightness? Where is the air barrier in such a construction? Somewhere between the underside of the joists and the finished floor? How would I create an effective air barrier? Foam floor insulation against the underside of the subfloor and joists? Taping up the subfloor? Is it even possible given the penetrations for ventilation & plumbing?
Also on a side note, I am aslo not entirely clear where the air tightness resides for a unvented crawlspace without a concrete slab, does the 20 mil poly on the ground function as both vapor and air barrier? If so how is the air barrier made continuous (overlap & tape?)
Lot’s of questions, the more I read the more questions and doubts.
Cheers,
Tim
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Tim,
First of all, I urge you to change your screen name so that it doesn't read "User-6824154." Here is a link to an article that explains what you need to do: How the GBA Site Displays Readers’ Names.
Q. "How do I combine a vented crawlspace with the principle of airtightness? Where is the air barrier in such a construction?"
A. If the floor assembly is insulated with fluffy insulation like fiberglass or mineral wool, there should be two air barriers: the subfloor and the crawl space ceiling (generally drywall, OSB, or foil-faced polyiso, if legal). For more information on this issue, see How to Insulate a Cold Floor.
Q. "Is it even possible given the penetrations for ventilation & plumbing?"
A. If you have an unconditioned vented crawl space, I strongly advise you to keep ventilation ducts out of the crawl space. Plumbing pipes are unavoidable, but remember that, during a cold snap, these pipes can freeze -- which is a strong argument in favor of unvented crawl spaces. For more information, see Building an Unvented Crawl Space.
Q. "I am also not entirely clear where the airtightness resides for a unvented crawlspace without a concrete slab. Does the 20 mil poly on the ground function as both vapor and air barrier? If so, how is the air barrier made continuous? Overlap & tape?"
A. Good question. The answer (which I didn't emphasize in my article) is that the polyethylene that covers the dirt is both a vapor barrier and an air barrier, as you correctly guessed. My article advises, "Seal the seams of the vapor barrier material with a compatible tape or mastic; many builders use duct mastic embedded in fiberglass mesh tape." Many housewrap tapes also work on polyethylene.
Tim, can I ask what you decided to do? I'm also designing a new build in Washington state and have the same question(s) you posted above....thanks in advance!