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I've asked this question before:

What are the pros and cons of a vented vs unvented crawl space?

However, now that I'm going to get air conditioning the question comes back again and again people seem to have different opinions.

I'm the DC area where the summers are hot and humid.

I've got 2" rigid foam insulation on the perimeter walls of my crawl space and an interior perimeter drain system covered with gravel. I'm going to cover the dirt with 2 layers of 6 mil plastic.

Now, I've got a few bids for AC and the AC guys have different opinions on the question of conditioning the crawl space. The guy that I may end up going with thinks that I should vent the crawl space in the summer and isolate it from the basement (my space underneath is divided: 1/2 crawlspace 1/2 basement).

However, there's this article:

http://www.buildingscience.com/documents/reports/rr-0401-conditioned-crawl-space-construction-performance-and-codes/?topic=/doctypes/researchreport

that talks against the idea. My bidders think it's OK to condition it (small register + return at the opposite end).

FWIW, I may also add a radon exhaust fan hooked into my french drain.

Given all the above info and your experience, what decision would you make if you were in my shoes?

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I don't live in a hot climate, but I do know one thing, heat rises and cold falls. i can't understand the justification of putting AC into a crawl space that is not air tight. I answered your question a long time ago about insulating and venting the crawl, and I still stand by that advise. I can not believe the air conditioning a dirt floor crawl space is going to positively effect the temp in the house above by any measurable amount. I think it will a waste of energy and money to AC the crawl. I COULD BE WRONG, but I'd love to hear a compelling argument for doing it.

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  • Putting registers in the crawl space doesn't make sense to me either. Maybe a return? Hmm, the negative pressure will pull cold air as well which, again, would be a waste. I'm still confused about this because of all those articles like the one I cite above. This is exactly why I post the question here. I highly appreciate your answer and I'd be very curious to see other answers with the oposite opinion coming from experienced people like you. If I don't hear anything I'll probably ignore that theory.
    – Peter Q
    Commented Apr 29, 2011 at 3:06
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    I read the article, I can understand the theory. The main point is obviously that the crawl has to be air and moisture tight for including it in the HVAC system. The real advantage seems to be lowering the humidity in the crawl and treating it like a full basement. Actually, not many full unfinished basements are heated and cooled. I can see some advantages, but not sure how they weigh against the cost of construction and added energy costs. Still a toss up in my mind. Not convinced yet.
    – shirlock homes
    Commented Apr 29, 2011 at 11:15
  • Regarding construction cost, I'd actually have more work to isolate the crawl from the basement than if the crawl was to be vented. I'm adding the poly plastic on the dirt regardless. I'm thinking wouldn't the radon exhaust act as a vent since it's sucking air via the perforated drainage pipe which would be under the plastic? Then the space above the plastic can be connected to basement part. Note that my basement would be finished, so there's going to be drywall between the crawl and the living areas.
    – Peter Q
    Commented Apr 29, 2011 at 14:49
  • The radon system vents under the poly, but we're talking whether to vent the crawl space or not. The radon system does not vent the crawl space. If you have floor and duct insulation, I would opt for smaller conditioned volume. Without it, you are sort of conditioning the crawl space anyway, so you may as do it effectively and seal it as a condition space. If still a toss up, I would ventilate the crawl space simply as a backup to the radon system. Despite evidence to the contrary, I don't fully trust radon systems in the long run. --just realized this is out dated, oh well.
    – bcworkz
    Commented Nov 24, 2012 at 23:00
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If you are putting air handler or ducts in the crawlspace, you should seal and condition it. This will save more energy than the additional energy used to condition the extra volume of space. It will also result in better comfort (ducts will do a better job of delivering conditioned air) and protection from moisture than having them in a vented crawlspace.

You already have insulation at the crawlspace walls. If you add vents, it defeats the purpose of that insulation. It is much more difficult to insulate the crawlspace ceiling successfully.

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See http://www.advancedenergy.org/portal/crawl_spaces/pdfs/ASHRAE%20IAQ%20Paper%20Revised%202008-07-15.pdf

And http://www.greenbuildingadvisor.com/blogs/dept/musings/building-unvented-crawl-space

And especially: http://aceee.org/files/proceedings/2004/data/papers/SS04_Panel1_Paper07.pdf (Moisture Solution Becomes Efficiency Bonanza in Southeastern United States Bruce Davis and Cyrus Dastur,, U.S. Department of Energy, National Energy Technology Laboratory #DE-FC26- 00NT40995)

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