I find it very confusing to read and hear conflicting advice on crawl space: vent vs air seal, different ways to do insulation, vapor barriers vs not, etc.

Mine currently has some moisture and has a 6" square hole in the exterior wall and 2 large 3' square openings to the basement next to it. What should I do?

BuildingScience.com recommends conditioning the crawl space: http://www.buildingscience.com/documents/reports/rr-0401-conditioned-crawl-space-construction-performance-and-codes

One concern I have with laying plastic on top of the dirt is creating conditions for mold.

I'd like to hear independent opinions from DIYers rather from websites and people in the industry since those usually have an agenda.

How did you do your crawl space?


Sounds like the vapor barrier and insulation at the wall would be the better approach.

According to this: http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11810 my zone (anything north of North Carolina) is OK for the vapor barrier.

Further questions:

  • 2
    What climate are you in? I would think it might depend on how severe your winters are, especially when you get into how much insulation you want to try and add when sealing the space.
    – auujay
    Commented Dec 27, 2010 at 15:16
  • 2
    Washington DC, USA. Hot, humid summers, occasional heavy rains during summer, spring and fall, winter is cold but not extreme (last year we had 4 feet of snow though)
    – Peter Q
    Commented Dec 27, 2010 at 16:09
  • See advancedenergy.org/portal/crawl_spaces/pdfs/…
    – Bryce
    Commented Oct 19, 2013 at 19:25

3 Answers 3


Most crawl spaces are vented if they have an earth floor or are prone to moisture. If you insulate between the floor joists with a moisture, mold and vermin resistant insulation (foam as we discussed before) you would still want some ventilation. The only time I would seal the exterior walls would be if I also used a pretty darn water tight moisture barrier over the floor, tuck taped to the knee wall at the bottom of the joists. You would be creating a dry cell and blocking movement of cold air through and under the house. This is typically done with a rubber/neoprene type compound sheet commercially, but can be done effectively with a couple of layers of good 6 mil poly. This poly is avail at Lowe's, HD, etc. in 10 and 20 foot wide rolls, 25 to 100 foot in length. You can use Tyvek tape to seal the seams and around posts etc. Leave yourself a way to enter the space to check humidity levels occasionally for reassurance.

  • 2
    Shirlock, thanks. Would you say that one is easier to do than the other?
    – Peter Q
    Commented Dec 27, 2010 at 5:30
  • 2
    If plastic cover is used, wouldn't mold appear due to vapor being trapped by the plastic?
    – Peter Q
    Commented Dec 27, 2010 at 5:33
  • 5
    If mold was present it would form on the underside of the plastic, this is where the Tyvek tape and making sure the plastic is air tight come in.
    – Tester101
    Commented Dec 27, 2010 at 17:44
  • 3
    Tester is correct. For the plastic cell to work properly and not trap excessive moisture, it needs to be very well sealed. We are doing a lot of this type of enclosure in the northeast right now. Commented Dec 27, 2010 at 18:58
  • 2
    I might add, in your part of the country, DC area, I might be more prone to look at a good insulation job and vents. During the winter months when it is not as humid and wet, you could close the vents to prevent cold drafts under the house, but if your insulation job is good, a little air flow would not make a lot of difference in your heating costs. Commented Dec 27, 2010 at 19:01

Now, this applies in Canada, where everyone has a foundation of some kind (or a slab, but the under-slab crawlspace would be very shallow, indeed), as opposed to being on stilts, and while I can't find the CMHC reference guide that I remember, from memory, in short:

Treat a crawl space like a very shallow basement.

This applies for new construction, and I suppose also for upgrades.

http://www.cmhc-schl.gc.ca/odpub/pdf/65916.pdf isn't the document I remember, but covers the steps for "Crawl Space Remediation Strategies".


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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    – isherwood
    Commented Apr 25, 2023 at 14:44

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