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

Question

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?

Edit:

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:

• Should I have the 2 inch rigid foam at the walls and vapor barrier on top or underneath or...? I cannot find this detail on the Internet.

• "Conditioning" the crawl space means I have to ensure air intake and outtake for it, doesn't it? I currently don't have AC, BTW, only window units.

• AskTheBuilder is talking about using high performance vapor barriers here: http://www.askthebuilder.com/B403_High_Performance_Vapor_Barriers.shtml Does anyone have experience with those? Where do I buy that stuff? I only found this: http://www.insulationsolutions.com/products/viper_CS/information.html

Answer 1

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.

Answer 2

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".

Answer 3

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)