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moist crawl space

Posted by ihhann (My Page) on
Tue, Jul 21, 09 at 1:11


My crawl space is moist, i.e., the air is damp and the foundation walls show signs of wetness. A mold inspector has not found mold and the subfloor is dry. I am planning to dry out the crawl space and I have a few questions regarding your product and the various approaches that I've read about:
1. I've read about two alternative approaches to dry out the crawl space:
a) Use a dehumidifier such as the Dri-Crawl Space Dehumidifier
b) Open up a HVAC duct and an air return to dry and warm the crawl space through conditioned air. This will also reduce overall heating/cooling cost since it the air from the crawl space will have a similar temperature as in the floor above.
- Is option (b) the cheaper solution that is recommended for less moist crawl spaces? Is option (a) recommended for severe cases? We live in Maryland (West of DC), so it gets humid in the Summer, but it's not as wet as in the Eastern Shore.
- Does option (b) require insulation of the foundation walls, while option (a) does not?

2. Based on what I read, the best practice indicates that one should close the crawl space vents and to seal the crawl space. But given that moisture will still be underneath the vapor barrier, will this not lead to wet foundation walls and wet studs underneath the vapor barrier?

3. What is the best material to seal the crawl space. In one article, Tim Carter (author of 'Ask the Builder' column) referred to high performance polyethylene as the best product. Other sites are recommending higher end products such as the 'Crawl Space Vapor Barrier,' which is made of 7 layers. Can you tell me what the advantage of the Crawl Space Vapor Barrier over the high performance polyethylene is? I'm not planning to use the crawl space for storage.

Any help would be greatly appreciated.

Follow-Up Postings:

RE: moist crawl space

For the definitive independent building science viewpoint, always check first. (See link for info on conditioned crawlspaces.)

Closed conditioned crawlspaces in the US southeast save energy as well.

Experiments in colder climates show improved performance with insulation on the ground too, typically EPS (expanded polystyrene).

given that moisture will still be underneath the vapor barrier, will this not lead to wet foundation walls and wet studs underneath the vapor barrier?

There should be nothing under the vapour barrier but the earth itself and the below grade portion of the foundation or piers. The most commonly-used material is 6 mil poly overlapped and weighted down; tighter systems up to 20 mils thick are available, but I haven't been able to Google up any independent testing on them. However, you will still need a mechanism to reduce the moisture level from moisture that inevitably gets past the barrier. A dehumidifier is the preferred method.

Here is a link that might be useful: See RR-0401 (Conditioned Crawlspaces)

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