0 Helpful?

Will moisture collect behind poly and foam on crawl space walls?

I recently installed a vapor barrier -- 6 mil plastic or maybe 3 mil -- over an entire dirt-floor crawl space and up to the sill plates. We also placed foam board over all exterior walls of the space.

Although this is done to keep moisture out of the space, I worry that moisture will collect behind the foam board and become undetected and create more problems.

Asked by Anonymous
Posted Aug 8, 2010 5:24 PM ET
Edited Aug 9, 2010 5:13 AM ET


6 Answers

Select your preferred way to display the comments and click "Save settings" to activate your changes.

Are the walls concrete, wood, or both? What makes you think moisture is coming thru them? Where is the house located? What else is in the crawl space?

Answered by David Meiland
Posted Aug 8, 2010 5:32 PM ET


Foam board won't keep moisture out of the crawlspace, but it will insulate the walls, keeping the space semi-conditioned (assuming it's not vented) and that will bring the temperature closer to the mold zone.

Answered by Riversong
Posted Aug 8, 2010 8:19 PM ET


Moisture won't "collect" behind the poly and foam. But you are right that the environment behind the poly will be more damp than the crawl space air. That's what you want.

Some builders used to install clear poly on crawl space floors. If homeowners saw water droplets under the poly, they would occasionally freak out. The solution is simple: switch to black poly, so the homeowners can't see what's going on.

We have all kinds of barriers in our homes, including air barriers and moisture barriers. The idea is to separate the indoor air (which should be at a comfortable temperature, and dry) from the exterior conditions (which are often very cold, or very hot, and wet).

You're worried about the conditions on the outside of the barriers you install. Don't worry. Those conditions are different from the interior air -- that's why you installed barriers. You want the interior air to be good.

Concrete walls are not harmed by moisture.

Answered by Martin Holladay
Posted Aug 9, 2010 5:25 AM ET


I like using white boat shrink wrap for under camp conversions I have done. White is nice for being able to see in these spaces.

This film is a heavy duty shrink polyethylene available stock in BLUE, WHITE, or CLEAR in 6 MIL and 7 MIL, from 12 feet wide to 36 feet wide. This film is commonly used to shrink wrap boats during the winter in cold weather areas. Other applications would be any large item that would be exposed to the weather in warm or cold conditions. All films have UVI protection (ultraviolet inhibitor) to prevent the polyethylene from breaking down from exposure to ultraviolet rays. It also has EVA (Ethyl Vinyl Acetate) added to the film to keep the polyethylene soft and flexible in sub -freezing temperatures. Shrink wrap tape is available for seaming sheeting together. Boat Shrink Wrap is applied by draping the sheet over the boat and by applying heat with a hand held propane powered heat gun. This film is a one use application. See BoatShrink.com.

Answered by aj builder, Upstate NY Zone 6a
Posted Aug 9, 2010 10:13 AM ET


Martin - interesting point you bring up with black poly. We have been using black poly for years but just last week made the decision to start using clear (in retrofit situations anyway). We somewhat recently just starting continuing our vapor barriers up the foundation walls (vs. stopping them where the soil meets the foundation) and we leave a 3" inspection gap at the top for our termite inspection gap. We thought going to clear on these areas would help us identify if excess moisture is getting through the foundation wall as our typical retrofit does not always involve installing exterior waterproofing and drain if there is currently not one. We also thought that the clear may look better and cleaner to a homeowner - when you look into a crawl space with black poly - you cannot really see anything - just looks black (duh) - we thought the clear may reflect some light better and brighten up a dark crawl. We are also looking into the white, we think that would look best but that seems much more expenisive through our surrent sources. All of our retrofits are on vented crawls built in the 50s and 60s - not all have problems and not all owners want to close them up so we recommend at the minumum to install a 100% vapor barrier - in these cases, we also liked the black better than clear as it did not allow weeds to grow near the vents where sunlight comes in.

We may need to reconsider changing - any other thoughts as to why black is better.

Answered by Danny Kelly
Posted Aug 10, 2010 12:07 AM ET


any other thoughts as to why black is better

Because Black is beautiful?

Actually, in addition to creating a moisture barrier, it's valuable to radiantly decouple the floor framing (and insulation) from the ground temperature, which is a major cause of condensation and mold growth in a crawlspace.

Short of a radiant barrier, a white groundcloth would perform this task far better than black, which is highly absorptive and emissive of infrared.

Answered by Riversong
Posted Aug 10, 2010 12:40 AM ET

Other Questions in Green building techniques

Conventional heat pumps with mini-split low temp efficiency?

In Mechanicals | Asked by Mai Tai | Jun 22, 18

How to keep pine siding from greying

In General questions | Asked by tech1234 | Jun 23, 18

Natural Gas wall furnaces

In Energy efficiency and durability | Asked by frasca | Jun 24, 18

Unvented roof — vaulted ceilings — moisture, rot, etc.

In General questions | Asked by Meyersrl | Jun 22, 18

Air Barrier Location

In Energy efficiency and durability | Asked by Dan C | Aug 6, 17
Register for a free account and join the conversation

Get a free account and join the conversation!
Become a GBA PRO!