Vapor Barrier in Crawlspace

yoyomaSeptember 30, 2008

I'm currently remodeling half my house and the floors and sub floor has been removed to reveal the crawlspace. There was a torn, old plastic vapor barrier on the ground and I asked the contractor if he could remove it and replace it with a new one. He said ok, but when I checked back, they showed me how the spots that were not covered were bone dry and the spots that were covered were quite wet, almost muddy, even though it hasn't rained in months. And so they recommend that a vapor barrier not be installed because they feel it will trap the moisture and create that muddy ground. They also said they never see vapor barriers in crawl spaces on jobs they work on. I'm in Nor Cal, in a moderate climate.

Does this make sense? Thanks.

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The reason the ground under the vapor barrier was damp was that it impeded the rising soil moisture at the soil level and kept it out of your house. In the areas where the VB was torn or nonexistent, the soil moisture rose up and entered the air in the crawl space under your house and either exited through your foundation walls, or continued on up and entered your living space.

So where do you want to keep the moisture? Ideally you don't want it trapped in your crawl space (hard on all the structural parts down there) nor do you want it entering your living space creating moisture and dampness around your stuff.

Even if you live in desert, there is constant rising soil moisture; it's just a question of how you want to manage it. In addition to actually blocking it at the soil level (and it is somewhat redirected when it hits an impermeable barrier), another technique is to actively, and constantly vent the air in your crawl space to carry the moisture on out. This technique carries with it some additional issues. And costs since it usually relies on constant use of small fans and electric vent dampers - though I think you could probably set up some non-electric devices relying on changes in wind direction, or air pressure.

Personally, in my old house, I opt for carefully-installed vapor barrier films in my crawl spaces. And since you're dealing with a crawl space that sounds like it is not normally accessible, I would use specifically designed VB material, not just construction poly which as you can tell doesn't last all that well.

In fairness, I should point out that there some conflicting opinions about managing crawl space vapor issues in old dwellings (mostly related to passive or forced ventilation systems, but also to just laying film on the surface of the soil. I suggest you do some casting about on the net to read the various theories. You might also chat with your local municipal building or code officials. They will know of any local reason why VBs are not the thing to use.



    Bookmark   October 1, 2008 at 9:17PM
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Thanks for the info. It turns out that the contractor's workers at the house recommended this. The next day when I came back they were laying a VB. I asked them what happened and they said the GC told them to lay it in order to protect the hardwood that will be installed. I asked them how thick it was and they said 4 mil. Having researched a bit online I told that it would have been better to use 6 mil, but they said that 4 mil is plenty thick, and many others use just painters plastic which is just .350 mil or something like that. They already were 90% done so that's that. They also didn't go up 6 inches the walls, and stapled the VB, and they didn't want to seal the seams until I insisted, but it's better than nothing.

I found a bit of wood debris and nails in the 10% they didn't cover yet, but said they raked. I guess you can't get everything up.

    Bookmark   October 1, 2008 at 9:51PM
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Venting the crawl space would be a good idea if you can do that. Since you're in the middle of a renovation, it might be worth considering.

    Bookmark   October 1, 2008 at 10:29PM
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It is already vented properly so that's good. Interestingly, based on what I've been researching online, it seems that not venting outside is recommended these days and instead use a crawlspace dehumidifer.

    Bookmark   October 2, 2008 at 12:02AM
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4 mil is as good as 6 mil except it tears more easily. Any sheet plastic vapor retarder should be protected by another layer like smooth surface roll roofing if people are going to be crawling around on it.

    Bookmark   October 2, 2008 at 1:15PM
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