Vapor Barrier Only on Crawlspace Walls


Old 03-15-07, 05:32 AM
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Vapor Barrier Only on Crawlspace Walls

I am trying to reduce humidity in my crawlspace and wanted to get your thoughts on attaching a 6 mil vapor barrier to all of the crawlspace walls. The ground (and the bottom 12 inches of the walls) in the crawlspace is already covered with a vapor barrier and the floor above is insulated with kraft-backed batt insulation betwen the joists. The foundation vents have been sealed. So I am thinking the crawlspace humidity, which can sometimes get up to 65%, is just water vapor coming through the cinderblock walls. There are no leaks or water in the crawlspace, which I inspect frequently. The house is in Virginia where the humidity is generally pretty high.

I don't think painting the walls with waterproofing paint will help so I ruled that out. I don't think I need the additional insulation on the walls so I was leaning against insulating with a vapor barrier, instead just going with the vapor barrier by itself. The only downside I can see is that it will be more difficult for our terminte guy to do his annual inspections of the crawlspace, but I don't think that's a show stopper.

Your thoughts please.
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Old 03-15-07, 05:48 AM
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Putting poly on the floor and up walls is fine, but unless your sill plate is pressure treated, you don't want to attach it to the wood on top of your foundation. To do so would subject the wood to the wet conditions that exist behind the vapor barrier (when the conditions are right water will condense on the back of the plastic.) Instead, brush on some mastic and the plastic will stick into it. If needed, a strip of wood can be applied on top of the plastic with masonry nails or screws.

The Aug 2004 Journal of Light Construction had a short article about removing mold/drying out crawlspaces, and this advice is taken from that article.
Old 03-15-07, 01:11 PM
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What's the humidity upstairs in comparison?
Old 03-17-07, 10:59 AM
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Response about Upstairs Humidity

Generally around 45%. We do have insulation with kraft paper vapor barrier between the crawlspace and the living space floor.


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