vapor barrier in crawl space

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Old 02-03-14, 01:59 PM
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vapor barrier in crawl space

I have a crawl space that is part of my basement, meaning it is open to the actual basement. It is dry in there, but cold. In the past I've had problem with water seepage in the main part of the basement. I have it fixed now. My question is if I put a vapor barrier on the floor of the dirt crawl space will I only be creating more of a headache? The barrier will keep moisture from rising, but where is that moisture going to go?? If it is just going to push it down then it will again try to push through the inside wall again. I've read horror stories about walls buckling or cracking.
This past week I've read no less than 200 articles about insulating this space. No 2 agree on a "proper" procedure. The wall is a block wall, the kind with holes, not solid block. I'd like to close those off on the top because I can feel the cold air coming out of them where the sill plate doesn't cover. If I do won't it just push moisture into the sill plate? I have no idea of what to do about the walls or rim joist that would be up to code. When the house was built, nothing was done the way it should have been by today's standards.
Right now I'm thinking of just putting some 6 mil plastic down on the crawl space floor. Nobody goes in there and it's not used for storage. Is that a good idea?
 
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Old 02-03-14, 02:59 PM
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vapor barrier

You may be getting a little confused over issues of liquid water and water vapor.

You mentioned that you had a problem with seepage in the basement in the past. Was the source of the water coming in through the wall that divides the crawl space and basement?

By installing a vapor "barrier" you are slowing the passage of gaseous water into the air of the crawl space. This does not mean that doing so will increase a flow of liquid water to a wall. The flow of water vapor is from a greater concentration to an air source that provides the capacity to absorb moisture. That would typically mean your basement air is warmer and thus can absorb additional vapor from the soil adjacent to your foundation.

Unless you have a membrane on the exterior of the foundation that is designed to prevent vapor passage you WILL be adding vapor to the basement on a perpetual basis. Water vapor molecules are smaller than liquid water molecules and thus can pass through a variety of things that people think of as barriers. This will be happening regardless of the treatment you may put on the inside of the wall with very few exceptions. Even products such as "Drylock" are a water barrier but NOT a vapor "barrier". If your footer is poured directly on soil,(no vap./water barrier below it) it too will act as a pathway for vapor to travel into your home on a constant basis and there isn't much you can do about that.

Your observation about moisture migrating toward the sill is somewhat correct but there are a variety of factors that will determine how the sill will be impacted. Some of the more critical issues are: what is the sill material (press. treat. or standard framing), what is the sill sealer material and what can you add between the sill and top of block such as aluminum or vinyl fascia or coil stock, what is the amount of foundation you have showing above grade to the point of the sill and what is covering the foundation above grade ?

You did not mention if there is insulation or duct work in the crawl space, these can also impact an answer.
 
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Old 02-03-14, 04:12 PM
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Hi Calvert
Let me start with the last part of your answer. There is no insulation of any kind in the crawl space. There is 1 heat duct and 1 cold air return going through that space.

Your right I think a lot of people (myself included) get the 2 confused. I'm learning. I'm going to send you a PM so we can discuss the exterior water issue more.
George
 
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