Encapsulating a Crawl Space

by Chris
(Indianapolis)

I am thinking of sealing my crawl space, according to the information in your Crawl space heater article.

In that article it states to not use a vapor barrier. If I seal my crawl space per this advice, will I have to remove the fiberglass insulation already installed in the floor joists of the floor above? It is R-19 and has the PAPER VAPOR BARRIER already attached to the pink fiberglass insulation, and is facing up against the floor above.

Answer from Green Energy Efficient Homes

It is generally not a good idea to have a vapor barrier on both sides of an insulated wall, floor or ceiling, because moisture that gets trapped in the cavity has no way of escaping. So if you were starting with an uninsulated floor upstairs I would suggest, as stated in the article, not to add a vapor barrier.

However, given that you already have the vapor barrier installed above the insulation, and that the vapor barrier below the insulation will be on the bottom and walls of the crawl space, not immediately under the insulation, I don't think you should have any problem with leaving the insulation as is. It's unlikely you will get much moisture build-up inside the insulation, and the floor can vent any water vapor build-up downwards into the crawl space below. While this will potentially increase the humidity in your crawl space (especially since it is sealed) it is unlikely there will be much humidity build up. I would just recommend checking the humidity in the crawl space periodically with a humidistat - perhaps a remote probe one, if such can be had - especially in the first year. My hunch is you won't have a problem - but it's worth double-checking.

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Encapsulating a Crawl Space

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Sep 06, 2010
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Thanks for the advice. Also.
by: Chris

I am also in the process of removing the carpet on the floor above the crawl space and replacing it with a laminate wood floor. Should I put down a visqueen vapor barrier as recommended by the flooring company, or NOT as you suggested in your reply to my question. Thanks, Chris D.

Answer from Green Energy Efficient Homes: I'm not sure where you have vapor barriers at present. You definitely do not want a vapor barrier on BOTH sides of insulation. I recently had a new addition put on my house and the builder put spray-foam on the underside of both the roof sheathing and the subfloor, then insulated below each with batt insulation. The building inspector wanted him to then put vapor barrier below the ceiling batts because he didn't think the spray foam counted as a vapor barrier (it does, but building inspectors sometimes take a long time to adjust to new realities). He also wanted vapor barrier on the subfloor before the oak floor was laid down. In both cases my builder was convinced this was the wrong thing to do - for the ceiling, doing so encloses the insulation in a vapor barrier on both sides, giving nowhere for any introduced moisture to escape, and in the floor, doing so encloses the subfloor in a vapor barrier, making it unable to dry out should it somehow get wet, which will weaken the subfloor.

That being said, this is just my in-expert opinion based on what my builder tells me and without my really knowing what all the layers of your construction are from ground to above the floor. Your flooring company may understand your particular situation better.

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