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Ask Mr. Tight-Watt: Insulating a crawlspace  

Ask Mr. Tight-Watt: Insulating a crawlspace

Find out the best method for sealing and insulating a crawlspace.

Ask Mr. Tight-Watt

Have a crawlspace? This Smart Choices newsletter reader wants to know about recommendations for insulating and covering a crawlspace. Energy-efficient home building expert Bill McAnally provides the science behind the recommendations.

Question: What is the purpose or science behind putting 6 mil plastic on crawlspace walls, then installing rigid insulation over that? Some experts advise running poly 6 inches up the wall, some all the way up to the sill plate. I can understand installing 6 mil plastic across the floor, which some experts also recommend, but don’t see why applying it to the walls is a good idea. What gives? –Steve, a Smart Choices reader

Answer: Bill McAnally provides this response:
Steve has a good question that comes up often. Here’s the science behind it:

First, remember that foam is an insulator that keeps the crawlspace a conditioned space, as long as the crawlspace is air-sealed from the outside. It prevents the cold, which enters your home through exterior walls, dirt floor and rim joist spaces, from cooling the crawlspace.

In addition, because the bottom of a wood floor or bottom of other floor covering in the area above the crawlspace could be a condensation point, insulation can prevent the area above the crawlspace from having moisture issues due to the difference in temperature between a warm floor on the first floor of your home and cool air below that comes from your crawlspace.

However, foam alone will simply slow vapor, not stop it completely. Foam is only a vapor retarder, even if all the seams are sealed.

The exception to this: If you use blown closed cell foam, with a fire retardant sealer applied over the foam (depending on the product) it may be a vapor barrier, and you may not need the poly layer described below.

A plastic, or poly, layer provides a vapor barrier. As a vapor barrier, the poly stops moisture vapor and soil gasses from entering the crawlspace. The plastic layer must be sealed on all seams and perforations and should go up the wall and be sealed to the sill plate. The vapor barrier should be 8-10 mil in thickness.

Do you have a question for Mr. Tight-Watt? Ask your question here.

Find answers to other questions Mr. Tight-Watt (and experts) have answered, here.

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