Q: My husband is building an addition to our home in the San Francisco Bay Area and was told by the city building inspector that he installed the flooring insulation incorrectly.

We installed the paper side facing the soil; he wants my husband to either remove the paper vapor barrier or turn the insulation around so the vapor barrier is against the floor. To do this we have to remove the netting that supports the insulation and reverse the insulation batts.

From your article, it appears that moisture comes from the ground, not the building, and we installed the insulation correctly. Can you give us your opinion?

A: Unfortunately, we agree with the building inspector. In a temperate climate such as the San Francisco Bay Area, the paper or foil-faced side of fiberglass insulation is always turned to the heated/cooled interior portion of the structure.

You are correct saying that moisture migrates from the soil into the crawl space. We see how you could reasonably assume that the first line of defense would be to put the vapor retarder toward the moisture. But in this case, rational is wrong.

By placing the paper toward the crawl space, you’ve created a situation where condensation is likely to form in the fiberglass batts between the subfloor and the crawl space. Eventually, this will create problems. Probably the best you can hope for is some mold forming, but the worst is a severe moisture condition allowing fungus and dry rot to flourish. The only option we see is for you to redo the job.

We disagree with the inspector that a possible solution is to remove the paper from the insulation. Although this might pass muster for code purposes, it’s next to impossible to do and maintain the integrity of the insulation. The best solution is to remove the netting and insulation you’ve installed and reinstall it correctly with the paper facing toward the interior of the addition. It really shouldn’t be too tough and won’t require that you mess with the floor. It will require some time under the house, which is never fun. …CONTINUED

A do-over can be a pain in the neck, but this one is necessary.

Q: I have a 2,600-square-foot rancher whose previous owner never put down a vapor barrier, but there are 6 to 8 inches of gravel in the crawl space. I am looking for some help on how to get a vapor barrier put down.

One company says it can spray foam under the floor in the crawl space, which will expand about 3 inches on the floor joists and will both insulate and create a vapor barrier. Do you think this spray foam is OK? Will it work?

A: Given your situation, expanding foam insulation is the best solution. Remember that a vapor "barrier" placed on the dirt in a crawl space is really a vapor retardant.

Installing a polyethylene sheet on the ground is an attempt to reduce moisture migration into the crawl space and ultimately into the house. We don’t quite understand why the previous owner dumped 8 inches of gravel under the house, but we’re sure it does little if anything to mitigate moisture penetration into the crawl space.

Because expanding foam is impervious to air flow and water, it will provide both insulation and a vapor retarder. The one caveat we have is to make sure that the crawl space has a sufficient number of foundation vents to move the moist air from under the house to the outside. The formula is a minimum of 1 square foot of foundation vent for each 150 square feet of crawl space.


What’s your opinion? Leave your comments below or send a letter to the editor. To contact the writer, click the byline at the top of the story.

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