Insulation advice varies by climate

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

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