Planning for vapor barrier in crawl space
Warlicki
| Posted in General Questions on
I am planning on insulating a crawl space. It’s not conditioned but is connected to a conditioned full basement. I’m in Climate Zone 6A (Madison WI).
There is a vapor barrier on the floor of the crawl space and the flooring installed above the crawl space has a vapor barrier under that it, per the installation instructions for that product.
I’m concerned about how to insulate the floor joists because I don’t want to leave fiberglass or spray foam exposed, but sandwiching it between two vapor barriers (one existing under the floor and one under the insulation) seems like a recipe for condensation.
How would you approach this, or is it just not possible to insulate the floor joists?
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Replies
Insulate the walls of the crawlspace, not the underside of the joists. If the crawlspace walls go down to or below the level of the frost line, insulating the walls alone should be sufficient. If the walls are short and don't go down to the frostline, then an insulating "skirt" that extends from the wall out along the floor for a bit can help.
I like to use polyiso for this application, but any rigid foam will work. Check if you need a thermal/ignition barrier over the foam in your area. Sometimes fire retardant paint is enough, but your local building department people can tell you what they require. There are polyiso variants like Dow's Thermax that are rated to be left exposed if you want to simplify your installation a bit.
Since you said your crawlspace is attached to a conditioned basement, insulating the perimeter of the crawlspace to bring the crawlspace into the conditioned space is the only option I would consider here.
Bill