Q: I am evaluating the most feasible method to insulate below the wood floor of a post-WW II house with a crawl space. I have approximately 16 inches of clearance between the bottom of the floor joists and the ground.
A: The preferred method to insulate a subfloor is to use unfaced batts. You can use faced batts, but they don't have rigid ends and are harder to keep up. Plus, you really don't have a vapor barrier (which, yes, does face the heated side) because a true vapor barrier maintains its integrity -- in other words it has no breaks.
If you have 16 or 24 inch on-centers, the professionals use something called lightning rods, which are rods about the thickness of a metal coat hanger with pointed ends about 2 inches longer than the interior joist space. You can stick these up an unfaced batt and then install a lightning rod every 18 inches or so to hold up the insulation. This is a fast and easy installation.
You could also nail plywood or lattice boards to the bottom of the floor joist, but this is a lot more expensive than lightning rods. Some people use netting, but that is a lot more work.
Using rigid board is an unnecessary expense and will not give you that much greater R-value.
If you decide to hire an insulation expert to install that for you, visit Service Magic, submit a service request and be matched with the ideal service professional in your area.
by Kathy Maynard, reprinted courtesy of Service Magic
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