I'm looking at the insulation in a crawl space below the first floor of my home. Joists are I-beams (12" deep, model TJI 350), 12" on center. Insulation is R19 fiberglass bats, tight-fitting side-to-side, but sitting on top of twine at the bottom of the I-beams. There is several inches of air -- 6"? -- above the top of the insulation, below the subfloor. That's not standard -- best practice seems to be insulation flush with the subfloor. So I'm thinking about what to do about this.

I have two basic ideas. One, there's room to install much more insulation, probably doubling the amount in this area, via another layer of R19 batts right below the current batts. Two, I could install metal insulation clips to push the insulation flush with subfloor. The insulation clip cost and installation time is not trivial, and more insulation wouldn't necessarily cost that much more or be that much more trouble. Finally, it's perfectly easy to do nothing.

How to think about the upside? How bad is it to have a large air pocket between the insulation and the subfloor?

2003 home in greater Seattle.

1 Answer 1


Leave it alone.

There's nothing wrong with this, especially in a subfloor.

More air is more insulation, and in this case the stratification means here's not going to be much in the way of convection currents to mess it up.

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