A GBA reader who calls himself “Mr. Mike” is working on an 11-ft. by 14-ft. addition to his house in central New York that sits some 5 feet off the ground. The space beneath the addition is a great place to park a lawnmower, but it’s also open to the cold.
The floor joists are supported by two beams, each a tripled 2×10. To insulate the floor, Mr. Mike put two layers of R-13 unfaced fiberglass batts in the joist cavities, ran a 3/4-in. layer of Cellofoam foil-faced rigid insulation over the bottoms of the joists, then covered the insulation with 1/2-in. oriented strand board (OSB) to keep out pests. The subflooring is 3/4-in. tongue-and-groove plywood.
All seemed to be well until Mr. Mike got ready to install strip flooring in the addition.
“When I pulled the existing carpet up to redo the floor, I made a couple of access holes to inspect the cavity and found signs of frost and condensation on the foam insulation — but only in certain areas, closer to where the addition is attached to the house,” Mr. Mike writes in a post at Green Building Advisor’s Q&A forum.
The flooring manufacturer has recommended a layer of #15 asphalt felt between the strip flooring and the subfloor, but Mr. Mike is concerned this may trap moisture in the floor system, making the situation worse than it already is.
Should he remove the foam and re-install the OSB? Add more foam to the bottom of the floor system? Insulate the tops of the 2×10 beams with foam?
Those questions are the start of this installment of Q&A Spotlight.
Make the foam thicker and seal the air leaks
To David Meiland, this problem seems to be caused either by the insufficient R-value of the rigid foam…