I have a mud room/enclosed porch on my house (climate zone 5); it’s about 14 x 6. The two inner sides where it joins the house are on a brick foundation; the two outer sides will have Hardie board put over them once we finish repairs underneath.

For insulating the floor, I want to have the following layers: subfloor, air gap, rockwool, then foil-faced iso facing down, sealed with tape at the seams and spray foam around the edges.

The underside has dry, loose soil under it, and the two outer sides have been open to the air for several months. We haven’t had excessive rain in the area at all. There’s about 16″ of space between the bottom of the joists and the soil.

But, the floorboards and joists are showing between 16% and 22% moisture content. Is the wood absorbing the moisture from the brick, or as it evaporates from the soil? I am leery of air-sealing under the joists if it’s just going to trap the moisture in. But if that’s what will let it dry to the inside, I’ll go ahead and do it. I just need to know which course to take.