Moisture tends to condense on subfloors that are cooled by air conditioning in summer. Insulation can actually make the problem worse by keeping the subfloor cool. Although research is needed, there are several possible approaches to reduce condensation on the subflooring:
1. Make sure rain drains away from the house on all sides. Your yard should have a 5% slope all around the house.
2. Make sure the ground under the house is higher than surrounding grade.
3. Cover the ground in the crawl space with plastic sheeting (as long as water cannot pool on it – need #1 & #2 first).
4. Insulation Options:
• If you want to insulate with batts (lowest cost option, but risky in air conditioned homes with crawl spaces – moisture problems are common), the kraft paper should be up and the batts held in full contact with the subflooring – with NO sagging anywhere. Not easy. More doable if using batts that fill the joist space.
• High density closed cell spray foam with a low perm rating is a good solution. There’s not enough research yet, but moisture models, building scientists and others are using it with good results. It’s the most expensive option.
• Low-medium density open cell spray foam with a vapor barrier paint skin applied after it cures is another option. It’s not well researched yet, but is being used and modeled successfully. It is lower cost than closed cell foam, and has the plus of flexing with the building, but can be tricky to get good paint skin coverage.
• Foil faced rigid foam insulation (2 in. thick) applied beneath and spanning the floor joists, held on with cap nails and seams taped, edges sealed with a foam sealant – is a tested good solution. It should be sealed airtight. Code officials may require a type of foam that meets fire resistance standard, or disallow it without additional batt insulation agains the subfloor; interpretations vary. Cost is in the same ballpark of spray foam, depending on installation ease. Not feasible when crawlspace is low.
• With any of the above, it’s best to protect the insulation with something like fiber cement board, hardware cloth, or other sturdy material to keep critters out.
• A different approach is to insulate on top of the subfloor with rigid XPS foamboards, use spray foam between studs of walls and partitions. Don’t know of anyone whose done it, but it’s theoretically an effective solution to avoid moisture problems, and would provide comfortable, resilient floors.