Rim joist condensation in winter – GreenBuildingAdvisor

I’m looking for advice on proper rim joist insulation to help solve a small condensation issue. For reference, this is a 2 story brick veneer home, built in 2000, traditional vented crawlspace, located in central Indiana, climate zone 5A.  The crawlspace is approximately 2000 square feet and 4′ tall with 2′ below grade and 2′ above grade. Typical pea gravel with vapor barrier, sump pump in the lowest part of the space, rigid foam on walls, outside grading is good, crawl space floor stays pretty dry. Vents are closed in the winter and open in the summer. I’ve only lived here a few years, and have monitored the crawlspace for the past couple seasons to understand the fluctuations throughout the year. I keep a temp gauge down there. This past winter we had a 2 week stretch of temps in the teens and single digits at night. The temp stayed around 45-50 degrees down there.

Hopefully that’s enough background info. My issue is condensation in the corners of the crawl during those very cold times in January and February. Nearly all the rim joist stays bone dry with no evidence of moisture ever….except for any interior corner. There will be condensation for about a 16-20″ span in the corners, on the rim joist and a little on the underneath of the subfloor. The joists are still very solid, not enough moisture to start any rot…but the subfloor will get a little spongy and soft in those corners…enough to make me think it could be an issue years down the road. My theory is there are cold spots forming in the corners, with the temp and dew point getting just low enough in those isolated spots to start condensation.

Full encapsulation with sealed vents would be ideal…but not in the budget in the near future. I’m considering insulating the rim joist just in the corners for now….maybe insulating about 2′ in each direction of each corner. 2″ rigid foam, such as Owens Corning XPS with spray foam around the edges seems to be the consensus I read here. Is there any reason to put foam on the underneath of the subfloor in those areas or should the insulating board on the rim joist be enough to keep the temp just warm enough to stop condensation? I’m just looking for thoughts and ideas…I’m planning to tackle this project this fall before winter.