I just found frost and water condensation on my rim joist. I live in Montana in a 30 year old home. Is their any thing I can do at this time or do I have to what tell summer to fix the problem as the rim and sub floor are wet? I would like to spray foam the area to fix the fiberglass insulate problem but how dry does it have to be to it? Please help me I don't want my house to fall down. Thanks

Is this in a crawl space or basement? Obviously you are reaching the dew point and the water vapor is condensing and freezing. In a crawl space, make sure to cover the ground with 6 mil polyethylene and keep the vents open. In a basement, run a dehumidifier. Is the floor insulated? Many state energy codes require that the rim joist be insulated with at least R-13 insulation. In a basement, you could also put a small space heater, electric, not gas, to help dry it out. After the wood is reasonably dry, install fiberglass but remember that the space needs to be kept as dry as possible or you will make the situation worse by having wet insulation in contact with the wood. Not a fun job in the winter!