I have to post a bit of an update: After thorough examination, I've discovered that the back side of the house does not extend out beyond the block wall, so a shallow rim joist cavity. Also, the outside walls all have exterior rigid insulation, including over the rim joists, so according to building science, I don't need to put XPS on the inside of it. However, the front of the house extends out past the sill plate, and another board is attached to the bottom of the joists outside, but with a 2" gap from the sill plate. There is aluminum soffit on it, which I can touch through the gap from the inside, by reaching under a tar paper of sorts, which is on top of the sill. Lots of air leakage in these cavities. Also, the sills have a woolly type gasket under them, but still have air leakage, so I will seal those up. With that added, I would imagine that I need to cut XPS to fit the bottom of these cavities for sure, and seal them up. But, do I still need the ones on the rim joist face, even though the outside is insulated? Also, I still have that wall with the inaccessible cavities, filled with fiberglass. This wall does extend out from the block wall, and also is insulated on the exterior. I can't feel near the draft I could from the front wall, except a bit from under the sill in spots, which I can squeeze a great stuff straw into. Building science shows that I should insulate the top of the block wall that is exposed, but if I were to do this on this particular wall, it would fit tight up against the last inside joist I mentioned above, effectively blocking the entire wall worth of rim joist space from the interior air. Would this not create a big problem? I am a bit puzzled on how I should deal with this area. Joe

I understand what they are saying, but...I have done a lot of research on this and including on these forums rigid foam seems to be the way people are going with this. You can check out some great videos in you tube also. Can we know for sure our basements have no humidity or moisture issues? I know I have to run a dehumidifier in mine in the summer, but over all it is pretty dry. The other issue some folks have raised is even with the rigid foam you have to make sure it is pretty tight so that moisture does not build up behind it and cause rotting of the rim joists. So much to think about. That is why I may just go with the spray foam despite the cost. I have some pretty big holes where the vents are going through the floor. I can feel the cold on the floor in my living room and dinning room. If you google rim joist insulation you will find a lot of information and videos.

my home is 24 years old. We do not have roxul around here so far as I know...PA, USA......I had fg in there for 5 years....crap! I can caulk w/fp caulk...big gaps though....might cost big $$$$$ The fp was wood burning...we put a vent free log set in. If you leave the flue closed it get warns the first floor great but it smells and then the stat never goes off so the upstairs gets cold...we knew all this going in...usually use it with the flew open a bit...adds some warmth but mostly used for ambience... See picture looking up...any other thoughts....

The rim joists are already insulated with spray foam. My issue is directly under the porch, because the basement/garage go all the way to the front of the porch underneath. It's appx. 105 sq ft. of space that not insulated.