Insulating rim joists

My house was built in 1986. The exterior has a 1" sheet of polyisocyanurate foil-faced insulation board that covers the wall and the rim joist, however the rim joist was covered with a separate piece and I don't think that the horizontal joint was taped. From what I have been reading, if you have 2" of polyisocyanurate on the exterior, no further insulation is necessary. The floor around my exterior walls is cold. Since I have only 1" foam on the exterior of the rim joist, how should I insulate the interior of the rim joist? I was thinking of caulking each bay, then 1-1/2" foam board + mineral wool bat. Is the 1" exterior insulation a sufficient air barrier to eliminate the need for more foam board on the inside, so I can just add the mineral wool?

 

No you usually want a minimum of 2" inside the rim joist, more in the frozen tundra. I wouldn't count anything outside the rim joist.

 

Thanks, I'm concerned that having foam on both sides of the rim joist could promote moisture becoming trapped and rotting it. Is this a valid concern?

 

IMO... Not really. The more foam you have the better since it's making a thermal break on either side of the wood which is where most condensation comes from... a big temperature difference from one side of the wall to the other. The foam on the outside is not air tight so it can always dry to the outside. When you air seal the foam on the inside that is your primary barrier to moisture. Your biggest worry would be if rain got behind the foam outside, regardless of whether or not you have foam on the inside or not.

 

Thanks. I feel like this is a real grey area and have found much conflicting information.

 

I prefer if people avoid posting commercial youtube links here. There is no perfect solution that works flawlessly in all seasons. You could put 4" of foam outside and that uninsulated sill plate and foundation are still going to be cold in winter. The little bit of foam he shows on the exterior is inadequate in any northern heating climate. So IMO you still need minimum 2" inside to isolate the cold rim joist for winter months to prevent frost.

 

Thank you for commenting and I understand your reasoning for removing the link.

 

I've weighed all my options and decided that your approach is what i'm going to go with, thank you for your help. I plan on using mineral wool insulation to cover up the 2" foam board to provide fire protection. One aspect I haven't been able to verify is how thick must the mineral wool be to be considered adequate. I would like to take the 3.75" bats and slice them down the middle to save on cost. Would 1-7/8" thickness mineral wool be adequate?