Insulating rim joists
Hello. I’ve been reading q&a on this website for a long time and have always found the content very helpful. This is my first time posting a question and hope someone has some information for me.
I’m finishing my basement and planning the insulation portion. My basement is block walls and in the rim spot where a rim joist would normally be it is also a cement block. The house is well built and is of double brick construction ( brick wall on the exterior a small gap and an inner cinder block wall. ) the joists are resting on the basement block wall and they are about 2 feet above the outside grade. After a great deal of research specific to my location i have decided on the following approach for the basement walls: roxul board (r6 I believe) fastened up to the block walls.. A 2×4 wall built up against the roxul and the cavities insulated wth r14 roxul. Then a vapour barrier on the interior. Specifically a smart barrier which apparently allows drying to the interior while not allowing vapour to penetrate the walls from the interior. For the rims I plan to do 2″ rigid insulation board with “great stuff” foam sealing it in place around the edges. Here is my question: the normal rim joist bays are simple, however on 2 sides of the basement where the joists run parallel with the block wall I can’t insulate with the rigid in the same manner. The last joist is only about 1″ max away from the block wall and I have no idea what’s behind that joist. I considered simply installing the rigid on the inside of that final joist effectively treating it like a rim joist but I underdstnd that would be leaving the joist unheated and would become somewhat of an exterior joist and exposed to the cold. I know some would say spray foam up in the 1″ gap but I am avoiding spray foam at all costs.
I Know this was a lengthy post and I apoligize but I’ve been toying with this for a long time and wanted to properly explain it.
Thanks in advance. If necessary I will submit a photo.
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