I am currently planning on insulating my basement and after doing some research and looking at cost I decided for the walls I was going to use 2" foam board then frame 2X4 walls and insulate with Roxul. However, the box sill is where I need help. I was planning on getting the box sills spray foamed, but I was wondering if this was a good idea in my case.
My house was built in the 1940's and the sill sits directly on top of the concrete walls. Being an older house there is obviously no foam sill gasket between the wood and concrete and the wood is untreated. So, if I seal with closed cell spray foam will the sill prematurely rot because of it's inability to dry out, since I would assume the sill is absorbing some moisture from the concrete even though the concrete appears dry and is roughly two feet above grade. On the outside the sill is only partially exposed at the basement windows and sits behind cedar shingles, foam, and vinyl siding everywhere else.
If spray foam is not a good idea and I have to use rigid foam cut to fit is the foam placed directly against the sill with roxul in front on the warmside or should the roxul be behind the foam on the cold side and touching the sill? Or any other options?
I look forward to your help. Thanks.