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Can sprayfoam cause premature rotting of basement sill?


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.

Timothy Miller
Re: Can sprayfoam cause premature rotting of basement sill?

Howdy the spray foam open or closed cell will not rot anything.... moisture and fungus cause rot. Are the sills above grad 6 " * is better.
Consider treating the wood with TIMBORE an be worry free

Re: Can sprayfoam cause premature rotting of basement sill?

There could be moisture in the concrete. Capillary action can bring water quite high in the wall. It is possible for the spray foam to prevent the top of the wall from drying if it is enclosed with spray foam. Builders in the know (hardly anyone) would put a waterproof barrier on top of the wall. Check for moisture. Is there sill seal on the top of the wall? Might be enough.

Why Roxul in the wall? Roxul is the way to go if on the exterior. On the interior I would stud the wall out and spray with closed cell foam. The Roxul will have gaps between pieces and allow air movement

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