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Reading around, it seems like most people suggest to put up XPS insulation when finishing a basement. That's great and all, but how do you put 2" of rigid foam behind pipes that already exist in the ground, sometimes as close as 0.5" or less to the foundation wall?

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Cut the foam into manageable pieces and slide into place. Use cans of spray foam insulation to fill the cracks in the sheets of foam caused by cutting.

Or spray foam the entire cavity and forget the rigid foam.

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So how does it work if the basement is supposed to have R10/13 insulation but you can only fit < 0.5" behind the pipe? That's R3 at best. – 2 Left Thumbs Jul 22 '14 at 16:25
I'm sorry, you didn't ask that question. You asked about putting foam behind the pipes. If you want to bring your insulation into code compliance, I suppose you could bump your basement wall out 6", or relocate the pipes. – The Evil Greebo Jul 22 '14 at 16:27

Your inspector should be able to grant a waiver for existing conditions if code compliance is a concern. If that is the case, I would wait and ask him/her for a solution that is acceptable. Hopefully something a bit shy of bumping the basement wall out 6", LOL.

You could put the 1/2" foam behind, than cover the pipes with split foam pipe insulation. I try to stay away from the spray foam (messy, smelly, cant see condition of pipes, etc.)

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