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Old 12-04-2014, 12:09 PM   #1
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Crawlspace Insulation with Rigid Foam Board

While I've been working in the main basement insulating the rim joists, see here,

I've been thinking about tackling the crawlspaces as well, air sealing especially as the one room that gets the most wind is usually very cold.

We live in Climate Zone 5, usually humid in the summers, and cold in the winters.

The one crawlspace is 18' wide x 30' deep. The height is about 4-5 feet, although where it meets the basement interior wall, it is 8' (won't be insulating that wall) and has batt insulation in the floor joists. About 15% of the batt insulation is falling down (it was held in place with chicken wire). We also have a bit of a mouse problem and with it falling down, I'd like to just get rid of it and insulate the walls, plus air seal and insulate the rim joists. There is plumbing and HVAC running through the crawlspace.

We currently have a radon mitigation system installed, so there is a plastic barrier on the floor and about 1 foot up the walls. Water issues have been taken care of in the past with outside drains. There are currently two vents (10"x18") that I recently sealed.

I can purchase polyiso rigid foam board with the thermal barrier (John Manville CI Max) locally.

A few questions,
1) Is pulling the batt insulation and going with the rigid foam the best practice? I'm not opposed to spray foam either, but have no idea the cost.
2) Will R13 (2") polyiso be sufficient?
3) Installation instructions say you can use power driven masonry nails with caps to install directly to the foundation wall or add furring strips and screw to the strips. There will be an air gap if I use the furring strips, is that good or bad?
4) How high up should I go with the rigid foam? Even with the top of the foundation wall, or a couple of inches below it?

Thanks for the help and suggestions!
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Old 12-04-2014, 07:42 PM   #2
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Spray foam is the easiest in terms of being able to get it done but will also cost you the most money. Rigid foam works very well. The best thing to do is to convert to a conditioned crawlspace if there is no persistent moisture issue, but you'll need to make sure that the radon mitigation and vapor barrier across the floor is perfect.
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