Flash and batt insulation

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Flash and batt insulation

I have a 100+ year-old home in climate zone 6A (Wisconsin) that needs new sheathing and siding, so I figured I may as well reinsulate the wall cavity as I pull and replace the sheathing (currently has R13 batt in a 2x4 wall cavity). After considering a variety of options, I keep coming back to flash and batt and wonder if this will work for me. Most (all?) flash and batt applications are applied from the interior--that is, 1-2" of closed-cell spray foam is applied to the exterior sheathing, batt insulation is installed, and drywall is put up. I will need to do the opposite--2" of spray foam applied to the drywall side of the cavity, then batt insulation, then plywood sheathing, WRB, 1/4" vented drainage plane, then clapboard siding.

The more I read about water vapor buildup, the more confused I get, but it seems to me that in my climate applying the 2" of spray foam to the drywall side, where it will act as an air and class-II water barrier (since it has low permeability) is the proper practice anyway. Is this correct? The batt insulation and the vented plywood would be the more vapor permeable side, but this is still not the most breathable of buildups. Am I going to create a mold/rot issue?

Also, any concerns about the drywall cracking and popping from applying 2" of closed cell foam to it?

Thanks for the help. This has been stumping me for a couple months, and I'd like to get a move on with the project before it gets too cold.

Asked by Eric Rinehardt
Posted Fri, 07/27/2012 - 12:32
Edited Fri, 12/21/2012 - 14:01