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Add exterior foam — but paper-faced batts?

Kyle Hollasch | Posted in Green Building Techniques on

I am about to replace the windows on one side of my house, which for various reasons will require removing my existing siding. I also have in my possession a whole bunch of 3 inch foil faced polyiso foam board, the perfect opportunity to add an R19 layer of foam on top of my existing 2 x 6 walls with fiberglass batts. Problem is, I’m sure the batts have kraft facers on the interior. So I’d be creating a vapor barrior sandwich.

So what are my options? Here are 5 that come to mind,

1 – I could of course do nothing, which dramatically simplifies things.

2 – I could remove the sheathing, cut out the kraft paper, put the batts back in, and then add the foamboard, but boy that’s a ton of work.

3- If I only needed an inch or maybe 1.5 of XPS or EPS, that might be just permeable enough, but in my zone 5/6 area with 2×6 walls I would need 2.5 inches of XPS to avoid condensation, which is then basically impermeable.

4 – Just put up the foam, make sure its flashed well with a WRB, and hope I don’t get any water infiltration.

5 – What if I drilled a number of 1/8 inch holes from the interior, through the drywall (and kraft paper) in each stud bay? Easy to patch, and I’ll be painting anyways. That should add some permeability just in case water ever gets in there. Of course, more vapor will get in there, but with R19 foam on the exterior I should have no condensation.

Thoughts?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Kyle,
    Don't worry so much. The kraft facing is a so-called "smart" retarder with variable permeance. It won't trap enough moisture to cause any problems.

  2. Expert Member
    Dana Dorsett | | #2

    When it's bone-dry a kraft facer runs about 0.4-0.5 perms, but well before there is enough moisture in the cavity to cause an issue it's permeance rises to something north of 5 perms, at which point the drying rate is limited by the vapor retardency of the interior paint. A couple layers of latex paint is typically 3-5 perms.

    With 3" of polyiso on the exterior of 2x6 /R19 in a zone 6 location the average temp of the polyiso during the winter months pushes it into a lower-performance range, bringing it's average performance down to something like R14, maybe even R12 during the coldest weather, but that's still more than sufficient for dew point control. During the shoulder seasons (or even warmer afternoons in mid-winter) it's performance rises into the R16+ range. At 3" you're good, but at 2" it's somewhat marginal, despite the labeled R value(which is it's performance when the mean temp through the foam is 75F, not 25F.)

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