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, which is a no go. So what are my options? I could of course do nothing, which dramatically simplifies things. 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. If I only needed an inch or maybe 1.5 of XPS or EPS, that might be permeable enough, but in my zone 5/6 area with 2x6 walls I need 2.5 inches of XPS to avoid condensation, which is then basically impermeable. This seems like too good an opportunity to miss, but that interior kraft facing is really a problem. Thoughts? Thanks, Kyle

No. With R19 on the exterior, the risk of condensation within the interior stud cavity is basically zero. On the other hand, if that space somehow gets wet, the chance of drying out is pretty much the same - zero. So be vary careful of your flashings, make certain there are no routes for water to enter and install your foam insulation.

Thanks Bob,

I plan on adding a WRB outside of the foam, so should I remove the existing WRB that is on top of the current sheathing?

Another random thought, what if I drilled 10 or so 1/8 inch holes throught the drywall (and kraft paper) in each stud bay? Very 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 as you said, with R19 I should have no condensation.

Thanks again,

Kyle