Greetings all,

First post by a longtime Taunton fan.  I subscribed to Fine Homebuilding and Fine Woodworking in the 70’s.  Helped get me started on the right path.  I took a detour into an unrelated career but always kept my fingers in the trade by remodeling and maintaining my own homes.

I’m adding a master suite over my garage.  It’s framed with zip siding.  I want to add foam board on the outside, and the gable is already trimmed to allow for 1″ foam.  I’ve since read on this site that I need at least R7.5 to keep the plywood above the dew point.

However, I recently saw on This Old House that they were using 1″ foam and spray foam inside with 2×6 walls here in zone 5.  I have 2×6 walls.  I’m hoping I can get away with this thinking that This Old House generally keeps on top of building science.

If I use closed cell spray foam with limited permeability can I get away this?  I’m in SE Mass which seems to be creeping towards zone 4.

Thanks in advance,
Scott