Is an exterior vapor barrier ok on the exterior in a mixed climate?
We are building in Charlotte, NC – mixed/humid climate. We learned long ago to stop installing a poly vapor barrier on the inside of the wall over our insulation – we have moved to kraft faced batts. Have also read to not have any vapor barrier on the wall in our climate whatsoever(interior nor exterior) so we have always used 15# felt on our exterior sheathing (typically OSB).
We are building a LEED-H platinum house – exterior wall is a 2×6 with 1″ DOW SIS – we are using climate pro blown in fiberglass (R 4.2/in.) for a total R28 wall. While reading the installation manual for the DOW product, we learned that it is indeed a vapor barrier so was mentally scrolling through all I have read about not installing a vapor barrier in our climate. I am thinking with our upgraded wall assembly we are going to be ok but want to double check myself.
During the summer we would be concerned with vapor diffusion through the brick veneer – with the increased insulation in the wall and the vapor barrier – i do not think any condensation can occur and the vapor should not pass to the interior – so we should be ok in the summer. My next thought would be – what about the winter – warm air trying to get to the exterior – I am assuming the 1″ SIS will have enough R value to avoid condensation from vapor diffusion coming out from the interior. Only time we should be getting any moisture in the wall will be if we somehow have a positive pressure inside the house but the wall should always be able to dry to the inside.
Am I missing anything? Always worried about trying something new in our climate. Thanks.