I'm constructing a small house in southern Missouri, climate zone 4. My question about vapor barriers is multifold:
- Should I install a vapor barrier at all?
- Should it be on the interior (warm-in-winter) or exterior (warm-in summer) side?
- What permeability/material is advisable?
As highlighted in existing answers on this site, this depends on where I am - however, having researched the recommended approach in my climate area, authoritative sources are giving me directly contradictory information, and I'm not sure which one to trust - the MO DoE, US DoE or the University of Missouri?
The University of Missouri extension and the MO Department of Energy say I should install one, and that it should be on the warm-in-winter side:
Vapor barriers are installed over the face of the studs or joists on the side closest to the inside surface of the home.
A vapor barrier should be placed on the "warm-in-winter" side of the insulation.
The Craftsman blog says I should install one, and that it should be on the exterior, warm-in-summer side:
if you live in a hot climate like I do here in Florida the vapor barrier should be on the outside of the wall assembly
And, finally, the US Dept. of Energy says I should not install any vapor barrier at all when building in Missouri:
Building scientists generally do not recommend putting a vapor retarder in walls in the mixed-humid climate. In the mixed-humid climate, walls should be able to dry to both the interior and exterior.