Architect/builder – GreenBuildingAdvisor

As an architect home builder I have built a few homes with the following roof assemblies on the front range of Colorado……from top down

1. TPO 60 mil fully adhered (glued w/low rise polyurethane) on….

2. 1/2″ Cement board fully adhered (not sure if it was needed for fire , hail, or just a hard surface to walk on.)

3. Two layers 3 1/2″ Polyiso fully adhered (glued) to…..

4.   Plywood decking on roof rafters, all exposed….. ceiling 

The house I a currently have in the works in on Point Roberts WA. a 4-C marine climate, I’ll probably use EPD so I won’t have to Heat weld the seams…  

The good folks at the Washington State Energy Code Dept, are telling me to put a  “SMART” AIR BARRIER THAT IS VAPOR PERMEABLE, preferably self adhering,  on top of the plywood deck that is the exposed ceiling. allowing any moisture in the system to dry to the interior.

What does the code and for that matter  building science say, if anything, about the Perm classification of this air barrier/ vapor membrane? 

I think I read something on GBA about taping the plywood joints is adequate!

Thanks 

Dana McBride 

see drawings attached