Whole Home Remodel – Spray Foam & IWS Issues? – GreenBuildingAdvisor

Hi all, 

In looking through the Q&A forum, I see a number of questions with very similar details to mine, but given the stakes I’d like to make sure I get this right for my client and have the right knowledge going forward.  

I’m doing an extensive whole-home remodel of a dated 60’s era ranch in climate zone 3A (coastal NC).  We are doing a couple of additions, a complete gut of the rest of the home, and eventually further improvements to the exterior.  With it also being in a high-wind zone, as part of the remodel, we are replacing the roof under the Fortified Roof program which stipulates full IWS over the entire deck.

The existing roof assembly was vented, though I’m not sure how effective it was as the vented vinyl soffit was placed directly over the original (no vent) plywood soffit which we have removed. My client would like to insulate the underside of the roof deck with spray foam – the air handler and duct work are in the attic (a ridiculous detail that I observe far too often in this region, but I digress).

I want to make sure that we aren’t running into a building science SNAFU by entombing the plywood roof deck with close cell on the underside, and IWS above.  I would imagine that there is not considerable vapor transmission upwards through a standard shingle roof anyway, but the extreme heat/humidity that we experience in this zone leaves me wary of doing so.  In reading some of the other forum topics, it seems that as long as the attic is conditioned, we should be in good shape. Is this the case?

Thanks in advance for some insight on this matter – been a long time FHB enthusiast, and GHB lurker, but nice to finally have something substantial to bring to the forum.