Rigid foam foil faced insulation

[Bruce09]

OK, so I've learned as I've went and sure not gonna win builder of the year but I have a question about polyiso board. I didn't put it down before the roof but I have some and figured it'd be better than none at all. Wen it's properly installed the foil side goes down on roof, then purl ins and roofing. I'm gonna put inside the building. Should I put it on top of rafters and do the foil side up white side in the shop? Would it be better to cut strips and place between rafters? And yes Don l, I realize this isn't the proper or easiest way, but I'm not easy nor proper myself so any help is appreciated..

 

[Dana]

Putting high R/inch foam between rafters is a waste of good foam, since the performance of R6/inch foam is being thermally bypassed by R1/inch wood. If it's moisture safe to install at all, it goes on the underside of the rafters, that's where it goes.

As long as the under side roof deck is vented soffit to ridge it's fine to put the polyiso on the underside of the rafters. If the roof deck is not vented underneath it's risky, since it creates a moisture trap between the #30 felt above the roof deck (which has very low vapor permeance) and the foil facers (which have even lower vapor permeance.)

Is there any fiber insulation between the rafters?

How thick is the polyiso?

Most polyiso is not fire-rated and would need either half-inch wallboard or OSB or something on the interior to meet most fire codes. Dow Thermax has fire rated facers, and would not need the the ignition/thermal barrier in many (or most) locations- you'd need to check with the local code folks.