Polyiso under metal roofing
First off, thanks for such a wealth of information! I live in central Indiana. I’m in the process of insulating my cathedral ceiling. I have access to cheap paper faced polyiso. My plan was to cut and cobble 3″ polyiso in rafter bays. The build up would be tongue and groove, 2×2 nailers with 1.5″ polyiso in between nailers for thermal break of rafters, 6mil Vapor barrier, 2 3″ sheets of polyiso with very thick paper fiberglass faced, 3″ vented airspace, 2×4 purlins, foil faced bubble wrap directly under metal roofing. Will I have moisture problems sandwiching vapor barrier between polyiso sheets? Any recommendations please? Wondering also if it would be better to go with vapor barrier, 5.5″ r-21 fiberglass, 3″ polyiso, 1″ air space, purlins, bubble wrap,steel? Thank you in advance for your time!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Damion,
There are several possible problems with your approach.
First of all, foil-faced bubble wrap is close to worthless. For more information on this issue, see Stay Away from Foil-Faced Bubble Wrap.
The main problems with your roof assembly is that it lacks a good air barrier. In your type of assembly, it wouldn't hurt to have two air barriers -- one under the insulation, and one over the insulation. But your assembly doesn't really have any.
Your ceiling (tongue-and-groove boards) is not an air barrier. This type of ceiling is notoriously leaky.
Your next layer -- rigid foam cut into strips and inserted between 2x2 nailers -- is the worst way to install a layer of rigid foam. Far better would be to create a continuous layer of rigid foam with carefully taped seams. Done properly, this could be an air barrier. Then the nailers (perhaps 1x4s) could be installed between this air barrier and your T&G ceiling.
Cut-and-cobble polyiso between your rafters has a fairly troubling history of failures and air leaks, so that's not going to be an air barrier. (Here is a link to an article with more information on this topic: Cut-and-Cobble Insulation.)
The 6-mil polyethylene that you plan to install is the closest thing you've got to an air barrier. But polyethylene makes a pretty lousy air barrier because of its fragility and the way it's easily defeated by fastener holes.
If your roof is already built, it's going to be hard to get a top-side air barrier. (Roof sheathing would have been really useful; taped OSB or taped plywood makes a good air barrier.) At the very least, you need to establish a bulletproof air barrier on the interior side, using gypsum wallboard, a European air barrier membrane, or a continuous layer of polyiso that has been carefully taped.
Finally, the R-value of your planned roof assembly is marginal -- either R-37 or R-45, depending on whether you give your polyiso an R-value of R-5 per inch or R-6 per inch. (Here is a link to an article about the R-value of polyiso: Cold-Weather Performance of Polyisocyanurate.) In your climate zone, you should really be aiming for R-49 or more.
Martin, thanks for the expedited response! Love all the info on this site,but its becoming a bit overwhelming as I try to build my own home! Bubble wrap was for condensation issues of steel roofing, not looking for r value there. Wouldn't poly sheeting be an effective air barrier if all rafters were taped after stapling? I understand the fastener holes being a problem,thats why I was adding 2x2 strapping to nail leaky t&g to. I am also foaming and staggering seams of polyiso in rafter bays. Why would taped polyiso be a more effective air barrier than taped polyethylene sheeting? Could I do both? Any recommendation on a European air barrier membrane? I have read all those articles, and have yet to find a roof insulation plan that someone will not disagree with. Im just trying to build something that will last without buying the bank! Thank you so much for all the wealth of information!
Damion,
Q. "Wouldn't poly sheeting be an effective air barrier if all rafters were taped after stapling?"
A. No, not really. If you really want to use a membrane (instead of taped polyiso or drywall) as your interior air barrier, choose something rugged like Intello Plus or Siga Majpell 5.
Q. "Why would taped polyiso be a more effective air barrier than taped polyethylene sheeting?"
A. It's thicker, less fragile, and less subject to job-site abuse.
Q. "Could I do both [taped polyiso and polyethylene]?"
A. Yes, as long as the polyethylene was continuous, and not cut up into narrow strips and inserted between furring strips.
Q. "Any recommendation on a European air barrier membrane?"
A. As I noted above, you might consider using Intello Plus or Siga Majpell 5. But most American builders just use taped drywall as an interior air barrier. Taped drywall is readily available and cheap.