Choosing Rigid Foam Board
Doelman
| Posted in Green Products and Materials on
We’re building an unvented roof with rigid insulation on the outside and fiberglass batts on the inside.
Would this board be appropriate to use? We’re in zone 3 so code only requires R-5 for the exterior insulation.
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I think when you say you're in CZ3 so code "only requires R-5 for the exterior insulation", you're looking at the tables for WALLS. For a ROOF, you need a lot more than R5 to make it a safe assembly. The usual rule is 40% of the roof's insulation needs to be on the exterior in a mixed interior/exterior assembly, but 50% is safer. That means for a typical R49 assembly, at least R20 needs to be on the exterior, which would be 4" of XPS (Foamular is XPS). You likely need more exterior rigid foam here than you think you do.
XPS is the least green of the common rigid foams too. EPS would be better, but I would use polyiso. If you can get reclaimed roofing polyiso in your area, that would likely be the cheapest way to go, and you'd need 3.5" for a safe assembly (3.5" would be around R20-22 if using polyiso).
Bill
Under IRC2015 we need R38 for our roof assembly.
Pulling from this article
https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs
We need 13% of our insulation on the exterior, which means R-5 rigid minimum. Am I misreading this or has the building science changed recently?
I refer you to Martin's comment #28 on that article, linked here:
https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs?_gl=1*d5wu2v*_up*MQ..*_ga*NTg3MDUxMjIuMTY2NjgxOTE5Mw..*_ga_SBNZMMC0G6*MTY2NjgxOTE5Mi4xLjAuMTY2NjgxOTc1Ny4wLjAuMA..#comment-13199
Martin mentions that with R38, you need at least R20 of that above the sheathing if using a mix of rigid foam and "fluffy stuff" like batts under the sheathing, which is the safer ~50% inside to outside ratio. If you were to use 40/60 as your ratio instead, you'd need just over R15 on the exterior, which would be 2.5" polyiso (R16).
Bill
Bill,
The OP is in Zone 3, so much milder than our climate, the fluffy/rigid ratio is much less. Down there you can even build unvented roofs with batts only using a diffusion vent.
As for the OP, I would use any type of insulation you can get for a good price. Around me the cheapest tends to be roofing polyiso (used for commercial flat roofs). I would avoid low density EPS as it won't hold up to foot traffic during install.
Akos, With apologies for the OP -- and reassurance that I am not trying to highjack his post -- might I call attention to a question I posted yesterday?
https://www.greenbuildingadvisor.com/question/glass-reinforced-facer-grf-polyiso-for-ceiling-roof-cavity
Basically, I am wondering whether "roofing polyiso" is ever used inside cavities, such as the ceiling under the roof of my future four-season room (which is currently a covered porch), and whether there are any drawback in doing so. I am finding many more opportunities to get pre-owned roofing polyiso than foil-faced polyiso. Thanks!
In that same comment he says "I stand by the recommendations given in the article." which would be R-5 exterior insulation in zone 3. His recommendation for R20 exterior is for Chicago, zone 5, very different climate than Central Alabama where I am in zone 3. I've never seen an ice dam in my life.
Sorry, I must have missed that you were in a much more Southern CZ. I'm used to thinking about the frozen North where I am, and where a lot of the posters seem to usually be from too. With higher average outdoor temperatures, you can get by with less exterior insulation -- a smaller fraction of the total being required on the exterior -- since the seasonal lows aren't as cold, and don't last as long.
I would try for reclaimed polyiso (the fiber or kraft faced stuff is usually the "roofing" polyiso, BTW), which is probably your cheapest option. Next cheapest would be EPS, but as Akos mentioned, you want one of the denser versions since the Type I stuff really isn't durable enough.
Bill
R-5 is enough in your zone IF you are also going with code-minimum insulation in the rafter cavities.
XPS, even the newer formulation you linked to, has more environmental impact than most construction materials--to the point that it's probably better to use nothing than to use that type of foam. Reclaimed foam or rigid wood fiber have no negative impact; polyiso, EPS and mineral wool have less impact (about 25-50%) than new formulations of XPS. (and much less impact than older formulations of XPS. )
HI Michael,
While the OP may not have access to it, the newest formulation of XPS available in my area (compliant with WA State GHG emission requirements) seems to be better than either EPS or polyiso. According to it's EPD, the blowing agent of that particular XPS product has a GHGe of <1.
https://www.soprema.ca/transparency-sopra-xps-epd/
Out of curiosity, in your view is the Sopra EPD inaccurate? I used it it below grade, thinking it was the lowest impact foam available to me.
Obviously, not so helpful if the product isn't available in the OP's area (my area has a similar situation with regard to wood fiber and reclaimed polyiso insulations).
Where do you see GHG emissions of less than 1? Page 18 of the document you shared shows that it's at least 2.06 kg CO2e, but other LCAs for similar foam show around 10 kg CO2e and I'm not sure what the differences are. They all use the same raw materials and blowing agent. I am not an expert in reading LCAs but I have read enough of them to know that everyone presents them slightly differently. GBA is working on an article that will explain the global warming impact of different types of insulation.
Thanks for the reply, Michael, and apologies for the sloppy writing on my part—should have been 'blowing agent with GHGe <1' rather 'product.' I've edited to reflect that.
I am also not an expert at reading LCAs, but it does seem this particular XPS (maybe others as well) has considerably less embodied carbon than EPS of more or less the same compressive resistance (e.g., Type IX).
Look forward to reading the article.
I do a lot of my work in CZ3 (NTX) and always use 1.5" R10 polyiso to match the height of the 2x4 roof perimeter nailers. It is double over the R5 in 2021 IRC R806.5
We had decided to go with 1” polyiso but that makes a lot of sense. I guess the cost difference isn’t worth the trouble ripping 2x4s to match the 1” insulation?
No, and you get the benefit of an extra outsulation value and you have 50% more nailing base... totally worth it. BTW, DuPont Thermax is a great product available in all markets.
Question on this roof assembly. We're doing unvented, 1.5" polyiso as you suggested, and standing seam metal, I'm getting some conflicting information on the actual assembly.
Since we're doing a single layer of insulation, I'm reading that we're supposed to spray foam the seams from some sources and others say we should use foil tape only when using polyiso, what do you use?
Then there's the underlayment and what I'm seeing is all over the place. I'm reading that you need to use peel and stick on the entire structural roof sheathing, some say you need to use it on the plywood on top of the insulation only, some say you only need peel and stick on the lower few feet of the roof around the eaves and underlayment on the rest. Then my roofing supply guy is telling me I need high temp underlayment on the entire roof, then my roofer is telling me I only need normal felt and no peel and stick at all. What do you guys do?