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Community and Q&A

Polyiso over roof deck or under rafters for conditioned attic in central Texas

C_Fox | Posted in General Questions on


New poster here, but I read most of the GBA articles on this topic. It’s been great how much I leanred. Thanks to all contributors here!

I just bought a central tx (zone 2A) home, built in 1983. My roofer tells me it has a pretty simple roof: stick-built, three main portions (central one is two-story living area, rest are single story), gable roofs, total of 40-square with 7/12 slope; if it helps, i attached a drawing that my roofer gave me. Right now, attic has blown-in insulation covering its floor. insulation is beaten down, too low R, very dirty. So, insulation would have to be replaced no matter what else. Instead of just replacing that insulation, Id like to make attic into conditioned (or semi-conditioned) space. Theres a few reasons for this: 1) attic holds furnace, water heater, and duct-work for second-floor of house, and i know its bad idea to have them in such hot unconditioned space; 2) use it for storage, since theres no basment, and to few closets & garage space.

I spoke to a few local isntallers about using spray cell foam insulation. I dont want to do that, even though I know its popular and insulates well. My reasons are mainly do to chemicals involved 1) dont like idea of “manufacturing” polyuruthain in attice right near were we sleep 2) worried about odors 3) nervous that i was told to leave house for 24 hours after install 4) saw on web about a few fancy ($$$) mansions that burned down in new england from “spontaneous combusion” when spraying foam. Also, the three installers i spoke with (found them by good ratings on angies list and yelp) recommedned open-cell spray and mentioned that they only install about R-20 to R-25 between rafters. From what i read here, open-cell in my area is a bit risky (not as bad as zone 5+) even though its common. Also, even the R-25 doesn’t seem to meet code as far as I know from here. That makes me more worred about dangers of spray-foam–if a bunch of local installers with good angies list and yelp ratings recomend things that dont meet code, then i worry even more about how they safely and profesionaly handle spray-foam with its dangesr, etc. Anyway, movnig on.

I read these GBA/FHB articles which were very good and helpful to me:

Long story short–as of now I decided to go with polyiso rigid foam. Good news–Theres a new (light-colored shingle) roof going up on my house as part of sale, so this gives us good options, i think. My polyiso options i think are:

  1. install 6″ of polyiso over roof deck (to make unvented roof)
  2. install 6″ of poyiso UNDER roof rafters (to make vented roof with unvented attic)

To be clear, option #2 is *NOT* “cut-and-cobble” as mentioned here on gba, but is instead continuous polyiso installed under rafters. In both options #1 & #2, we’d do 2 separate layers of 3″ polyiso, staggered seams, and everything properly taped down. So, i think we should avoid thermal bridging from rafters in both cases.

if we did “over roof deck” polyiso, i’m not sure if we’d do either regular foil-faced polyiso foam (with cap-nails for 1st layer, long nails for 2nd layer, then 2x4s or deck), or something like polyiso SIP or nailbase—happy to get input from folks here that know more. If we did “under-rafter” polyiso, we’d make sure to use something like foil-faced dow thermax to benefit from fire properties of thermax and to get some additional radiant barrier benefits; we’d also would use prefab vent baffles to ensure 2″ of good roof venting as i read in above FHB article.

In both cases, i’d still have to do something with the gables; probably just close there vents, and add similar layers of polyiso.

So, what do you think about which install is better? Any ideas on price difference between extra cost of polyiso while putting down roof, vs cost of extra project of putting continuous polyiso under rafters? If I do it under rafters, i can possibly save money by doing it myslef or doing it later and over time, so i’m not out all that cash at once, like over-deck polyiso while putting up roof.

Many thnx in advance for suggestions and help.

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  1. GBA Editor
    Martin Holladay | | #1

    From a building science perspective, either method would work, as long as you pay close attention to airtightness.

    Installing rigid foam above the roof sheathing is by far the more common method. If you want to install rigid foam under your rafters -- an unusual approach -- make sure that your plan meets the approval of your local building inspector, especially as concerns any requirements that the rigid foam be protected by an ignition barrier or thermal barrier (generally 1/2-inch drywall).

    Using a value of R-5.7 per inch, as polyiso manufacturers now advise, 6 inches of polyiso has an R-value of R-34. If you are in Climate Zone 2, the 2012 IRC requires R-38 -- so your plan to install R-34 is close, but not quite there. You might want to run that issue by your local building inspector as well.

  2. C_Fox | | #2

    Hi Martin,

    Thanks for the prompt reply. I really appreciate all the stuff you do to answer questions from home-owners./ I've gotten such misleading info when talking wtih local contractsers...

    Anyway, I guess I missed the lower (R-5.7) value of PolyIso. I saw yoru article on Calculating the minimum thickness of Rigid Foam Sheathing:

    The R-value shown on polyisocyanurate labels is usually equivalent to R-6 or R-6.5 per inch. However, the actual performance of polyiso decreases at cold temperatures. Concerns about the cold-temperature performance of polyiso are real, so GBA recommends that cold-climate builders use caution when choosing a rigid foam designed to keep wall sheathing above the dew point during the winter.

    Since i'm in tx, I thought i was safe to go with R-6 to R-6.5. Oh, well--thanks for setting me straight in the reply. In looking at your other articles, such as Cold-Weather Performance of Polyisocyanurate, I see lots more info about "true" R-value of Polyiso. we'll go with the R-5.7 value and maybe I'll just install 7" of Polyiso to get to an R-40 rating.

    A few more questions, please:

    (1) Not to put words in your mouth, but it seems like i might be better with PolyIso over roof deck, rather than hanging Polyiso from rafters. At least its more often done, so id think that means i'm more likely to find someone to do a good job. Any idea on whether its more labor (in general) to put the product over roof deck, or under rafters? Id guessing more labor under rafters, but what do i know.

    (2) I cant decide between: A) assembling the "sandwich" onsite: taking two layers of staggered-seam foil-faced polyiso, and adding top layer of decking; or B) buying a polyiso SIP? It seems like (A) gives me benefit of staggered seam and foil-faced radiant-barrier, while (B) is possibly easier for the roofer. Is (A) generally more expensive (due to higher labor); does (B) have any benefits over (A)? THanks!

    (3) How much do I have to worry about the added weight of this (7" Polyiso + extra layer of decking)? I tried to run the numbers myself: the new shingles weigh 275lb per square. I think that polyiso is around 2.5lb/ft^3 (145lb/roofing-square) and plywood/OSB is a little under 100lb/roofing-square, which means that a roofing-square worth of 7" polyiso is about the same as doing an "overlayer" of shingles (ie, putting second layer on without ripping off first layer). I think this means that I should be fine, but wouldn't mind a second opinion...

    Thanks again!

  3. GBA Editor
    Martin Holladay | | #3

    Manufacturers of polyiso have been adjusting their R-value labels in recent years in response to criticisms that claims of R-6.5 or greater were unrealistic. Lately it seems that polyiso manufacturers are beginning to settle at around R-5.7 per inch, which is a fairly realistic number for Texas. In colder climates, conservative builders figure that it's safer to assume that polyiso performs at R-5 or even R-4.5 per inch.

    Concerning your structural concerns, you should definitely consult an engineer. There are times in life when the best response to the feeling you are experiencing -- the feeling that you are overwhelmed with questions -- is to hire an experienced consultant.

    Concerning your questions about rigid foam plus site-added sheathing vs. SIPs vs. nailbase, the decision is yours, based on buildability issues and cost issues. You will probably have to do some research before you make a final decision -- or hire a G.C. you trust to help you settle on a system that will work for your project. Many of these options are discussed in my article, How to Install Rigid Foam On Top of Roof Sheathing.

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