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Community and Q&A

Unvented roof underlayment and rafter bay insulation

Stan Smith | Posted in Energy Efficiency and Durability on

Hi guys…first time poster, love the site its been a huge help. I’ve just finished framing my home in Montana and am ready to move on to the roof. It consists of multiple single pitch decks ranging from 1.5″ – 2″ in 12. Framing is 2×12 rafters.

Originally I set out to do rigid foam above the roof deck, but realized late in the game it would take 6″ to do it safely. Its too cost/time prohibitive for me to go that route. I’m now leaning filling the rafter bays with rigid foam directly under the roof deck and air sealing with spray foam. Basically flash and batt but with rigid instead of spray so I don’t go broke!

The next question is underlayment beneath the metal standing seam roof. I get the sense that as long as the conditioned space is air tight, keeping rain out is the main focus, not as much allowing the OSB to breath to the exterior. I interpret many building science articles to say that if you air seal the bottom face of the OSB, underlayment permeability doesn’t matter and in fact they recommend ice and water full adhering membrane.

Personally I don’t see how a roof deck could breath being under a sealed mechanical lock roof, but the thought of sandwiching osb between ice and water membrane and closed cell rigid foam is somewhat scary.

Any thoughts would help tremendously. I don’t want to get in a vented/unvented argument, I just want feedback from people that see the benefit of both methods. My worst case option would be to to a vented space in the rafters assuming I could fit enough insulation in the 11.25″.

Thanks in advance….

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  1. GBA Editor
    Martin Holladay | | #1

    You can use asphalt felt roofing underlayment; you don't need Grace Ice & Water Shield, except perhaps at the valleys and eaves if you live in ice dam terrritory.

    You method you have chosen is called "cut-and-cobble." Unvented cathedral ceilings using cut-and-cobble are risky, and some failures have been reported. Read more here: Cut-and-Cobble Insulation.

  2. Stan Smith | | #2

    Thanks Martin. You're a staple around here it seems. I fall into the category of having the money but being too cheap to pay for a spray foam guy. They want $6k for 2" around here which is standard, but not nearly enough. Probably doing the minimum to keep bids down. I'm am definitely not excited to put the time into the "Cut/cobble" method and I agree its probably hard to guarantee success. Crazy thing is the DIY spray foam kits don't seem any cheaper than hiring out the job. Any suggestions on the best value kit would help too.

    I guess at this point I'm exploring a vented option, I just don't know if I could fit enough insulation in the 11.25" cavity I have to work with. If I could get away with 2" of airspace I could probably make it work.

    As far as felt on the roof, the metal roofing companies all say not to use it since it will dry out very quickly and do nothing for me.

    Thanks again for your time. Its nice to have people that have tried this stuff before.


  3. Expert Member
    Malcolm Taylor | | #3

    You have probably already down so, but did you tell the roofing companies your pitches are that low? My suppliers won't warranty roofs below 2 in 12, although some will if the deck is covered with Grace Ice and Water.

  4. Stan Smith | | #4

    Hey Malcom,
    Yeah, they know the pitch. I'm actually going with 2" standing seam instead of 1.5" because they warranty it, and more importantly recommend it down to a 1/2 slope.

    I will probably pony up and put spray foam under there since I cant vent it and cut and cobble seems risky. The question ends up being, is it ok to sandwich the osb deck between spray foam and ice and water membrane?


  5. Expert Member
    Malcolm Taylor | | #5

    Indeed - that's the million dollar question. I wouldn't be comfortable doing so, but will defer to others with more experience.

  6. KEVIN ZORSKI | | #6

    Adeeb - One possibility is to lay 2x4 stock on top of the 2x12 rafters ( making small "t's" in cross section), then OSB nailed into the 2x4 face. This creates a 11/2" vented space. Then "cut and cobble" to the underside of the 2x4's. That leaves you the whole 11+ inches for insulation. I,too, have concerns about the 2/12 pitch. No one in Maine builds such a roof for a residential structure. I'd suggest 4/12 at least, even with standing seam. 4' of wet snow and ice can really raise havoc on a low slope. Best of luck to you with your building project.

  7. Stan Smith | | #7

    Thanks Kevin - I think if I went through all that trouble, I would just bite the bullet and do the spray foam. I think I'm leaning towards using a couple inches of rigid with a couple inch layer of spray over it at this point, but I'm going to get some estimates and see if its worth it. As far as low pitch roofs go, Americans are the kings of super steep pitch roofs, no doubt. If you go to forward thinking places like Scandinavia, Germany, Holland, Canada, etc you'll see a lot of different thinking on roof pitch and efficiency in general. The last time we got 4' of snow here was in the late 70's. Realistically 2' would be a stretch on a roof, maybe the ground if you get way up in elevation. You also have a much wetter climate in Maine than we have here in Western Montana. That said, I'm not sure why it would raise havoc on a residential roof but not on a commercial roof? Thanks for your time.

  8. KEVIN ZORSKI | | #8

    Adeeb- Commercial applications would use a built up roof designed for a snow and rain load, often with a central drain for water - a much more expensive proposition. They'll also sometimes use a single piece of EDPM rubber roofing with no penetrations. Can your low pitch standing seam remain tight when it's under water? Maybe us New Englanders are too steeped in tradition. I went to Colby College in Maine, and at that time they were building a new science building designed by a "hot" architect who was not from Maine. I was watching them build one day, and I overheard one of the female lunchroom workers say, "That roof's gonna leak- it's too flat." It took two more winters for her prediction to come true. I tend to respect natural forces, not try to outsmart them. Maybe that is "backward thinking" but maybe it's just common sense. All it takes is one severe event to wreck your day - and your house.And maybe, like you say,you don't get that kind of situation. Extreme weather does seem to be more common everywhere. Coldest February on record this year where I live - since early 1900's!

  9. GBA Editor
    Martin Holladay | | #9

    You are inspired by successful commercial roofs, but are apparently unwilling to accept the standard commercial solution to your dilemma -- which is to install the insulation on the exterior side of the roof sheathing. If you install rigid foam above your roof sheathing, all your problems will be solved. The OSB will be warm and dry.

    Other solutions are all less satisfying than the standard commercial roof solution.

    If you end up installing spray foam, open-cell spray foam is obviously preferable to closed-cell spray foam in this instance. If you go this route, don't forget the interior gypsum wallboard and the vapor-retarder paint.

    If you decide to encapsulate your roof sheathing between two vapor-impermeable layers -- peel-and-stick membrane above and closed-cell spray foam below -- you won't be the first person to do so. Just make sure that the spray foam contractor does the work during a spell of dry weather, so that the OSB is dry on the day it is encapsulated.

  10. Stan Smith | | #10

    Hey Martin,

    I'm not unwilling to do anything, I just want to get it right. I woke up this morning thinking about doing the exterior method, which is how I'm doing the walls by the way. The roof just needs more foam which adds quite a bit of work to it. I already have 2" of foil faced polyiso, do you think if I added 2" of the same it would give me what I need in Zone 6? Thats somewhere in the R24 range before any cold weather performance dips.

    The las concern with rigid on top is that I'm not totally set on metal roofing yet. What would your recommendation be if I use rigid and some other type of roofing that needs a solid deck, not purlins?

    I was going to start looking into a combo of rigid on top and Open on the bottom to try to get the cost and detail down if it would allow me to use less rigid on top. Any articles or ideas?

    Do you have any article links on encapsulating the layers?

    Thanks Martin

  11. GBA Editor
    Martin Holladay | | #11

    In Climate Zone 6, building codes require that rigid foam installed above roof sheathing have a minimum R-value of R-25 to make sure that the roof sheathing stays above the dew point during the winter. That's 4.5 inches of polyisocyanurate (before any consideration of the fact that polyiso performs poorly in cold weather).

    For more information on these issues, see:

    How to Build an Insulated Cathedral Ceiling and

    In Cold Climates, R-5 Foam Beats R-6

    For information on how to install a second layer of roof sheathing above rigid foam, see this article: How to Install Rigid Foam On Top of Roof Sheathing.

    For general information on methods of insulating a low-slope roof, see Insulating Low-Slope Residential Roofs.

  12. Stan Smith | | #12

    Thanks, I've read all of these already. So if you were in my shoes and had 2+ inches of polyiso already, what would you add to it to finish the system?

  13. GBA Editor
    Martin Holladay | | #13

    I would add 4 inches of EPS on top of the polyiso, followed by a second layer of roof sheathing.

  14. Stan Smith | | #14

    So 6" total plus deck. I guess the downside to this is just dead weight and time. You choose EPS over EPS for environmental reasons? Seems like XPS performs a little better which could reduce thickness.

  15. GBA Editor
    Martin Holladay | | #15

    Yes, I prefer EPS to XPS for environmental reasons. You can specify what you want, of course.

    Once again, as a general piece of advice to GBA readers -- advice I've given before: it's good to nail down insulation details -- the types of details that Adeeb is now struggling with -- at the design stage, not after the building has already been framed.

  16. Stan Smith | | #16

    Framing wouldn't have been any different, but yes you're right. I originally intended on doing exterior insulation, I just screwed up and didn't spec enough. Double checking before I started I discovered how much more I need on the roof. Live and learn.

  17. Stan Smith | | #17

    To finish this off for somebody down the road. I think the only way to do this right is to do R25+ foam on top of the roof deck. Spray foam on the underside of the roof deck has too many downsides in my opinion. I've decided to do a commercial membrane roof on top of 4" of ISO which gives me about R26. Standard around here is 2" and my roofer has seen failures because people are sandwiching the deck sheathing with foam on top and bottom. Said after 5 years you would walk right through the deck. Granted these people have recessed cans and a million other things ruining their ceilings.

    So we're turing this into something fun and we'll potentially turn it into a living roof down the road. So it will go as follows:
    -2" of spray foam on all the blocking between rafters and soffit
    - R39 batts in rafters
    - 4" of ISO on top of deck R26
    - Rubber membrane roof with a couple of internal drains
    - Walls have 3" of polyiso on the exterior with R19 batts.

    Should work out well. Thanks for everybody's input.

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