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Community and Q&A

Maximizing R-Value for Roof Without Using Exterior Foam

JohnsonDesign | Posted in Energy Efficiency and Durability on

I’m a homeowner building in a remote zone 5 location. My contractor, who is very good but also the only game in town, doesn’t have experience with installing rigid foam insulation. Our build is off grid and we are relying on solar, so we want to maximize efficiency. The rafters are 12” TGIs. I would like to install r-38 batts between the rafters, but that would require 6” of exterior foam to maintain the proper foam to fluffy ratio. The home is a simple single-plane shed roof. What do you recommend to maximize insulation without use of exterior rigid foam?

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  1. Expert Member
    Malcolm Taylor | | #1

    A very well performing, but a bit difficult to build option is to use R-38 batts as you would prefer, but then cover the TJIs with a taped WRB, and then fur up the TJI's with 2"x3"s attached to their top chords to create a continuous vent channel from eaves to peak below the sheathing.

  2. JohnsonDesign | | #2

    Thanks Malcolm. Sorry if this is an obvious question (I’m not a builder). So the 1x3s create a vent channel between the sheathing and 1x3s by preventing the batts from contacting the sheathing? Is that better than closed cell foam against the sheathing supplemented by a lower r value batt?

  3. Expert Member
    Malcolm Taylor | | #3

    The WRB stretched across the tops of the TJIs creates an air-sealed barrier for the batt insulation, an approach which, unlike using baffles, allows you to completely fill the cavity with the R-38 batts. The 2"x3"s above create a ventilated cavity which allows the sheathing to dry and also exhausts any moist air that makes it's way into the roof from the interior.

    There are a number of reasons to choose a vented roof assembly if possible including: Cost, long term reliability, and not having to use foam. To me going to an unvented assembly in most climates is something best reserved for situations where venting isn't feasible.

  4. Expert Member
    Peter Engle | | #4

    Note that you can get R-38C batts that provide this R value in about 10" of space. That would allow space for an additional 1-1/4" R6 batt in the cavity, bringing the total to R-44. Not quite what the IRC recommends in your climate zone, but close.

    Another approach is to use the R-38C batts with air channels fastened to the underside of the top chord of the I-joists, providing continuous eave to ridge ventilation. Then boost your R-value by adding foil-faced polyiso insulation to the inside of the I-joists, behind the drywall. Tape the seams and the foil facing becomes your air and vapor barrier. 2" of polyiso would give you another R-12, for R-50 total. One advantage to this approach is that you can get the roof installed quickly and normally, and save the insulation details for inside where it's warm and dry.

    1. Expert Member
      Michael Maines | | #5

      Peter, I was going to suggest your second approach. I have used it before with good results. I like 1/4" plywood for creating the vent channels.

    2. Tyler Keniston | | #6

      Peter, or others,
      Do you like the foil faced polyiso specifically because it has such low permeability, or is it just the higher r-value of polyiso? Other?

      And do you strap the foam before drywall or straight to drywall?

      1. Expert Member
        Michael Maines | | #7

        Tyler, polyiso has the highest R per inch of any readily available insulation, it's easy to get and easy to work with. It would be a slightly safer assembly to use a somewhat vapor-permeable product such as EPS or wood fiber, but they have lower R per inch, which usually matters on ceilings, and wood fiber is not an air barrier. I would strap the ceiling with 2x material so I could install low-profile LED lights and run the wiring on the interior side of the foam but if you're trying to save headroom and you don't have ceiling fixtures you could glue the drywall to the foam, and run long screws through to the framing.

      2. Expert Member
        Zephyr7 | | #8

        Foil faced polyiso is a vapor barrier (due to the foil facers). When this material is installed directly above the drywall, it provides a super tight air barrier, since the foil facer and foil tape are easy to seal, and also a sort of whole-ceiling vapor barrier. This can help in a vented ceiling to limit the amount of moisture that can get up into the insulation, but don't use it with an UNvented ceiling.

        If you need vapor open polyiso, use the kraft or fiberglass faced roofing polyiso. Polyiso without impermeable foil facers is more vapor open, typically in vapor retarder territory.


  5. JohnsonDesign | | #9

    Thank you for the collective wisdom.

    Peter and Michael, when creating the vent channel by fastening 1/4” plywood (or other material) to the underside of the top chords, what is your spacing on that material? I’m assuming it doesn’t need to be continuous, but instead just enough to keep the batts from contacting the sheathing?

    I’m also considering using a 14” I-joist to give more room for insulation. We had discussed an unvented assembly with 14” joists with 2” closed-cell foam against the sheathing, supplemented by batts. Would you recommend the between-joist venting approach (with the suggested foil-faced polyiso vapor barrier) over the unvented closed-cell foam + batts approach? To my novice mind, it seems like the closed-cell foam with the batts Peter suggested would be a simpler and less labor-intensive installation achieving a similar r-value, but maybe at a slightly higher material cost.

    We’re building in the very arid high desert (6500’ elevation), if that makes a difference in evaluating any increased risk with an unvented roof.


    1. Expert Member
      Michael Maines | | #10

      Brett, the vent channels should be continuous and ideally air-sealed with tape or sealant, though some of us think that's less important than others. I suggest reading this article and the related links:

      If you are considering an unvented assembly, check out R806.5 and table R806.5: Those are considered the bare minimum for safe assemblies. 2" of closed-cell foam may be enough but it depends on your climate zone.

      1. JohnsonDesign | | #11

        Thanks Michael.

    2. Expert Member
      Peter Engle | | #12


      The high desert location does matter, as it makes any roof design much less risky for condensation potential. There's just less moisture in the overall enviroment to cause trouble. That said, if you have a very tight house and/or a lot of interior moisture sources, you can still get in trouble.

      I always (or almost always) prefer vented roof solutions to unvented ones. In my experience, they are just more resilient in all conditions. With your simple shed design, a vented roof is easy to accomplish, and inexpensive to do really well. Also, I try to use as little spray foam as possible, and it's just not necessary in your roof.

      If you have the room, the simplest and often the least expensive approach is just to make the I-joists as deep as you need, install the 1/4" plywood spacers for air channels, use an interior air/vapor retarder membrane, and blow the entire cavity with cellulose. This is a no-foam approach, very "green" and one of the most affordable options. In a desert climate, I'd even be tempted to skip the membrane and just do a great job of air-sealing the drywall, with vapor-retardant latex paint on the drywall for moisture control.

      1. JohnsonDesign | | #13

        Thanks a bunch, Peter. This is just the sort of advice I was hoping for.


      2. Expert Member
        Malcolm Taylor | | #14


        I've never done either opti0n, so I'd be interested in your experience. Which do you think is more labour intensive: A full WRB and strapping on the top flanges of the I-Joists, or plywood baffles in each rafter bay? I guess the third option would be to use strips of WRB as baffles.

      3. JohnsonDesign | | #22


        I was doing more research today and ran across this article. In the comments, there’s discussion about the value of venting low slope roofs. I neglected to mention that this is a 2/12 roof. Does the low slope affect your opinion about venting/not venting in this situation?


        1. Expert Member
          Peter Engle | | #23

          Malcolm (#14): I hate the thought of using WRB on the top flange without sheathing. This is just an accident waiting to happen. Also, you've got the challenge of keeping the I-joists from rolling over when you walk on them. Even with blocking at eave and ridge, they will be super-wobbly. I certainly wouldn't want to be the guy trying to install the 2x3 spacers on top. As Aaron says below (#15), Ben Bogie described a pretty efficient way to cut and fit plywood to the underside of the flanges. That would be my choice.

          Brett: 2/12 roofs don't get as much benefit from venting as steeper roofs. But in your case, I expect it would be enough. You benefit from the desert climate and the thick top flanges on the I-joists, with close to 2" air gap that's well-formed and very smooth. And, in most arid areas, there's usually quite a bit of prevailing wind to drive air movement even if thermal convection doesn't do it.

          1. JohnsonDesign | | #25

            Thanks again, Peter. I'll discuss with my contractor again.


          2. Expert Member
            Malcolm Taylor | | #26

            That's my feeling too. Ben Bogie also used the membrane and furring detail on one job and said it wasn't too bad to build, but although I'm comfortable on most roofs, the idea of building that makes me pretty nervous.

        2. Expert Member
          Deleted | | #24


  6. Aaron Beckworth | | #15

    I’ve read several recommendations similar to Peter’s (comment #12). In a recent Q&A Spotlight Ben Bogie recommended installing 1/4 or 3/8 inch CDX plywood to the underside of the top flange and air sealing them with a combination of tape and sealant. Ben also suggested gang cutting several sheets at once to increase efficiency.

    The question I have is how best to cut vent openings in the rim board. I picture the I-joists fastened to a rim board at the top plate. For the sake of the air barrier formed by the CDX vent baffles it would be best to limit the vent openings to no greater than the depth of the top flange. Would it be easiest and/or best practice to bore holes through the rim board or some other method? Would it be best to do this before the rim board is installed or after?

    1. Expert Member
      Malcolm Taylor | | #16


      It depends on how the overhangs are supported. Usually the I-joists continue out, so then rather than a rim-board there will be blocking between the rafters that can be kept down several inches. If there is a continuous rim-board it gets a bit dicey as the attachment is through the top flange of the I-joists, so you don't want to be removing too much material in-between.

      1. Aaron Beckworth | | #19


        That is not what I was hoping to hear. The idea that I’ve been thinking through is adding ventilation to an assembly used by South Mountain Company on one of their housing projects described in the JLC article, High-Performance Homes on a Budget.

        Martin Holladay wrote about their approach in his piece, Airtight Wall and Roof Sheathing.

        I like the simplicity of the approach, but will need ventilation to meet code here in CZ4B.

        1. David Argilla | | #20

          Why not just add the rafter tails on top of the structural sheathing like in the second image from your linked GBA article, but continue 2x4's up to the peak and let that space be the vent channel. A little extra cost for the second sheathing layer, but much simpler than adding faux tails to the wall. Also allows for a longer overhang.

        2. Expert Member
          Malcolm Taylor | | #21


          I don't mean to be discouraging, just to urge caution. You need to leave sufficient meat not only to secure the rim-joist, but also to deal with the lateral loads the applied rafter tails will add - especially as unlike conventional roof framing, the sheathing doesn't contribute to the overhang strength.

          One way round that would be to extend the roof sheathing down the overhangs, and box in the underside of the rafter-tails so it wasn't visible.

          Stepping back though - the primary reason to have applied overhangs is to maintain continuous air-barrier at the sheathing. If you are cutting holes and putting vent channels, it defeats the whole purpose. Why not just extend the I-joists and be done?

  7. Expert Member
    Akos | | #17

    I would skip any vent baffles and go for HD batts (either FG or MW). The wind washing effect lot of places refer to is the older style loose fill fiberglass. HD batts loose very little effective R value from wind washing. If you take a bit of care when installing the batts to keep it out of the vent channel you should be fine.

    One item to watch is to order batts for metal studs. These are slightly wider and will fit snug between the I-joist webs, regular wood stud batts will have a gap which is pain to try to fill. Lot of times is less lead time to go with two layers of standard batts, I usually use a 2x6 and 2x4 batt for R31 roof.

    Depending on how wide your flanges are, you'll have to do a bit of trimming to get the batts to sit flush to the ceiling. Instead of trimming, sometimes it is simpler to either not insulate there or fill this space with some squished R8 rolls.

    1. Expert Member
      Malcolm Taylor | | #18


      That's how my own house is insulated, with a 3" airspace. It saves an immense amount of time.

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