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Community and Q&A

Vapor-impermeable or semi-permeable rigid-foam under rafters in unvented (hot-humid) attic…

C_Fox | Posted in General Questions on
I’ve been reading GBA for going on a few years now, and have learned so much!  Thanks to Martin, his colleagues, and all the contributors for making me much, much more knowledgeable about building science and green building tech.
I’m continuing garage renovation that I began a year ago, and have previously posted here about.  Everyone here has been tremendously helpful in getting me to where I am now.
I’ve consulted a couple key GBA resources (in addition to responses to my earlier posts here, from Martin and others):
  * “How to Build an Insulated Cathedral Ceiling” by Martin Holladay <;
  * “BSC RR-0108: Unvented Roof Systems” by Joseph Lstiburek <;
Also, I’ve also been in contact with my building inspector, who’s approved what I’ve done so far, and also told me that he’s okay with below potential directions I might choose in response to this question.
Some background: I’m converting my 500sf garage attic (under raftered, gable roof with recent new asphalt-shingle roof) into conditioned storage space.  This is in Central Texas, within Zone 2B (Hot Humid).  Here’s where I am so far:  (1) High-SEER ductless mini-split has been installed for HVAC needs; (2) thermostat and humidistat control of HVAC have been added, along with constant thermometer / hygrometer data-logging (for long-term trends to improve conditions and increase longevity); (3) stone-wool insulation has been installed between the rafters to serve as bulk insulation from interior attic, to exterior environment; (4) decided NOT to add soffit vents, ventilation baffles, and ridge vent, due to discussion by Lstiburek in RR-0108, and received approval from inspector for this code deviation; (5) now trying to decide on final addition of rigid-foam insulation underneath rafters.  For this project, it’s infeasible to add rigid-foam in any location except underneath the rafters (primarily since I just put a new roof on the house).
So… The final step of this project is for me to install some additional rigid-foam insulation, underneath the rafters, for two reasons: (1) to increase overall insulation; (2) to reduce thermal bridging effect of wooden rafters.  My uncertainty is whether or not it is better to install vapor permeable (or, at least, semi-permeable), or vapor impermeable rigid foam.  According to my research, typical EPS rigid-foam (such as Owens Corning’s Foamular brand) has a water vapor permeance of around 1.10, while typical alum foil-sheathed PolyIso rigid-foam (e.g., R-Max Plus-3) has a water vapor permeance of around 0.03 (surely due to the foil, more so than the PolyIso itself).
My main question is:  In this location (ie, on the interior, under rafters) do I want to use rigid-foam that has a high-permeance, or a low-permeance?  And, if high permeance, would a mid-level permeance be acceptable (given that that’s mainly what’s available around here)?  Finally, would I be better off with no rigid-foam if the available options are insuitable?
I’ve re-read Lstiburek’s RR-108 article, and noted Figure 3 which shows allowance for unvented roof in my area of Texas.  It obviously does not required rigid-foam above the roof-deck, but also does not mention any rigid foam below the rafters.
Finally, to reiterate, my inspector has nominally blessed any of the above options for rigid-foam install.  Look forward to any & all GBA community insights, to help me decide!

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  1. Expert Member
    Dana Dorsett | | #1

    Figure 3 of RR-0108 notwithstanding, the IRC prescribes a minimum of R5 above the roof deck for unvented roof assemblies. Setting that aside...

    >"My main question is: In this location (ie, on the interior, under rafters) do I want to use rigid-foam that has a high-permeance, or a low-permeance? "

    The short answer is "none of the above".

    Installing continuous rigid foam under the rafters (even an inch of Foamular) creates a potential moisture trap.

    The same or better thermal performance can be had by extending the rafters with "Bonfiglioli strips":

    If you have the room for 2" of polyiso + 1x furring Bonfiglioli strips that would add 2.75" of depth, which is an easy compression fit for 3" thick "Save'n'Sound" fire and sound abatement batts, which would perform at about R11 at that thickness. The 2" polyiso would be R12, the 3/4" furring another R1, so overall it would be about the same as adding 2.5" of EPS. If there's only enough room for 1.5" foam it's still a fairly easy compression to the 2.25" depth and R9.5-ish performance on the batts, R10 for the strips.

    BTW: Foamular is XPS, not EPS, and is blown with HFC blowing agents, making it one of the least green (by a LOT) insulation materials in common use. EPS and polyisocyanurate are blown with FAR more benign pentane.

    Full foam would require some sort of ignition barrier to meet fire codes, such as half-inch gypsum, etc, but with only strips of harder-to-ignite polyiso behind 1x furring and some rock wool the inspectors would probably give it a pass.

    To keep the thin batts in place, running some PERFORATED aluminized fabric type radiant barrier stapled to the strips would restrain the batts without creating a moisture trap. Most perforated RB runs about 5 perms- a Class-III vapor retarder, comparable to a single coat standard interior latex paint on sheet rock. The RB would add another ~R1 of performance, and would protect batts a bit, and unlike foam it has sufficient fire ratings to be left exposed in an attic (which is it's primary application.)

  2. C_Fox | | #2


    Wow, great reply. really helped me think through this alot. Spent all evening thinking more about it, and changed my plans to closely follow your suggestions. More below.

    (1) First off, thanks for pointing out how much worse XPS is (than EPS/PolyIso) for all of us, and for correcting me that Foamular is XPS, not EPS. I'm glad that I haven't ever used XPS on any of my projects --- only ever used PolyIso (usually the R-Max that's carried locally). I'm going to continue to use PolyIso here, especially since you've shown how to do it, without worrying about vapor permeance.

    (2) Thumbs up for reminding me of fire ratings of PolyIso vs other rigid foam. Glad to stay with polyiso and then skip the drywall cover. I read up some more on this, and it's interesting to read about diff between thermoset and thermoplastic. For others who might come across this thread, see here: (but note that this particular link is to polyiso mfgr, so perhaps biased).

    (3) The "Bonfiglioli strips" approach is super cool. I'm doing that. Probably going to use two sheets thick worth of PolyIso for total thickness of 1.5" (as 2" thick causes some space limitations).

    One quick follow-up question: Do you think there's any advantage to cutting the polyiso for the "Bonfiglioli strips" at 2" wide (WIDE, not thick), rather than just 1.5" wide? My only thought is that letting the strips "hang-over" the sides of the rafters by about 1/4" on each side might help out a tiny bit more to prevent side seepage at the rafter thermal bridges, since the RockWool insulation fill (between the rafters) pulls away a little bit at the ends? I think I'm asking this question clearly, but maybe it's too muddled to make sense. Any ideas?

    Anyway, thanks again, Dana --- such amazing help!


    1. Expert Member
      Dana Dorsett | | #3

      Making the strips wider than the rafter edges creates a fit problem with batts, potentially creating thermal bypass air channels at exactly the place where you're trying to create the thermal break. In fact it's better to use 3/4" plywood ripped to 1.5" wide than using 1x3 furring, though with a batt knife and careful tucking you can still get it to fit reasonably. If it's never going to need to support gypsum board, even half-inch plywood would be fine.

      It's dead-easy to cut clean precise-width strips of polyiso of thicknesses up to 2" or greater using a steel wallboard taping knife that has been sharpened on the edges, and a straight edge, as shown here:

      1. C_Fox | | #5

        Hi Dana,

        Great stuff, and great link. So, 1.5"-wide PolyIso it is!

        I had planned to actually use my table saw to cut the strips, but perhaps the "sharpened putty knife" approach is even easier. I've never cut foam with my saw before, and imagine that it might make a big mess, but I figure I'd get nice uniform cuts at least. Do you have any experience with this?

        One other question that perhaps I should have mentioned earlier.

        My plan was to finish off the install with a decorative layer of 1/4" plywood veneer (left plain, withOUT paint or stain). In this case, with the new plan as above, it would be mounted atop the Bonfiglioli strips.

        My understanding is that this thickness of plywood, in general, has a water vapor permeance that is semi-permeable (likely 5+ perms), and in general has a higher wet-bulb than dry-bulb permeance. (In particular, my plan is to use the trade-name PureBond plywood product as it's supposedly LEED-conformant and more green due to being formaldehyde / urea-free).

        At these supposed levels of permeance, it seems like the 1/4" plywood would be somewhat similar to your above-mentioned perforated radiant barrier (perf RB), as a Class III vapor retarder. Do I have this right?

        Thanks again, you've been a great help with all this!

        Take care,

  3. Deleted | | #4


  4. Expert Member
    Peter Yost | | #6

    Hi CJ -

    In Climate Zone 3B, the importance and impact of air leakage trumps by far thermal bridging by conduction. I am not saying there is no benefit to interior rigid insulation as a thermal break, just that focusing on air leakage is a better bang for the buck (especially with your latent load and its energy and IAQ penalties).


  5. C_Fox | | #7

    Hi, I'm posting a follow-up along with thanks to Dana and Peter for all your helpful comments! I've since put in the Bonfiglioli strips, and those are working great. I'm also tracking down any remaining air leakages, and have that under control. Thanks again for all your help. It's been great.... Best wishes, -CJ

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