GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon
Musings of an Energy Nerd

Sandwiching Roof Sheathing Between Two Impermeable Layers

Should we be scared of a “foam sandwich” with sheathing in the middle?

If you have installed closed-cell spray foam on the underside of your roof sheathing and vapor-impermeable roofing or Grace Ice & Water Shield on the exterior, then your roof sheathing can't dry inward, and it can't dry outward. Is that a problem?

Many builders have heard the phrase, “Walls should be able to dry in at least one direction.” The rule usually makes sense. If you want to install rigid foam on the exterior side of a wall — foam that will prevent outward drying — it’s usually a good idea if the wall can dry to the interior.

What about insulated roof assemblies? Should the same rule apply?

If a building has an insulated roof assembly — either a cathedral ceiling or an insulated low-slope roof — there are several ways that the roof sheathing may be able to dry.

1. In a conventional vented roof assembly — one with a ventilation channel between the underside of the roof sheathing and the top surface of the insulation — the roof sheathing can dry by giving up its moisture to ventilation air that enters at the soffit vents and leaves at the ridge vent.

2. In an unvented roof assembly with rigid foam above the roof sheathing, the roof sheathing can dry toward the interior by diffusion, as long as the insulation between the rafters is vapor-permeable, and as long as the ceiling materials are also vapor-permeable. Fiberglass batts, mineral wool batts, and cellulose are all vapor-permeable. Gypsum drywall is also vapor-permeable. This approach works well, as long as the builder remembers not to install any interior polyethylene. (Of course, in the illustration above, the second layer of roof sheathing — the outermost layer — can’t dry inward. But that’s a discussion for another article.)

[Image credit: ATAS International]3. Ordinarily, an unvented roof assembly with closed-cell spray foam installed on the underside of the roof sheathing (for example, a flash-and-batt assembly) does not allow the roof sheathing to dry inward — at least, not much. However, this type of roof assembly may be able to dry outward if one…

GBA Prime

This article is only available to GBA Prime Members

Sign up for a free trial and get instant access to this article as well as GBA’s complete library of premium articles and construction details.

Start Free Trial

42 Comments

  1. User avater
    Armando Cobo | | #1

    AAARRRGGGHHH!!! Frustrating...nice job on mix signals. So we've been telling people for years NOT to trap Plwd/OSB between two layers of impermeable or low-permeable materials, but holy and behold, now you, Joe and others say is OK? and later...
    1. "If you’ve got a choice, it’s probably a good idea to design your roof so that it can dry freely in at least one direction."
    2. "Eventually, of course, the roof will leak and get wet; that happens to all roofs. When the roof leaks, it’s time for a roof repair or new roofing."
    3. So if "We have lots of historic experience with commercial roofs that can’t dry out in either direction." we should be OK to follow same with homes?
    4. What about code R806.5.5? Does it need to be changed to reflect that you can't, BUT...
    5. Is anyone accounting for realities on jobsites vs. control lab experiments?

    1. User avater GBA Editor
      Martin Holladay | | #3

      Armando,
      Like any other specification for a building, this takes judgment. Should you buy a siding product made out of sawdust and glue, pressed together to look like clapboard? I dunno. It's up to you.

      Should you sandwich roof sheathing between two impermeable layers? It's up to you. I think there are better ways to design an unvented roof -- for example, by using rigid foam above the roof sheathing, instead of closed-cell spray foam on the underside of the roof sheathing. I'm not a fan of spray foam. But if you love spray foam, I think it's possible to do it safely.

      Q. "So if 'We have lots of historic experience with commercial roofs that can’t dry out in either direction,' we should be OK to follow same with homes?"

      A. Yes.

      Q. "What about code R806.5.5?"

      A. What provisions of the code are you concerned about?

      Q. "Is anyone accounting for realities on jobsites vs. controlled lab experiments?"

      A. Can you be more specific? What are you worried about? Workers make mistakes every day of the week, so it's always best to specify simple assemblies that are easy to inspect. That doesn't mean that workers won't still make mistakes.

    2. Bradley Weingartner | | #14

      Armando, I agree with your sentiment here. This is akin to somebody saying "this is how we've always done it" as a valid excuse for poor practice.

      Martin: When Joseph Lstiburek says "Commercial roofs", exactly what does that mean? I don't believe comparing a built up membrane roof to something like an asphalt shingle roof is fair or meaningful. I don't believe comparing a steel structured commercial building with a stick frame house is meaningful either.

      The long term performance to the former does not guarantee performance of the latter.

      1. User avater GBA Editor
        Martin Holladay | | #16

        Bradley,
        Q. "When Joseph Lstiburek says 'commercial roofs,' exactly what does that mean?"

        A. In this context, I assume he means something like "low-slope roof assemblies on large commercial buildings like retail establishments, warehouses, and factories."

  2. User avater
    Mark Walker | | #2

    What about a vented layer on top of the foamed-below sheathing?

    1. User avater GBA Editor
      Martin Holladay | | #4

      Mark,
      Read my article again. I discussed that approach in the paragraph that begins with the number "3" in the section under the heading, "Ways that roof sheathing can dry."

      I wrote, "This type of roof assembly may be able to dry outward if one of two approaches is used: ... (b) above-sheathing ventilation channels are provided (either by installing 2x4s, 16 inches on center, perpendicular to the ridge, along with a second layer of sheathing, or by installing metal roofing on 1×4 or 2×4 purlins that are parallel to the ridge). If you choose approach (b), remember that the rate of outward drying will be limited by the permeance of the roof sheathing and the permeance of the roofing underlayment. Board sheathing is more vapor-permeable than OSB sheathing."

      I also discussed this approach in my 2011 article, "How to Build an Insulated Cathedral Ceiling." In that article, I wrote:

      "If you prefer, you can locate your ventilation channels on top of the roof sheathing rather than under the roof sheathing. If you decide to do this, make sure that any roofing underlayment that you install above the roof sheathing is vapor-permeable — for example, #15 asphalt felt, VaproShield SlopeShield, or Solitex Mento — and that your local building department accepts this approach to roof venting. If you install ventilation channels on top of a vapor-impermeable synthetic roofing underlayment, the flowing air won’t be able to help dry out the roof sheathing.

      "If you plan to install ventilation channels above your roof sheathing, it’s best to choose a roof sheathing that is vapor-permeable (for example, fiberboard). If you use plywood or OSB, there’s a small chance that the sheathing can still accumulate worrisome amounts of moisture over the winter; this is especially true for north-facing roofs.

      "You can create 1 1/2-inch-high ventilation channels above the roof sheathing with 2x4s installed on the flat, with the 2x4s located above the rafters, 16 inches or 24 inches on center. Although this approach is less fussy than installing vent baffles underneath the sheathing, it usually costs more, because most types of roofing require a second layer of plywood or OSB on top of the vent channels.

      "In some cases, these ventilation channels are installed above a layer or two of rigid foam. It’s also possible to purchase nailbase (a type of SIP with OSB on one side instead of two) that includes integrated ventilation channels between the OSB and the rigid foam; one brand of these panels is Cool-Vent from Hunter Panels.

      "If you are choosing to build a vented roof assembly, don’t forget to include soffit vents and ridge vents."

  3. User avater
    Armando Cobo | | #5

    Obviously all depends on installation quality with any product, but since we know that water leaks on the roof will happen eventually, why give options for builders to apply a roof system that sooner or later the owners are going to have to change after the warranty expire... costing them tens of thousands of dollars.
    Somebody is going to take today's blog and say, "hey, if we screwed up in the design, its ok, we'll fixed with a terrible suggestion".
    I going to start working with a new builder to design their new homes, but he took me to visit a jobsite designed by someone else, were the impermeable WRB and tile roof has been installed, and they were going to install 5.5" R50 ocSF under the roof decking. IMO, his options at this point are to install 11" R38 ocSF or 2" R13 ccSF + 7.5" R25 ocSF under the roof decking, he knows that both have risks. What a trap... it seems to follow today's blog.

    1. User avater GBA Editor
      Martin Holladay | | #6

      Armando,
      If you believe strongly that roof sheathing should be able to dry in one direction, that's fine. This article lists several ways to do that -- and I'm sure that you are familiar with all of these methods. Nothing in my article prevents you from designing roofs that allow sheathing to dry in one direction.

      Whether you like it or not, lots of builders install closed-cell spray foam on the underside of the roof sheathing. While it's true that all roofing eventually fails, requiring a re-roofing job, I haven't seen any evidence that the presence of closed-cell spray foam changes the average cost of a re-roofing job. Roof leaks and the eventual need for re-roofing are facts of life, for any building.

  4. Malcolm Taylor | | #7

    "Builders who end up with roof sheathing that can’t dry inward are usually victims of their own poor planning."

    Like any other part of a building, consciously opting for something risky when designing an assembly just doesn't make sense. Especially with roofs, where the consequences of failure can be so catastrophic.

  5. User avater
    Paul Kuenn | | #8

    Would you trust any roofer? I don't! In two months I've had to strip shingles and replace OSB on 5 homes. OSB had been placed as solid sheathing on top of 100+ year old boards. A few misplaced nails in vents and flashing is all it took to completely rot the sheathing because all 5 homes had closed cell foam sprayed on the interior. Obviously it never dried at the leaks. Two of the 5 had been done within 5 years, one at 10 years and the worst two were done 15 years ago. When I find my charge cord I can upload some great photos. None of these used the same roofer. Keeps me busy!

    1. User avater GBA Editor
      Martin Holladay | | #9

      Paul,
      We all know that a stupid roofer who puts nails in the wrong place can cause roof leaks. That's not news. The real question is: Do you think that leaks cause more extensive sheathing rot before they are noticed when the roof assembly includes closed-cell spray foam?

      When I was a roofer, we saw lots of leaky roofs, many caused by stupid roofers, that had extensive sheathing rot -- and this was back in the day before spray foam was common. Roofers have been replacing rotten sheathing for hundreds of years.

      1. Bradley Weingartner | | #15

        I really think this hits on the main concern I have with the idea of sheathing being sandwiched: "Do you think that leaks cause more extensive sheathing rot before they are noticed when the roof assembly includes closed-cell spray foam?"

        In my mind it comes down to the distinction between a catastrophic roof failure vs. a transient roof failure. If water is coming in each and every time it rains, and goes undetected for any length of time, I think the end result is mostly the same regardless.

        If water only trickles in under just the right conditions, whether that be rain fall rate, wind, or some other factor - which anybody who's ever spent time in attics know is a bit of a fact of life with most human built roofs - which assembly will be safer? My bet would be on the assembly that allows free-drying in some direction.

        But that said, is it possible that closed cell foam, while potentially accelerating sheathing rot may help prevent collateral damage as well? It wouldn't really surprise me either way I don't think.

        1. User avater GBA Editor
          Martin Holladay | | #17

          Bradley,
          Clearly, there are a lot of factors here. I agree that many owners -- myself included -- feel more comfortable if they can enter their attic and inspect the underside of the roof sheathing. Almost everyone agrees that this type of roof assembly -- a vented unconditioned attic with insulation on the attic floor -- is particularly forgiving of roof leaks.

          I still advise GBA readers to build this kind of roof assembly if possible.

          That said, architects continue to design buildings where the approach that I prefer is impossible. We get questions every week of the year from owners of buildings designed by the type of architect I'm talking about. And GBA needs to advise owners of these buildings on how they can insulate their convoluted, valley-infested, hip-plagued roofs, many of which have roof sections that are either low-sloped or cathedralized.

  6. Mike Last Name | | #10

    Hi Martin,

    I've read all these roof insulation articles and I'm getting more confused the more I read. In previous articles, you warn not to use open cell foam on the underside of a low slope roof because moisture can reach the sheathing from the inside of the house. Now it seems like using closed cell under the roof is bad when there is TPO on top because there is no path to dry. Can you help me out here?

    Edit: I have a 100 year old brick row house with a lofted front facade. There is no possible way to install all of the rigid foam needed to meet code above the sheathing. It would look crazy on the front of the house.

  7. User avater GBA Editor
    Martin Holladay | | #11

    User-7396286,
    First of all, can you tell us your name? (I'm Martin.)

    If you are seriously interested in installing a continuous layer of exterior rigid foam above your roof sheathing, but are worried that your fascia will look too thick, see the illustration below to see how thick rigid foam can be disguised. [This illustration came from a Building Science Corporation document.]

    As the article on this page makes clear, you don't have to lose sleep over the use of closed-cell spray foam on the underside of roof sheathing, even if your roof sheathing can't dry to the exterior, as long as the roof sheathing is dry on the day that the spray foam is installed.

    You are correct that closed-cell spray foam on the underside of roof sheathing is safer than open-cell spray foam on the underside of roof sheathing -- because the use of open-cell spray foam is associated with damp roof sheathing.

    1. Mike Last Name | | #12

      Ah, I see the username is anonymous. I'm Mike, thank you for your reply.

      Your explanation makes sense and I still plan to use closed cell under dry sheathing. Also, thanks for the explanation on the new fascia. That also makes sense.

      One final question: Does the recommend advice for polyiso thickness above the sheathing apply in the case where there is a TPO-polyiso-sheathing-closed cell foam sandwich?

      I'm in Zone 4A and every roofer I've talked to uses TPO on top of 1" of polyiso regardless of interior insulation options.

      Thanks again!

      1. User avater GBA Editor
        Martin Holladay | | #13

        Mike,
        Q. "Does the recommend advice for polyiso thickness above the sheathing apply in the case where there is a TPO-polyiso-sheathing-closed cell foam sandwich?"

        A. The rules about the minimum thickness (R-value) for polyiso installed above roof sheathing always apply, regardless of the type of roofing that you intend to install. For a full explanation of code requirements for minimum R-values for polyiso installed on the exterior side of roof sheathing -- code requirements that align with the recommendations of building scientists -- see this article: "How to Install Rigid Foam On Top of Roof Sheathing."

        According to almost all building codes, if you install polyiso on the exterior side of the roof sheathing in Zone 4A, the polyiso needs a minimum R-value of R-15 -- equivalent to about 3 inches of polyiso. Installing only 1 inch of polyiso would be a code violation, regardless of what type of roofing (TPO, or something else) is installed.

        The relevant code provision is section R806.4 of the 2009 IRC (or later versions of the code).

        All of that said, if you add closed-cell spray foam on the underside of the roof sheathing in addition to a continuous layer of polyiso on the exterior side of the roof sheathing, then the R-value of the closed-cell spray foam can be added to the R-value of the polyiso to determine compliance with the R-15 minimum requirements.

        So if you have R-6 polyiso above the roof sheathing, and R-9 closed-cell spray foam under the roof sheathing, the combined layers of air-impermeable (foam) insulation would comply with the requirement to install at least R-15 of foam insulation. The remainder of the required insulation (either R-23 or R-34, depending on whether your target for total R-value is R-38 or R-49) would need to be some type of fluffy insulation installed in direct contact with the cured spray foam.

        [Later edit: Just because this type of polyiso / sheathing / spray foam sandwich is possible, doesn't mean it's a good idea. It's always better to install an adequately thick layer of rigid foam on the exterior side of the roof sheathing, rather than a layer of rigid foam that is too thin.]

        1. Mike Last Name | | #18

          Wow ok thanks again! I want to say I understand. I had to read the code a few times but it does make sense for the most part. I may still be missing the underlying reason that the code is written the way it is despite reading your articles and the IRC.

          I believe that you changed your advice between what you wrote in the above comment and the 2015 Article "How to Install Rigid Foam On Top of Roof Sheathing" where you stated:

          "Option 2: You can install some of the insulation above the roof sheathing, and the rest of the insulation underneath the roof sheathing (and in direct contact with the roof sheathing)."

          "If you follow Option 2, a wide variety of insulation materials can be installed under the roof sheathing. Among the possibilities: fiberglass batts, mineral wool, cellulose, or open-cell spray foam. (Closed-cell spray foam is not recommended for this purpose, since closed-cell spray foam prevents the roof sheathing from drying toward the interior if it ever gets damp."

          As well as:

          "One more caveat: If you are planning to thicken the insulation installed under the sheathing in order to achieve a total R-value that exceeds code-minimum requirements, you’ll need to also thicken your above-sheathing foam layer to keep the ratio of above-sheathing insulation to below-sheathing insulation in the proper proportion."

          In your comment in the May 2019 article you said I can get away with 1" polyiso, sheating, R-9 of closed cell (hitting R15 total) then following up with batting to get to R49.

          I can't use more closed cell instead of batting to get to R-49? (I might not have enough physical room for the batting vs closed cell which is higher R per inch)

          I'm still missing something. Sorry! You're a patient dude it seems!

          1. User avater GBA Editor
            Martin Holladay | | #19

            Mike,
            I stand by the advice I gave in the older article, "How to Install Rigid Foam On Top of Roof Sheathing."

            To summarize the advice:

            1. If you want to install rigid foam above your roof sheathing, make sure that the rigid foam is thick enough to prevent moisture accumulation in the roof sheathing. (The contractors in your hypothetical example who install only 1 inch of polyiso aren't following my advice.)

            2. Once you have installed an adequately thick layer of exterior rigid foam, the remainder of the needed R-value of your roof assembly should consist of vapor-permeable insulation installed in direct contact with the underside of the roof sheathing. Examples of vapor-permeable insulation include fiberglass, cellulose, mineral wool, or open-cell spray foam.

            In the answer to your specific question about a sandwich of polyiso, roof sheathing, and closed-cell spray foam, I explained whether and how the sandwich would meet code requirements. But I didn't recommend this approach.

  8. Mike Last Name | | #20

    Mike here again...I'm probably clogging up the board at this point, so feel free to delete if need be.

    I don't understand why it matters if the roof sheathing is below the dew point if it's entirely encapsulated by TPO on the top and closed cell spray foam underneath. The top surface of the sheathing may be below the dew point but it's not in contact with outside air or water vapor. Maybe it sneaks in from where the perimeter edges of the roof somehow?

    The ceiling rafters are buried into double wythe brick on both sides and the roof deck sits on top of them, encapsulated below with closed cell spray foam. The roof deck is on top of the joists and encapsulated above in a 1" polyiso TPO layer. (100 year old brick row house).

    Again, newbie here but reading a lot.

    1. User avater GBA Editor
      Martin Holladay | | #21

      Mike,
      If you are the type of homeowner or builder who has no qualms about building a roof assembly with roof sheathing that can't dry out in either direction, you can go ahead and sandwich your roof sheathing between TPO-covered polyiso and closed-cell spray foam. As I wrote in this article, builders have been doing it for years, and these roofs work.

      That said, what if you are the type of builder who thinks ahead, and comes to GBA for advice before you finalize your details? You might write, "I live in Climate Zone 4, and I plan to install polyiso above my roof sheathing. How thick should my polyiso be?"

      I would advise, "Choose polyiso that is at least 3 inches thick, to keep your roof sheathing above the dew point in winter. Then make sure that whatever insulation you install on the interior side of the roof sheathing is vapor-permeable."

      Evidently, you've already got a roof with polyiso that is too thin. (It's 1 inch thick.) That forces you to supplement the too-thin foam with interior closed-cell foam, and you end up with roof sheathing that can't dry inward. It's not ideal, but you've run out of options -- because your polyiso is too thin.

      1. Mike Last Name | | #24

        Thanks. I'm in the process of finalizing roof details and will install within a week.

        You wrote: I would advise, "Choose polyiso that is at least 3 inches thick, to keep your roof sheathing above the dew point in winter. Then make sure that whatever insulation you install on the interior side of the roof sheathing is vapor-permeable."

        Except open cell spray foam apparently as the advice on that has changed? So we're down to air AND vapor permeable options (fiberglass, mineral wool, cellulose) on the interior side of the assembly. Your article above says open cell is fine but your comments in this thread say to avoid it.

        1. User avater GBA Editor
          Martin Holladay | | #25

          Mike,
          When open-cell spray foam is installed on the underside of the roof sheathing, you can get problems with damp sheathing. (The sheathing gets cold in the winter, and the open-cell spray foam is vapor-permeable -- so interior moisture can pass through the open-cell foam by diffusion, reaching the cold sheathing.)

          Instead of allowing your sheathing to get cold in the winter, you can install an adequately thick layer of rigid foam on the exterior side of the roof sheathing. This approach keeps the roof sheathing above the dew point for most of the winter. The sheathing is now warm and dry. (In your climate zone, this approach requires at least R-15 of exterior rigid foam.) If the sheathing isn't cold, you won't have moisture accumulation.

          Once you've protected the sheathing by installing an adequately thick layer of continuous rigid foam on the exterior side of the sheathing, you are now free to install vapor-permeable insulation on the interior side of the roof sheathing -- for example, fiberglass, cellulose, mineral wool, or open-cell spray foam -- without worrying about moisture accumulation.

          1. Mike Last Name | | #30

            I finally get it. I'm kinda slow. Thanks!

  9. yavaid | | #22

    Hi Martin,
    This is Naveen. Have got Prelim-plan and had requested a cathedral ceiling(to have a peaceful marriage.. :P .. my preference is 9'(2X8) or 10'(2X8) ceiling with R-60 blown-in cellulose), ofcourse will seal the 2nd floor ceiling for air tightness. Since its going to be cathedral i was thinking of 14" truss and have 12" CC-SPF and will have Zip sheathing and have a Ventilation above Zip Sheathing with 1X2 purlins and install fibre based shingles. I am in Zone4a.
    The more i read and listen to Joe i think i get it, however when i read other papers and articles, i get worried. In short given that there will be roofers failing's... does the above roof system have any chance of surviving over a long period? in other words what is wrong here?

  10. User avater GBA Editor
    Martin Holladay | | #23

    Naveen,
    Please describe your roof assembly in more detail. Will your insulation consist of R-60 cellulose, 12 inches of closed-cell spray foam, or both?

    1. yavaid | | #26

      ok, Top to bottom this is the sequence of layer
      1. fibreglass shingles
      2. 1x2 purlins
      3. Zip sheathing
      4. 12" CloseCell spray Foam
      5. Drywall

      Thanks

      1. User avater GBA Editor
        Martin Holladay | | #27

        Naveen,
        I'm guessing that you forgot to mention a layer. You can't install fiberglass shingles on 1x2 purlins, so I'm guessing that there will be a layer of plywood or OSB between the purlins and the shingles.

        The type of assembly you are talking about is the same assembly mentioned by Mark Walker in Comment #2, above. Directly underneath Mark Walker's comment is my answer (Comment #4). If you read my Comment #4, I think you'll find the answer to your questions.

        Installing a vent channel above roof sheathing only works well if (a) the roof sheathing is fairly vapor-permeable (Zip sheathing isn't), and (b) the roofing underlayment is fairly vapor-permeable.

        1. yavaid | | #28

          So i would use fiberboard instead of Zip sheathing and then use OSB/plywood on top of 1X2 purlins to place the shingles? or should i go the route shown in the picture attached(seems less expensive)

          i am trying to build a house as tight as possible without going Passive house route, so i am kind of restricted in terms of checking truss regularly(to top it going for cathedral ceiling for family room alone. What would i have to be cautious about?

          Thanks

          1. User avater GBA Editor
            Martin Holladay | | #29

            Naveen,
            There are lots of ways to build an insulated cathedral ceiling. The way you have chosen is neither the least expensive, nor the easiest, nor the best performing, nor the greenest, way to build.

            For lots of information on all the different ways to create a cathedral ceiling, you might want to read this article: "How to Build an Insulated Cathedral Ceiling."

  11. yavaid | | #31

    @Comment#29,
    If i am not confused and clear after having read that and few other articles from JLC i wouldnt be asking the questions i have asked earlier.
    Question is what is the simple way to build unvented cathedral ceiling( i have a dormer for a 15' section of 45' ridge.), which is preferably green too.

    1. User avater GBA Editor
      Martin Holladay | | #32

      Naveen,
      Q. "What is the simple way to build unvented cathedral ceiling?"

      A. I think the simplest way is to install an adequately thick layer of rigid foam on the exterior side of the roof sheathing, supplemented by fluffy insulation (fiberglass, cellulose, or mineral wool) between the rafters. Ideally -- especially if you are a green builder -- you will use recycled (reclaimed) rigid foam. For more information, see "How to Install Rigid Foam On Top of Roof Sheathing."

  12. WESLEY YOUNG | | #33

    I have been reading many related articles and I am in a unique situation as I am converting a wood framed dirt floor Quonset Hut in climate zone 5 into a Workshop/small apartment. My plan was to tighten up the roof to prevent leaking and spray 4" closed cell spray foam the underside. I am starting to think it may be better to take the metal roofing off as i have to replace the nails with screws and butyl tape the seams and install a plywood sheathing on top of the pearlins. I feel like this might add a layer of protection between the roof and the wood frame of the building.
    At this point I was hoping to just add an underlayment on top of sheathing and reinstall metal roofing. I would at some point in the future like to replace the roofing with a nicer metal roofing but would like to get by with this for a while. I'm hoping this approach would work just fine with the right underlayment. Any suggestions?

    1. User avater GBA Editor
      Martin Holladay | | #34

      Wesley,
      It would be easier to give you advice if you asked a specific question.

      Most metal roofing can't conform to a curve, so that is problem #1. Curved metal roofs exist, but most roofers have never encountered one -- so you'll need to get advice from someone with relevant experience.

      Installing sheathing on a curved surface is also challenging. In some cases, you can't install 1/2-inch sheathing -- so you may be forced to install two layers of 1/4-inch sheathing with adhesive between the layers. This approach may raise engineering questions.

      1. WESLEY YOUNG | | #35

        Martin,

        Thank You for your response. I am not concerned with the ability to bend even 1/2 sheathing to a 19' radius so thats not really my question. My question is with 4" closed cell foam under the sheathing creating a vapor barrier what type of underlayment would you use to then install the metal roofing on top. Should the underlayment allow the sheathing to breath and if so would that still be good to repel water from the outside? Or should I use something that would seal the whole thing better protecting it from outside water intrusion Like adhesive backed water and ice type underlayment?

        1. Malcolm Taylor | | #36

          "I am not concerned with the ability to bend even 1/2 sheathing to a 19' radius"

          You should be. The structure of your building is an arch. The forces it is resisting are primarily outward. Adding the plywood, which wants to remain straight, significantly increases these forces.

          Most quonset huts rely on the metal structure itself as the structure. Before committing too far on the interior and insulation, I'd see if the existing roofing can be effectively made weatherproof. My guess is the difference in having to replace the existing roofing with new custom bended stock, and replacing the whole thing with a load-bearing metal hut wouldn't be that significant.

          1. WESLEY YOUNG | | #39

            Hello Malcolm,

            Thats a aspect I have not yet considered. I do however have a structural engineer that would be happy to go over that issue with me. I am thinking I may not need 1/2" for sheathing due to the width of the existing purlins the space between them is around 12". i am thinking of using 3/8" sheathing oriented horizontally in the direction it tends to bend anyhow. I have sheets of this sheathing on hand and find that it conforms to the arch of the building with little or no effort. If it is still a concern I would be fine with possibly adding some rafter ties maybe cables around 8' high from side to side to counter that. Any thoughts?

          2. Malcolm Taylor | | #40

            Wesley,

            3/8" will work like a charm. I wonder though - if you are going to add 4" of closed cell foam, what's the advantage of adding the plywood? A synthetic underlayment will both protect the roof and keep the foam from adhering to the metal. That might be all that is necessary.

            There is a good chance the building is fine as it is structurally. Monkeying with arches by adding ties might cause more problems that they solve. It's definitely worth a quick chat with an engineer.

          3. WESLEY YOUNG | | #41

            Malcolm,

            Now thats interesting! The building has been as is since 1942 when it was built. One of the issues I have is that they installed the roofing in opposite directions so that the ribs on either side didnt line up at the seems. The top is overlapping only by about 3" then they beat it down with a hammer to make it meet. I plan on addressing this by removing the roofing I think the panels are a little under 12' meaning its takes three pieces to get to the bottom of one side. I will take a section cut it in half for the bottoms and end with a section centered on top to eliminate the seam at the peak. Then butyl tape and screws. One of the reasons I wanted to use roof sheathing is to give the foam something to stick to besides the metal so it would be replaceable down the line. the other part is it would make the building more resistant to sway which it has a very slight lean to it and i'm not sure if thats been there since the beginning or happened over time. I guess I would question if just the spray foam alone would add enough rigidity to stablize it if it is starting to lean. And would the sheathing be worth it as an extra layer of protection in the sandwich?

          4. Malcolm Taylor | | #42

            Wesley,

            Thinking about it more I'm going to walk back on my comments about the advantages of a load-bearing metal structure. The wood arches and purlins provide a good base to support whatever insulation you choose, and as you say, filling in the voids with foam will add quite a bit of rigidity to the structure. Unless you are in a high seismic or wind area I'd bet it is enough on it's own.

        2. User avater GBA Editor
          Martin Holladay | | #37

          Wesley,
          Leaving aside (for the moment) the structural issues and the issues surrounding the need to bend the plywood and the metal roofing, the answers to your questions about "does the plywood sheathing need to breathe?" can be found in the article on this page.

          It's certainly better if the plywood can dry out in at least one direction, but that won't be possible if you install closed-cell spray foam on the underside of the roof sheathing. That said, people do it all the time, and the plywood is going to be fine as long as the plywood is dry on the day that the spray foam is installed.

          The vapor permeance of your roofing underlayment is irrelevant, because there won't be any outward drying. You can select any type of roofing underlayment you want.

          1. WESLEY YOUNG | | #38

            Thank you Martin that is helpful.

Log in or become a member to post a comment.

Related

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |