GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Insulating shiplap vaulted ceilings

Laura62584 | Posted in General Questions on

Hello!  Ive been reading alot of Q&A posts on here as Ive been renovating my house and most recently doing research on best methods for insulating my non vented vaulted shiplap ceilings.  Originally the house had shiplap ceilings with a wood shake shingle roof allowing any moisture to seep out wherever it pleased.  A couple years ago I ripped off the many layers of old (leaking) roofing when I bought the house (wood shake with 1/2” ply over it and two layers of asphalt 🙄) and installed new 19/32” ply over the shiplap, peel n stick and my new Taylor Metal Roof.  Now Im insulating to try and keep the house more comfortable.  My rafters are roughly 3x6s spaced anywhere from 16 to 30” apart, mostly 22”ish.  I really didnt want to drop my ceilings at all so polyiso was the decision I made with spray foam around it.  I was originally going to do solid polyiso between the rafters but recently decided to go with 2” polyiso against the shiplap, Rockwool R15 to fill the rest and a layer of 1”polyiso attached to the bottom of the rafters for thermal bridging.  All in all giving me R34 which is more than code here.  All seams will be taped and any gaps spray foamed as I go.  I was even going to do a separate vapor barrier if needed on the inside of the 1”.  Im using foilfaced RMax thermasheath.  My question is that once I get it all sealed up and I go to put my shiplap ceilings in underneath all this, how do i keep the screw/nail holes holding the new ceiling to the joist from leaking vapor/air?  Thats alot of penetration into the foil facing/vapor barrier.  Im in Los Angeles CA so moist air is not a huge concern for the most part here, but Im paranoid about a rotting roof deck.  Im up in the higher elevations too and we get snow in the winter as temps dip to high 20s.  Our zone is 9a/b I think.  Thanks for any helpful tips!


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Akos | | #1

    The plyiso between the rafters doesn't buy you much as the high R value is short circuited by the thermal bridging of you rafters.

    Polyiso bellow the rafters is a good thing, this is where you want it.

    Also the cut and cobble solution you are proposing can create issues as it is impossible to reliably air seal the foam. It just not the same as spray foam.

    For the rafters you are best to go with fluffy insulation or open cell spray foam with a vapor diffusion vent at the ridge. You can read about it here:

    It works well in your climate and avoids trapping moisture inside your cathedral ceiling.

    Because of the random rafter spacing, SPF might be the simplest.

  2. Laura62584 | | #2

    My roof is unvented, thats not changing. Im putting the 1” RMax under the rafters to stop the thermal bridging. And Rockwool is going between the rafters just under the 2”. My question is how do I stop air/vapor leakage due to screws/nails that penetrate the vapor barrier of the Iso and any additional vapor barrier I put up? Im putting shiplap up under the 1” Iso.

  3. Expert Member
    Peter Engle | | #3

    Laura, As mentioned above, the 2" polyiso between the rafters, up against the T&G is not the best idea, as its R-value is largely bypassed by the thermal bridging and air leakage around the panels. When you put the 1" polyiso under the rafters, it turns into a truly bad idea, as you have vapor barriers (the foil facings) top and bottom in the cavities, creating a moisture sandwich. If any water finds its way in there, it never dries out and that can rot the rafters. In your dry climate, vapor barriers are probably not needed on inside or outside of the assembly. Many spray foam contractors would be willing to do the whole roof with open cell spray foam, trimmed back to the surface of the rafters. Vapor diffusion vents may or may not be necessary, depending on several local factors, and could be added later if necessary. If you wanted to bump up the R-value and reduce thermal bridging, I would use EPS on the underside of the rafters, as it is vapor permeable. I lived in LA. Even up in the hills and in winter, it would be considered a dry climate. You don't really have to worry about roof condensation issues until you're up in Wrightwood, or anywhere else above 4'000-5'000 feet or so. Sure you get some cold and snow, but it's warm(ish) and dry most of the time.

  4. user-6184358 | | #4

    You are correct in applying the rigid against the t&g ceiling. It is used for condensation control. It needs to be R-5 min or R-10 based on your CA climate zone. See the Ca building code section 1203.3 Interior environment. The comments above are true in that in a wall application rigid foam between the studs is of no advantage, due to the thermal bridging of the lumber. I would prefer the rigid foam the way you are doing it - in that the spray foam won't have a chance of being done improperly and creating a toxic mess.
    The best way to prevent air movement into your new ceiling is to install drywall and tape it then install your tongue & groove ceiling. The boards will leak air into the ceiling at the gaps between boards not the nail heads. Second best would be a fabric or plastic smart vapor barrier.

    1. Expert Member
      Akos | | #7


      For condensation control, the rigid insulation needs to be ABOVE the roof deck. Installing it underneath is not the same thing.

      The reason the insulation needs to be above the roof deck is for it to keep the sheathing above dew point and avoid condensation. If it is on the inside, the sheathing will stay too cold and any moisture from air leaks can condense on it causing roof rot. Cut and cobble is impossible to air seal properly, so there will be air leaks and condensation, but because the foam doesn't allow for enough drying, it will create a moldy mess in no time.

    2. Laura62584 | | #8

      Thanks Tim. My concern wasnt so much with the gaps of the T&G leaking air because i know whats behind them will be completely vapor tight, I was just worried that air could move down the shank of the nail or screw. Probably minimal if it did, but just a concern I thought id ask about. Im taping all my screws with foil tape that holds my foam up. Ill think about the drywall but is that necessary with a complete vapor barrier already in place?

  5. user-6184358 | | #5

    Search this site on how to insulate a cathedral ceiling.

  6. Expert Member
    Akos | | #6

    Some of the discussion went down a couple of paths. To clarify your options are:

    -diffusion vented ceiling with batts in between the rafters with rigid insulation underneath. The foam/drywall is your main air and vapor barrier and the ceiling can dry towards the outside through the diffusion port

    -unvented roof with batts or SPF with drywall and a smart vapor barrier on the inside. The assembly can now dry towards the house interior. You can also put a layer of rigid insulation on the inside but it needs to be permeable (unfaced EPS or permeable polyiso), it can't be a faced product.

    -unvented roof with rigid insulation on top of your shiplap with batts/spf between the rafters. This is generally the most robust and energy efficient setup, but is probably not an option since you have a new roof.

    Cut and cobble in your case could be risky assembly as it is neither air tight or permeable enough work.

  7. Laura62584 | | #9

    Ive done SOOOO much research, Ive read every article, every post about every opinion on how to insulate an unvented cathedral ceiling. There are ALOT of different opinions on what works and what doesnt. From the information available Ive drawn my best conclusion on what will work for me. I dont have the option to foam board under my newly installed $20k roof or I would. I read a how-to from Johns Manville on foam board between the rafters for unvented ceilings and its a credible source. That coupled with the seemingly reliable opinions of people all over the internet, this should work. My 1” foam attached to the bottom of my rafters should be my main vapor retardant. I was just worried about the fasteners that hold my finished ceiling puncturing it.

  8. Expert Member
    Akos | | #10

    I wouldn't trust manufactures recommendations, their main goal is to sell as much product as possible, even when not suited for the application.

    My go to place is BSC ( These folks get paid to build and monitor test assemblies and houses. You can search through their site for information about unvented roofs. Here is a good one:

    The assembly you are suggesting creates a vapor barrier sandwich with your roof sheathing in the middle. The roof will most likely fail.

    1. Laura62584 | | #11

      If I was creating a sandwich with my roof sheathing in the middle that would be the ideal situation because that would mean I had foam on the roof deck and interior of the house. That would be best case scenario but unfortunately I was talked out of putting foam on the sheathing by my terrible roofing contractor.

      In this article it explains how to put the foil faced foam boards in between the rafters and spray foam them into place. Im going a step further and putting an inch of uncut 8ftx4ft 1” thick foam on the underside of my rafters to make the area even more impenetrable to moist air.

      Im mostly worried about keeping moist warm air in the winter away from the shiplap and plywood so it doesn’t condensate. Ive had minimal condensation over the last two years where my skylight is. Not on the glass but what I Suspect is between the shiplap and plywood. Its only been a few times and it only happens when i leave the heat on over night in the winter. With the insulation im putting up my hope is that there is no way for that warm air to reach my boards enough to cause a buildup of moisture. Ive got a friend who is doing the same method on his ceilings that arent vented and hes gotten the ok of the building inspector on his. So Im pretty confident its not going to fail. I wasnt going to put up drywall but I may do that now after reading about the benefits of it. When I redo the outside of the house I may leave a small air gap at the eves to allow any moisture that could possibly get through to dry out, but theres not a sizeable gap between the shiplap and the foam. Definable not an air channel that would act as a vent. I had thought about putting in a 1/4” spacer but was talked out of it.

      1. Expert Member
        Akos | | #14

        Vapor barrier sandwich is a bad idea.

        The common example would be a headlight with a small crack in it, like you commonly see on cars with fogged headlights. The enclosure of the headlight is a vapor barrier in this case. Even a small crack allows a bit of water to leak in but because the shell is vapor tight, it can never get out. Eventually this process fills the light with water. The same thing will happen with the roof as the foam is never truly air tight.

        The only way to make it work is to allow for a bit of venting. This would mean spacing the foam slightly off the roof deck and adding either a vapor diffusion vent at the ridge or installing soffit to ridge vents. Code calls for 1" gap but even a small gap of 1/2" works provided your ceiling is air tight.

        You can build whichever way you want, the only thing I can say is the unvented assembly from JM is bound to fail.

        1. Laura62584 | | #15

          Unfortunately the diffusion vent on the ridge isnt a possibility for me. Id have to redo my whole roof.. I get the vapor sandwich being a bad idea, my original plan was iso board filling the cavity and the 1” underneath the rafters.. i read an article on someone else doing theirs this way and they were concerned with the foil facing on each layer of iso trapping air/vapor. But they were told its not a problem because the lowest layer of foam is the vapor barrier and the entirety of the cavity is filled with foam. Anyway, I just recently decided to go with the rockwool in between the layers of foam because I have a bunch of it.. and its easier to work with. So Ill go with an unfaced vapor permeable layer of 1” foam with a vapor retardant on the inside which will allow any moisture that gets into the cavity to dry to interior. Now Im concerned mostly about the ridge. And ive been looking at the diffusion vent idea.. i wont be able to do that on the outside, but im wondering if i can somehow do it so that any moisture that gets in can dry to interior. Im thinking if i stop my foam boards shy of the ridge (a 2x4s width because thats the width of the center wall) fill that area with rockwool and use the vapor retarder to regulate the moisture that may get up in there then it would at least have a chance to dry if it gets wet.. Not sure that would be the most energy efficient but short of not insulating at all I think its my only option at this point.. I dont get why people keep saying to use spray foam but that foam board and spray foam around it wont work. The can foam is one-part vs 2-part poly foam kits but its still vapor tight. And done right should keep moisture away from the deck the same as any poly foam kit could. I get human error and such, Id never trust a contractor to do it thats for damn sure, but Im going to seal it up tight myself.

          1. Expert Member
            Akos | | #16

            A vapor sandsiwtch is fine with closed cell spray foam against the roof deck, anything else is trouble.

            I recently added a ridge vent to my garage, pretty straight forward. Adding a permeable membrane (ie DELTA-FOXX) under the new ridge cap would not be that much extra work.

            For best result, the BSC folks recommend a diffusion vent with a smart vapor retarder on the interior (ie MemBrain).

  9. user-6184358 | | #12

    Your description indicated a peel & stick under your metal roof. Do you know what brand and if its vapor permeable?

    1. Laura62584 | | #13

      Ive checked with the roofer and its mostly vapor impermeable. Im now debating if I should leave the 1/4” gap between the foam and the shiplap and open that area up at the eves when i do the exterior of the house. But i keep reading how the foam should be up against the shiplap.. theres so many different opinions.. I just dont know who to trust..

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |