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Roofing insulation

Manny_Lourenco | Posted in Energy Efficiency and Durability on

Hey guy, I I’ve read so much that I thought I had it down pal, after posting a question I think I’m even more lost now. So this leads me to my roof. The good thinking is that the house is not built yet so I can still do what is needed. I will have a flat roof with 1/4”per foot slope. I have 14” roof trusses which give me plenty of options for interior insulation. So I know I must keep the sheathing warm, so I must get the right ratio of exterior insulation to interior insulation correct??? so  that the interior moisture won’t  condensation on the interior plywood. I’m in zone 4 can some one help clarify please…

I was going to do two batts of rockwool R-23 for a total of R-46, 3/4” sheathing, two layers of  staggered 2” EPS or XPS with out the foil facer for a total of 4” and then 3/4” roof deck with a white cool roof membrane . But I was told that it much insulation on the inside is to dense and should add more to the out side which would make it better at keeping the sheathing warmer. I think I understand some what, however can someone please help clarify thanks again. Also the attic is finished with drywall and will be conditioned . Thanks again

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Manny.

    You are right that there is a ratio of exterior foam insulation to interior fibrous insulation and the ratio helps to keep the sheathing warm and therefore dry. I believe that you have a good ratio in mind, but the IRC calls for R-49 in Climate Zone 4. You should look into what R-value your local building inspector wants you to hit. These roofs dry inward, which also makes mineral wool a good choice because it is vapor open. Remember, air sealing your drywall ceiling is also necessary. Here are some articles that will help you understand all of this:

  2. Expert Member
    Dana Dorsett | | #2

    >" two batts of rockwool R-23 for a total of R-46, 3/4” sheathing, two layers of staggered 2” EPS or XPS with out the foil facer for a total of 4” and then 3/4” roof deck with a white cool roof membrane ."

    First, a cut'n' cobbled foam UNDER the roof deck is almost certainly going to eventually create a failure somewhere in the roof deck from air leaks around/through the foam. It's far better to put the foam ABOVE the roof deck, where the roof deck is quite air tight, and minor convection through the roof deck and foam to the membrane roof doesn't come with a structural rot risk.

    For an under-deck solution it has to be fully adhered spray foam to be sufficiently air-tight over the long haul.

    The IRC prescriptives for zone 4 require at least 30% of the total R be either air-impermeable insulation (read "closed cell foam) on the underside, or above the roof deck. But with a high SRI cool roof the roof deck runs subtantially cooler in winter, so it absolutely needs to be more than that! Shoot for at LEAST 40% and up to 50% wouldn't necessarily be insane.

    With 4" of 2lb roofing polyiso (not EPS, not XPS) above the roof deck you'd be at a labeled R23 above the deck. Derated for wintertime temps it will be good for at least R20, which would allow you to install up to R30 rock wool under the roof deck.

    If you filled the full 14" cavity with 1lb blown fiberglass or 3lb cellulose you'd have about R50 under the roof deck, and would need at least 4" of roofing iso to hit the code prescriptive, but in all seriousness even 5" is probably not enough to adjust for the cool roof effect- 6" is safer.

    If going with closed cell foam under the roof deck, 3" of HFO blown closed cell foam (R20-R21) and 11" of cellulose or 1lb fiberglass (R40) would meet the prescriptive, but 4" HFO blown foam (R28) and 10" of fluff (R37) would be better to avoid wintertime moisture accumulation in the fiber insulation.

  3. Manny_Lourenco | | #3

    Dana Dorsett

    Never a cut and cobble. Under the first roof sheathing there will be two layers of R-23 for a total of R-46, then the first roof deck then 2 layers of 2” staggered extruded polystyrene (the pink foam) for a total of 4” then there will be another 3/4” roof sheathing and then the roof. In essence a sandwich but on the outside. The rockwool will be on the inside.

  4. Manny_Lourenco | | #4

    Here is a picture

    1. Expert Member
      Dana Dorsett | | #13

      A picture is worth WAY more than the fewer than 1o0 words description of the stackup! :-) (The ambiguity of calling the nailer deck a "roof deck," and the structural roof deck being called "sheathing" was throwing me off.)

      The R20 (R18 warranteed) climate-damaging pink stuff is barely going to cut it for dew point control at the structural roof deck layer with on R46 fluff in zone 4A., even if there WASN'T a high SRI cool roof.

      The chapter 7 IRC prescriptive for zone 4 is a minimum of R15 out of R49 total, which is about 31%, and with a cool roof even that isn't going to be enough. R18/R64 is 28%. If you go that route it will absolutely need an air tight, smart vapor retarder on the interior.

      If you go with 4" of 2lb roofing polyiso (blown with low impact hydrocarbons) instead of the pink stuff you'd have a labeled R23 out of R69 total, for 33%, which might be just enough, but I'd still recommend a smart vapor retarder behind the ceiling gypsum. A truly expert WUFI analysis of the stackup done by a competent engineer would be able to tell for sure if the vapor retarder was needed, but it's probably cheaper to just install the vapor retarder. (2-mil nylon is pretty cheap stuff.)

  5. Jon_R | | #5

    > another 3/4” roof sheathing

    This will be cold and at higher risk of rot (due to vapor diffusion and never perfect air sealing). Be conservative and use non-wood (eg, gypsum) cover board or eliminate it completely.

    > staggered 2” EPS or XPS

    If you use XPS, you should donate about $10 per 2" sheet towards some offsetting carbon reduction (which makes it non cost effective). Also consider fiberglass faced polyiso. And recycled foam.

    +1 on using de-rated R values and not building to code minimums.

  6. GBA Editor
    Martin Holladay | | #6

    Everything you need to know is in this article (one of the ones suggested by Brian):
    "Combining Exterior Rigid Foam With Fluffy Insulation."

    For this type of roof In Zone 4, a minimum of 31% of the total R-value of the roof assembly must come from the rigid foam layer. In your case, the total R-value of your roof assembly (nominal R-value) is 43+20= 63. Since R-20 is 31.7% of 63, your plan is fine.

    1. Expert Member
      Dana Dorsett | | #14

      He's installing R46, not R43 under the roof deck, and the pink stuff is only warranteed to R18, which is about where it will perform after the first decade or two of service.

      And, it's under a high SRI cool roof, which is going to reject most of the help from solar heating to dry out the structural roof deck.

      R20/R66 is 31%, the absolute bare minimum per the IRC prescriptives, which don't take into account the high solar reflectivity.

      R18/R64 is 28%, which doesn't even meet the prescriptive ratio.

      Definitely NOT fine!

      1. Manny_Lourenco | | #18

        Dana thanks for all your reply’s I’m coming to the conclusion that I’m going with Polyiso, or maybe semi rigid rockwool, up to an R-25 which is 35%( didn’t realize how bad XPS was for the environment, reconsidering even the basement slab insulation.)

  7. Jon_Lawrence | | #7

    Manny - get rid of the XPS (pink foam). Use roofing polyiso instead. You can create a vent channel on top of the polyiso so that cold 3/4" plywood can dry out if needed. I used this product:

    I spec'd plywood instead of OSB and 2" of polyiso. We added a separate layer of polysio first to get 4" with staggered joints. We ran out of the ThermaCal panels about 3/4 of the way through the job and the lead time to get more would have been 6 weeks since the plywood faced panel was a custom order. We decided to make our own using 1x3 for the furring strips and if I had it to do over again, this is the way I would have done it.

    So on top of the roof sheathing I would go with: 2 layers of 2" polyiso, 1x3" furring strips attached to the rafters using 7" HeadLok fasteners (just use 2 or 3 fasteners per board), then 3/4" plywood secured to the rafters with 8" HeadLok fasteners 16" OC (these go through the furring strip). Since polyiso does not compress like ComfortBoard, you don't need to pre-drill anything to get the fasteners to sit flush and not as important on a roof anyways.

  8. Manny_Lourenco | | #8

    Martin thanks for the replay does it make a difference on what percentage is on the inside vs the outside. I’ll have the R-43 on the inside and the R-20 on the outside. And will the outer plywood sheathing covering the polyiso be prone to wetting like the one on the inside? I thought that if anything got wet it would be the roof membrane and not the plywood or the polyiso on the outside? Same for the one inside if not warmer?

    1. GBA Editor
      Martin Holladay | | #11

      Q. "Does it make a difference on what percentage is on the inside vs the outside?"

      A. Yes. I urge you once again to read my article, "Combining Exterior Rigid Foam With Fluffy Insulation," where these details are discussed.

      In your climate zone, you need a minimum of 31% of the total R-value of the roof assembly to come from the rigid foam layer installed on the exterior side of the roof sheathing. The remaining insulation -- 69% or less of the total R-value of the roof assembly -- can be installed between the rafters, on the interior side of the roof sheathing.

      Q. "Will the outer plywood sheathing covering the polyiso be prone to wetting like the one on the inside?"

      A. No, because it is separated from the warm interior air by an air barrier. The warm interior air wants to condense or accumulate on the first cold surface it encounters, not on the sheathing installed on the exterior side of the rigid foam.

      Q. "I thought that if anything got wet it would be the roof membrane and not the plywood or the polyiso on the outside?"

      A. To avoid dampness, (a) you need roofing that doesn't leak -- this prevents wetting from the exterior, and (b) you need to keep the first condensing surface facing the interior warm enough to prevent moisture accumulation or condensation.

      1. Jon_R | | #16

        > warm interior air wants to condense or accumulate on the first cold surface it encounters

        Warm interior air will condense/accumulate on *all* sufficiently cold surfaces that it encounters. More condensation/accumulation will occur as it moves outwards and encounters colder materials.

      2. Manny_Lourenco | | #19

        Martin Holladay, that’s for such a great article, I didn’t realize that I had to sign up to get to read the full article, I must of been sleeping, lol. Now I’m signed up for the year. It like the world of information opened up. Thanks again. Thanks

    2. Expert Member
      Dana Dorsett | | #15

      >"I’ll have the R-43 on the inside and the R-20 on the outside."

      Still nowhere NEAR enough for a cool roof (even worse if using XPS for the R20.) Go for at least R25 on the exterior with R43 in the cavity (a 37% ratio) if going with a cool roof up top.

      1. Manny_Lourenco | | #17

        Yes I have come to that conclusion after signing up for GBA and reading the full article by Martin on combining foam with fluffy stuff”. Thanks again to all. Now I have to figure weather to use XPS or Polyiso. I know the polyiso doesn’t preform so well in the cold and would have to add more of it or I’m debating using polyiso for the first 4” and then a XP’s to cover it all for a total of 6”. I’m going to post a separate question on installing my parapet wall over the insulation so that there is no cold spots, around the perimeter of the roof.

  9. Manny_Lourenco | | #9

    Johnathan yes I have actually looked at these panels, only thing is I have a flat roof and it will not be vented, also since it is a flat roof the driving force to vent the under sheet is harder to accomplish, is your roof flat? Did you add a vent at the bottom and top.

  10. Manny_Lourenco | | #10

    Johnathan how much was each panel and where did you pick them up?

  11. Jon_Lawrence | | #12

    Manny - I saw your detail of a gable roof and figured that is what you were enquiring about this time. Flat roof is a little different. Above the sheathing you have your base layers of polyiso with the tapered layers on top to create the slope, then a coverboard and then your membrane. In my case we secured the base layers of polyiso with screws and plates, the tapered layer, coverboard and membrane with adhesive. I never really got a good answer about vapor retarder above the sheathing, but my sheathing for the flat roof is taped Advantech, which is low perm and airtight. All of my roofing materials either came from Allied or ABC, including the ThermaCal.

  12. seabornman | | #20

    Any commercial work with a membrane roof I have done simply places all of the insulation above the roof deck without any coverboard. The cost of the extra rigid insulation (not much more labor to lay down a thicker piece) is usually less than the fiber insulation and labor below. Makes it real easy to run systems below the deck and air sealing is not required. Look up the next time you're in a Chili's restaurant.

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