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Community and Q&A

At my wit’s end: Flat roof insulation

Tommy57 | Posted in GBA Pro Help on

I’m absolutely frustrated and am hoping there is someone who can help me with my current project…I’m a Paint contractor, but this is my own house. I have a 3-season room in a 1930’s home in Northern NJ(I believe Zone 5)that I’m converting into a full-time heated/ac room.  The room is 12′ x 16′. The ceiling is 8′ high, 2″ x 6″(true 6″ not 5.5″) rafters and is a flat roof with no attic. The roof is a black rolled asphalt? I don’t know the technical term I’m assuming it’s asphalt. My issue is how to correctly insulate the Ceiling. I’ve spent what seems like hundreds of hours online and had a few insulation contractors look at the room, and I can’t believe the inconsistencies that exist on this subject. I’m extremely frustrated. I just want to insulate this ceiling without creating any moisture/mold issues. I know for a fact the roof does not have any external insulation. I believe it consists of only the sheathing, a roofing paper type layer and then the rolled asphalt. The roof is only 4 years old so removing it is not an option. The room does not have any venting, nor is it possible to vent….Before anyone suggests closed cell spray foam I’m ruling that out. I have no confidence in the “healthiness” of the product. It might be a great product, but I don’t want it in my house. As I stated earlier I’m completely frustrated, and would sincerely and humbly appreciate anyone’s help on this subject.

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  1. rhl_ | | #1

    You need sufficient exterior insulation, typically rigid foam, but I guess you could use other stuff too. In Zone 4A you need R-15 if you are in Zone 5 you need at least R-20. Minimum code value is R-49. So the balance the R-29 needs to be on the inside. You can add more to the outside if you want. However if you add more to the inside you need to maintain the ratio of outside/inside prescribed by the above (code) values.

    1. Tommy57 | | #9

      Thank you for replying Ryan. Ripping the roof up to add exterior insulation is just not an option. What's your opinion on this: Making the rafters 7 inches deep and then adding (2) layers of R-15 ROXUL for a total of R-30 in the ceiling. The drywall will be my air barrier and the primer/paint my vapor barrier.

  2. Expert Member
    Akos | | #2

    Lot of the older 2.5 story houses with flat roofs aroud here (Toronto Zone 5) are insulated and vented as per diagram 1 :

    Typically it is 2x6 construction for a slight slope with 1x6 bellow for the drywall. Usually you can fit 5.5" batts in there and still have a decent vent space above. In your case you might want to go with 3.5" batts.

    This is not ideal but seems to work OK provided there are major holes in the ceiling like too many pot lights.

    Insulating from above with rigid is much better solution but means re-roofing.

    1. Tommy57 | | #8

      Thank you for trying to help me out Akos. I see example #2 best fits my situation. Unfortunately the "insulation space" in the diagram doesn't state how deep it is. I'm wondering if me stuffing 7 inches of ROXUL in the space would be sufficient in preventing condensation on the roof sheathing?

      1. Expert Member
        Akos | | #13


        #2 is risky no matter the depth of insulation batts. It only works with closed cell spray foam.

    2. Peter Yost | | #16

      Hi Akos -

      can you tell me where this image came from? Can't really tell from the link.

      Thanks - Peter

      1. Expert Member
        Akos | | #18
  3. GBA Editor
    Martin Holladay | | #3

    If you don't want to install spray foam insulation, your only option is to install rigid foam above the roof sheathing, along with new roofing. For more information, see "Insulating Low-Slope Residential Roofs."

    1. Tommy57 | | #4

      Thank you for replying Martin. This is unbelievable in that 3 different insulation contractors(all reputable) gave me 3 different opinions. First guy said, R-19 FG insulation with baffles. Second guy said, 1 inch gap at sheathing followed by 2 inch rigid foam followed by an inch gap space followed by a second 2 inch rigid foam board against the drywall. Third guy said, he'd spray 2 inches of cc spray foam and call it a day...This is why I'm so frustrated! I don't want to waste your time, but how many inches would you recommend of cc spray foam? And also, would making the rafters an inch deeper to add 2 layers of ROXUL (r-15 each layer for a total of r-30) make any sense. The drywall would be airtight, there isn't any recessed lighting only a ceiling fan and a central air vent which I can easily seal tight

      1. Expert Member
        BILL WICHERS | | #6

        If you can’t vent the space, a gap between the insulation and sheathing won’t help and will probably hurt. If someone suggested a ventilation space in a non-vented ceiling assembly then they probably don’t understand the physics of how the assembly actually works.

        In a non-vented “hot roof” assembly, you need to keep moisture away from the sheathing. About the only really safe way to do that is with closed cell spray foam. I have almost the exact same roof system you do in my sunroom. The roof is very shallow pitch, non-vented since there is no way to vent it due to other construction details, and adhered rolled roof on top. I have about 6.5” of closed cell spray foam which gives about R38 and a vapor barrier, as well as perfect air sealing. I’ve had no problems with it. It would have been cheaper to use less spray foam and batts to make up the difference (the spray foam is your vapor barrier and air seal), but I wanted the time savings of having the spray foam contractor handle it as a “once and done” project.

        My spray foam contractor was very good. I spent some time talking to him about some of the problems I’ve heard about with some installs. He said the temperature and pressure of the two foam agents in their gun is critical to getting a proper mix and chemical reaction when the two agents hit the surface they’re spraying. He constantly adjusted these things throughout the job as he was applying the foam. I had zero issues with improperly cured foam anywhere. I think you’ll find that if you use an expierienced spray foam contractor you won’t have any problems.


        1. Tommy57 | | #7

          Thank you Bill. What happens if you develop a roof leak? Where does that water go? Does the spray foam block it from entering the rafters?

          1. Expert Member
            BILL WICHERS | | #11

            You’re pretty much screwed if you get a roof leak. The water won’t be able to get out, will pool up on top of the spray foam, and soak into the sheathing. You need to make sure your roof is properly flashed and sealed EVERYWHERE.

            Don’t worry too much about leaks. Leaks always cause problems. Just make sure that the roof is sealed before you apply spray foam. I waited a month or two between roofing and insulating and watched very closely with a light (a bright work light like you’d use to check drywall finishing) every time it rained. I had other work to do so the waiting didn’t affect the project.


      2. GBA Editor
        Martin Holladay | | #12

        Q. "how many inches would you recommend of cc spray foam?"

        A. The answer is in this article: "Flash-and-Batt Insulation."

        1. Tommy57 | | #14

          Hmmmm. I appreciate the insight. I'm not trying to be difficult or prove anyone wrong I'm just trying to understand this. CC spray foam on the underside of the roof just doesn't make sense to me if it's going to hold water in the case of a roof leak. I'm still not understanding why tightly stuffing the 7" rafters with R-30 worth of ROXUL, where there would be no air flow, wouldn't be sufficient. I'm thinking if the drywall, my air-barrier, is air-tight then how is air going to enter the rafter cavity? Couple that with vapor-retarder paint to minimize vapor entering the rafter cavity.

          1. Expert Member
            Akos | | #15


            Lot of venting and roofing is a bit squishy.

            Unvented roofs with batts sometimes work.
            Vented roofs with batts mostly work.
            Sprayfoam or exterior insulation always work.

            One option is to go with unvented batts, if you have problems down the road add in the required amount of rigid insulation on the top.

            Your call.

          2. JC72 | | #17

            "I'm still not understanding why tightly stuffing the 7" rafters with R-30 worth of ROXUL, where there would be no air flow, wouldn't be sufficient. "

            Answer: R-30 by itself doesn't meet code. Code for the entire roof assembly is R-49. Nobody around here is going to suggest a route which clearly doesn't meet code. That is entirely your choice or the building inspector.

            "CC spray foam on the underside of the roof just doesn't make sense to me if it's going to hold water in the case of a roof leak"

            Answer: "Big leaks are always going to get noticed" **


          3. FrankFulton | | #22

            "One option is to go with unvented batts, if you have problems down the road add in the required amount of rigid insulation on the top."

            Akos, please say more about this approach. Are you suggesting that you could "encapsulate the mold" at a later date, if needed, as Joe L suggests? Or, do you think there is a risk of having to tear down the ceilings and foam anyway, a few years down the road?

          4. Expert Member
            Akos | | #23


            I haven't done this so I wouldn't recommend it, it was just one option.

            My thinking was that with a well sealed ceiling with a smart vapor barrier and with a forgiving decking like 1x6 boards, the amount of condensation on the roofing would not be large enough to cause issues and could dry towards the inside during the cooling season. Lot of ifs there.

            If it goes wrong, would mean pulling the roof off replacing the rotten decking and batts underneath, installing the correct amount of foam for condensation control and re-roofing. Sounds like a pain to me but could be an option for someone adventurous.

    2. charlie_sullivan | | #5

      I wouldn't quite say that's the only option--there's also the option of a complex venting system with a cupola added so there's some height difference in the venting path, as described in the article you link. But as you note there, it's hard to do right and is rarely done right.

      Edit: and that would also render the foam on top of the roof useless, so you'd need the insulation below to be thick enough to do the whole job.

      1. Tommy57 | | #10

        Thanks Charlie. I don't think I have the confidence in finding someone in my area to do that correctly.

  4. Tommy57 | | #19

    I appreciate everyone's advice. I just refuse to use cc spray foam. In my opinion it's the next "asbestos". No offense to anybody who uses it or believes in it. I'm sure it does the job it's suppose to, but I just have a gut feeling on the long-term unhealthiness of the product in a living space. Exterior insulation on top of the roof sheathing will require me to rip the roof up and re-do, which is out of the question. If these are my only 2 options then I'd rather leave the room as a 3-season room and accept defeat.....Like I stated earlier I'm a Painter not a Builder, etc, so if some of my replies on roof/ceiling assemblies seem to not make sense forgive me I'm just trying to understand what is obviously a very uncertain and inconsistent science....So I'm still trying to figure out how to insulate this ceiling. Does anyone have an opinion on 2 layers of 2" rigid foam (seams staggered) for a total of 4"(R-20 value), and then R-15 ROXUL between that and the drywall? I understand it does not meet code, but will it suffice? Does the 4" of rigid foam mimic what 4" of cc spray foam would do? Would it make more sense to put the rigid foam flush with the drywall as opposed to the roof sheathing? Still trying to understand.

    1. Expert Member
      BILL WICHERS | | #20

      If you’re thinking of cutting the rigid foam and gluing it between the rafters, then no, it isn’t safe. We call that “cut and cobble”, and it’s only safe with vented roof assemblies. With your unvented roof, a cut and cobble install will never seal well enough so you’ll end up with moisture getting to the underside of the sheathing which is bad.

      I really think you should reconsider your stance on spray foam. As long as it’s installed properly, the cured foam is a very stable material. Worst case, it would be something like the old lead paint — safe unless you disturb it. Even asbestos is considered safe if it’s encapsulated and not disturbed. From my years of working in the field, spray foam problems are very rare and always due to inexperienced installers. There are far more successful installations than there are problems, but the people with problems always talk about it which artificially inflates people’s perception of the amount of issues.

      Ultimately it’s your decision, but spray foam really is the only option for you that’s known to be safe long term, unless you want to replace your roof and put rigid foam on top. Two years ago, I was in the same place you are today. I used spray foam after finding no other options. I’ve had no problems with the installation.


      1. Tommy57 | | #21

        Thank you Bill. What you say makes sense.

  5. ryan_kohl | | #24

    I guess I'll be the guy to suggest adding XPS to the top of your roof. Add the requisite thickness of insulation in conjunction with probably R19 batts within the 2x6 joist space. The insulation would have to be weighed down, so you'd need to look into whether that is feasible, and the perimeter of the roof would need some alteration, but if you are looking to keep the roof, and avoid mold problems, thar be your answer.

  6. Premier_Building_Sup | | #25

    None of you three options from reputable companies meet unvented attic codes. unventeted attics must meet code R-806.4 ( I am not at my office at the moment so the code reference number might not be correct). Even if a company is reputable doesnt mean they understand all the codes. There are a ton of great companies that specializes in adding attic insulation to existing attics but have no clue on how a unvented attic should be insulated. It is possible but not easy to meet climate zone 5 energy code requirements with 6 inches of unvented attic space. Every home is different and if i had more information maybe i can help you with a solution that we can prove meets the current energy code to the local buildiing inspector. My first question is what city and state is the home located in? 2nd question is do you have a full set of plans for the home? Also do you k know the r-value for the rest of the home. Attic and walls

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