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Community and Q&A

Row House Roof: Insulation and Moisture Management

agm413 | Posted in General Questions on

Hi All, thanks for any forthcoming help and opinions! Zone 4, close to 5, NJ Possibly getting the chance to install a new TPO + Rigid insulation on the exterior deck of my row house flat roof in the coming weeks. The company I am working with typically does 2 layers of 1 inch poly iso with offset seams for r10 CI on the roof deck. If I want to add fiberglass below the deck, to avoid condensation is it safe to assume I need to stick around the r13 level of insulation to be safe and avoid condensation issues? Is it safe to go higher on the batt insulation at the risk of throwing the ratio of interior to exterior insulation off? I’d love to really fill in the rafter with something like r38, but don’t want to compromise the new decking or rafters. I could inquire about a 3rd layer of 1 inch, or two layers of 1.5 inch polyiso, but sometimes it’s may be best to stick with what they are most familiar with and do the best around that as to not compromise the installation.

As an update to this, the contractor got back to me with a quote including 1.5″ polysio on the roof deck, so now the quotes are as follows:

1) Mod bit rolled roof, no insulation – 10.6k

2) TPO over 3″ polysio (possibly with batts r19 under it, but need to have that quoted still) – 22k.

A pretty hefty upcharge for the extra scope. Would probably never get a payback on that, but would hopefully lead to a more comfortable second floor. The TPO is better for this roof since the slope is about 1-2%.

Main concerns now besides big price jump are: Thermal transfer through the party wall that makes exterior insulation less effective and I’d need to see a detail for insulating the underside of this large overhang (although it is on the north facing side, so least sun):

Not sure if people have any updated thoughts or experience between the two.

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  1. user-2310254 | | #1

    R-10 is insufficient. If you are assuming CZ5, you need at least 41% of the R-value on the exterior. If you are shooting for close to current recommendations (R-49 to R-60 depending on local code requirements), the contractor would need to install at least R-20 to R-25 on the exterior. The balance of the insulation on the underside of the roof deck can be fiberglass or cellulose.

    If you want R-38 in the rafters, you might want a bit more exterior foam to avoid potential moisture issues. For more on this topic, see

    1. agm413 | | #2


      Assuming they stick to the r10 as they planned, r13 is OK to do on the underside?

      Keeps it close to the 41% number that way

  2. user-2310254 | | #3

    II guess you have reservations about the contractor's proposal, or you wouldn't be posting on GBA.

    Let's have one of the experts weigh in.

    My non-expert opinion is that R-10 does not meet the minimum requirements for using rigid foam on the exterior of the roof in your climate zone. I suspect the roof sheathing will stay too cold during the winter and accumulate too much moisture.

    If this work is not done correctly, there is a risk you will have to replace the roof, insulation, and sheathing in just a few years.

    1. agm413 | | #4

      I am in a similar boat that I thought 2 inches for r10 was a bit low, but also interested to see if any experts have an opinion to either a) get to at least r15, or if r10 exterior +r13 interior is okay.

      1. Expert Member
        BILL WICHERS | | #5

        It's really the RATIO of inside to outside R values that is important here, since it's that ratio that determines what the temperature of the sheathing will be. Colder sheathing generally means wetter sheathing, which is something you want to avoid. If you can't exactly hit your target ratio, it is safer to have extra R value on the EXTERIOR side, which will mean WARMER sheathing.

        Allison Bailes wrote a good article about this stuff that discusses the recommended ratios that you can read here:

        I would try to go a bit more conservative on those ratios for extra insurance, and shoot for a 50/50 ratio in CZ5. That means half your R value on the exterior. If you're really right on the edge between CZ4 and CZ5, then the 40% (exterior) to 60% (interior) is going to be pretty safe too, so you might consider that if it ends up being cheaper to implement.

        With the 40/60 ratio, you need R20 of your total R49 worth of R value on the exterior of the roof sheathing. R20 worth of polyiso is just over 3 inches worth. The usual way to get ther would probably be to use two layers of 1.8" "roofing" polyiso (1.8" thick is a common thickness for roofing polyiso) for 3.6" total and a little over R20 of total R value.

        You contractor's proposed use of two layers of 1" polyiso is about R12, and really isn't enough. Two layers of 1.5" polyiso is about R18, which is much better, and probably sufficient for your CZ from the tables, but shooting for R20 or better is safer still, especially considering that you're almost in CZ5.

        Either CZ needs R49 total, so whatever is left needs to be made up on the underside of the roof. If you want to use R38 batts on the underside, then you need just over R25 on the exterior side to make the 40/60 ratio, and R25 worth of polyiso is about 4" (two layers of 2" polyiso will get you R26). Remember that the RATIO is important here for the mositure safety of the sheathing, but code sets the MINIMUM TOTAL R value for the assembly.

        One last tip: BE SURE your contractor STAGGERS the seams of the polyiso sheets. Joe Lstiburek of Building Science Corp learned that lesson the hard way, and wrote an article about it.


        1. Expert Member
          Michael Maines | | #7

          Bill, the 2021 IECC/IRC required R-60 in roofs for zones 4 and higher. I don't know NJ requirements though.

          1. Expert Member
            BILL WICHERS | | #17

            I don't think that's been adopted in many places yet, certainly not my area, but it is advisable that the OP check hist locale.

            R60 is going to complicate these types of assemblies where there is limited rafter thickness. I'll have to check if we can still save on thickness with continuous insulation by going with the assembly U value calculation instead of the "add all the rated R values" calculation.


        2. agm413 | | #9

          Thanks for the feedback Bill. I think your post sums up about where I am at with everything.

          Luckily the contractor had a lot of other details down like staggered seams, etc.. so last thing is figuring out how much insulation I can get them to do on the outside. Hopefully more than 2 inches !

  3. Expert Member
    Michael Maines | | #6

    In terms of resistance to moisture accumulation, the ratio of impermeable to permeable insulation matters a lot more than the total R-values. Like Steve said, in CZ5 you need at least 41% of the total R-value in the impermeable insulation.

    Because this more of a long-term durability issue than a code compliance issue, I use conservative values for long-term R-value; in the case of polyiso, R-5/in, which it appears you are already assuming. It's safe to increase the ratio of impermeable-to-permeable, but not the other way around. In other words, more foam is ok, more fiberglass is not. There are many examples of this rule being broken and the assembly doing just fine, but they often have leaky envelopes or direct sunlight can reach black EPDM and drive moisture back into the interior. I avoid foam when I can for environmental reasons but this is not something I think is worth taking any risks on.

    Depending on the scope of work, the building department might require you to meet current code requirements for total R-value, and you'll certainly see reduced energy bills if you can do better than R-23 in your roof. If you can upgrade to 3" of polyiso, at R-15, you could use R-21 batts at the interior for a total of R-36. That shouldn't be a big upcharge to reduce heat loss through your roof by an additional 50%.

    To safely use R-38 at the interior, for CZ5 your foam layer would need to be at least R-26.4, or 5.5". That would give you a code-compliant R-60+ assembly.

    If you are stuck with 2" polyiso on the exterior, another option is to use a combination of closed cell spray foam and fiberglass on the interior. Just make sure it's HFO-blown foam for somewhat reduced environmental impact (and improved performance) over legacy HFC-blown foam.

    1. agm413 | | #8

      Thanks for the comment !

      Since the roof is I'm the "flat" category I think there is more leeway in total r value for code compliance, but right now there is only r13 fiberglass just above the ceil9ng, so it isn't well insulated by any measure.

      Moving to the r10 (maybe more, will have to talk to them ) exterior r13 Batts will be a nice improvement.

      Luckily they are pretty detailed for all the other aspects of the installation like offset seams, etc. Just trying to nail down what is best for the insulation, and if they'll do the 2 layers of 1.5 or 2 inch polyiso instead of the two 1 inch layers.

      Only downside of the interior spray foam is dealing with the removing/replacing the interior finished ceiling.

      1. Expert Member
        Michael Maines | | #12

        There may be a local ammendment but I am not aware of anyplace in the IRC that allows a reduction in required R-values for low-slope roofs. There is this section: which allows a reduction to R-30 over 500 sq.ft. if certain conditions are met.

        I didn't realize you were doing all of the work from the exterior. It's best if the batt insulation is in direct contact with the roof sheathing. I'd suggest using faced insulation and putting the facing upward, at the top of the rafter bay.

        1. agm413 | | #14

          Much appreciated. I'll have some good conversations with them in regards to total r values and the ratios between the rigid and fiberglass.

          And yes they are doing a full roof replacement here, new decking and all, so good chance to start from scratch.

    2. agm413 | | #24

      HI Michael,

      As an update to this - they got back with the 2nd quote, so now the quotes are as follows:

      1) Mod bit rolled roof, no insulation installation - 10.6k

      2) TPO with 3" polysio (waiting for them to add in r19 batts to the quote) - 22k.

      seems like a hefty upcharge, but obviously the scope is better, The roof is also very low slope - probably between 1-2%, so that is another reason they recommended the TPO vs Mod bit rolled roof.

      2 worries about the expensive route with the foam: Diminished performance from no insulation on the party wall between units and there may not be insulation installed on the underside of the large overhang, which you can see here or attached:

  4. agm413 | | #10

    Thanks for the feedback Bill. I think your post sums up about where I am at with everything.

    Luckily the contractor had a lot of other details down like staggered seams, etc.. so last thing is figuring out how much insulation I can get them to do on the outside. Hopefully more than 2 inches !

  5. gusfhb | | #11

    The additional price for twice the insulation one would think is not a huge percentage of the price of the job.

    1. Expert Member
      Akos | | #13

      I'm with Keith. When getting this type of work done, a bit of extra rigid adds very little to the overall job cost. Going from 2" to 4" is no extra labor. The cheapest extra R value you can add to your place.

      1. agm413 | | #15

        Thanks for the extra opinions.

        Will definitely get the different price ideas for them. And agree that most of the cost is labor here, not the foam board.

  6. Jon_Lawrence | | #16

    I am in NJ CZ 4 and I have a flat roof section with exterior insulation. Mine was a new build so we followed code and I have a minimum 5" of polyiso at the edge of the roof, more at the far end because the drainage slop is made using all polyiso. The ceiling joists are filled with dense pack so my minimum ratio is about 35%.

    I assume since yours is a "renovation" you may not need to follow new construction code (check with you local inspector), but as others have mentioned, the cost of additional rigid insulation is not that much more, will reduce you heat losses, and will give you the flexibility to add additional insulation on the interior in the future if you decide too.

    How are you planning on draining your roof? Was there a slope built-in when it was originally framed or is there a center drain?

    Fluffy insulation in an unvented flat roof is code violation in our zone unless you have the proper ratio of exterior to total insulation. Did you demo yet? Any signs of rot on the roof sheathing.

    1. agm413 | | #19

      Good to hear from a fellow NJer.

      The roof has a very slight pitch to the back. In the 1 to 2% range from what they told me. There are gutters in the back of the house.

      Right now the only insulation is r13 Batts on the attic joists, so not against the roof sheathing.

      Not sure what they said about the sheathing, but the plan is to replace the sheathing, so it is a full tear out and replace.

      1. Jon_Lawrence | | #20

        So if you are going to replace the sheathing, you will be able to fill the cavity with mineral wool, which I would choose over fiberglass because the former will hold water in times of high humidity and the latter will wick it towards the sheathing. Just make sure your ratio of exterior/total r-value is correct.

        2% is standard slope for a gutter drained flat roof. In my case we built the slope with the polyiso. I bought my polyiso from Allied Supply. If you send them the schematic, they will design the sloped layout including crickets if needed. The panels are 4'x4' and labeled with a letter that matches the schematic so it is easy to know where to place each panel.

        1. agm413 | | #21

          Thanks for the info. I'll have to wait for their newest estimate and go from there - they'll likely price it out with their standard r10 rigid foam and I asked for too much interior insulation without telling them to up the rigid foam.

          They suggested a TPO system over a rolled mod bit system since the pitch is pretty low.

          What did you use and has it been good/worked out?

  7. Deleted | | #18


  8. AntonioB | | #22

    Some thoughts -
    1 - R10 + R13 is definitely underinsulated although the ratio in your area would probably work as far as condensation most of the time. However when you have your coldest periods, you still risk condensation because it will simply be difficult to keep such a low insulation roof assembly warm enough.
    2. But why go through the trouble and expense of all that work and NOT get the most insulation value you can? You're already paying for a large part of the labor. It's just a question of additional materials.
    3. I would shoot for R60, with R30 rigid on top and R30 batt between the joists. If you use tapered insulation to build your slope it's probably safe to go with an R30 average rigid over the whole roof surface, rather than considering the R30 a minimum.
    4. Regarding polyiso ... keep in mind that the R6.5 per inch and higher numbers that get tossed around do NOT reflect the significant loss in R value that polyiso experiences over time. Realistically, in a matter of 5 years it can be down to R5.5 per inch, which puts it near the R5 that some EPS insulations achieve. And no, EPS does not lose R value over time. It's stable. Using polyiso and R6.5 for your condensation calculations could result in problems a few years out, once the polyiso has degraded down to an R5.5.

    1. agm413 | | #23

      Appreciate the feedback - I am currently waiting for some of the updated quotes from the contractors to see what things will pan out to be as far as costs go, but you are definitely right that material costs are far lower than the labor and other flat costs (dumpster, just showing up with a crew, etc) since the roof is only 650 sqft.

      I've been using r5 for the polysio calculations to be conservative as well.

      I definitely agree that r23 is low for the climate zone - it is just what sort of ends up being the default "safe" assembly based on them using the 2 inch polysio.

      the toughest part of it all is since the house is fairly small and inexpensive to heat/cool for the most part, it's really just the abstract value of not having the upstairs be very hot or cold depending on the season and being comfortable on the 2nd floor.

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