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Community and Q&A

Flat roof assembly question

JTuretsky | Posted in GBA Pro Help on

I am replacing the roof on a brick masonry building in NYC within the 5 borough. The building was built in 1910/11. It is a flat roof, about 40 ft long and 25ft. wide. The previous roof (a tar and gravel system) failed and my brother and I removed it, and replaced the damaged roof deck. (a mixture of plywood replaced probably 30-40 yrs ago, and the original t&g boards) with 3/4 OSB. Toward the front 10 foot or so of the building the t&g boards were dry and strong so we left them in place. A little bit of repair to the cornice “box” was necessary. It is a hollow, wooden cornice that hangs off the front, with a framed out box behind it over which the roof membrane will wrap and come up to flashing on the top of the cornice.

The roof deck is framed with 2×8 joists, resting in pockets in the parapet wall. These joists are incrementally higher than the last and so on, to form the pitch of the roof. Below that level are 2×10 timbers, also in brick pockets, that are level and form the ceiling joists. So in the back the two levels of joists are quite close and in the front are about 2 foot apart. Previously there was a 2×4 and drywall dropped ceiling installed over (or if we are standing right side up, then under) the old plaster and stuffed with fiberglass batt. I now am aware that this created a condition in which the un-vented “attic” space above was cold and allowed condensation of rising air on the bottom of the roof deck. In the process of removing the old roof, we completely removed the entire ceiling and plaster and everything below. It is open and accessible from above and below. Aside from giving us a great deal more height, and making the windows look less cramped to the ceiling, we got 100 years of garbage and dust out of there.

The new roof system we have already purchased is a Firestone peel and stick 60 mil EPDM, and 2 inch poly iso foam insulation (also by Firestone). Here is my current cross section after researching matters.

From Top Down:

-60 mil EPDM Rubber membrane
-Single layer of 2″ ISO board (R12)
-3/4″ OSB (in the front 10′, 3/4 t&g pine)
-Original 2×8 ROOF Joists, convinced (perhaps scabbed out with strips of wood) to accept a deeper R38 insulation.
-Attic space, unvented
-Ceiling joists and sheet-rock ceiling

The second (top) floor is an apartment, the first floor is a garage we use for a shop. Hence my desire to augment the R12 ISO with fiberglass batts.

Here is my question, where do I put my vapor barrier? My best guess would be between the ISO board and the OSB, but 2 sources have recommended a smart vapor barrier under the fiber glass closest to the living space. This slightly confuses my admittedly novice comprehension of the assembly. What is my room for error on this, if I put in the wrong place or neglect to put it at all? Am I even in the right ballpark??

Could anyone offer any advice? As I said, the entire assembly is accessible from above and below at the moment. I can do anything within reason of cost. (I would like to avoid having someone come to do spray foam or blow in some fill if I can buy a material and install it myself, hence the fiberglass batts, but if that’s just wrong then I will do it right.)

I would like to use the ISO boards and do this “warm roof” approach because it seems to be less risky than venting the flat roof and I already purchased the material. Some unrelated information: we have not yet decided, but are entertaining the idea of using XPS to insulate the brick from the inside (just in the upstairs apartment), since I do not wish to cover the facade. But this is a ways away and hopefully will not impact the immediate decision.

Any input would be so much appreciated.

Well insulated regards,
Jacob Turetsky

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  1. user-2310254 | | #1

    I'm sure Dana and Martin will comment, but I suspect you don't have enough polyiso on the exterior since that material loses r-value over time and at temperatures under 50 degrees. So two inches is more like r-9. Have you considered using reclaimed foam? It is cheap(er) and would allow you install a thicker layer.

  2. JTuretsky | | #2

    Hmm, i should read into this. How much more iso are you thinking? I was taking my cues from the following article. R10 is recommended for my area. Is this degradation something that needs to be accounted in a chart like this? First i am hearing about this degradation.

    Thanks for the response though.

  3. Jon_Lawrence | | #3


    If the polyiso is foil faced, it is a class 1 vapor barrier. But even if it isn't, the EPDM sure is. Your roof has no potential to dry to the outside. The risk here is that moisture from inside gets into the roof cavity and there is not enough insulation on the outside to keep the OSB warm enough to avoid condensation. You want to keep moisture out of your roof, but you also want it to dry to the inside if moisture levels get high in the cavity. That is where the smart vapor retarder comes in.

    In addition, you want to make sure that the first condensing surface is not the OSB and in order to do that you need to make sure your ratio of exterior to interior insulation is correct. I believe the 5 boroughs are all Zone 4 and not marine and if that is the case then your roof needs 31% of the total R-value on the exterior. I agree with Steve that R-12 nominal polyiso won't get you there. Also, have you thought about DP cellulose instead of fiberglass. Check out a recent BSC analysis of various materials in non-vented and vented assemblies for more info.

  4. user-2310254 | | #4

    Jacob. Insulation Depot is in Bath, New York, but there may be someone even closer who could provide enough reclaimed foam for you to put all your insulation on the exterior. Your assembly would dry to the interior and be much safer from a moisture-management point of view. If you are not too far along, it would be worth considering and perhaps pricing out.

  5. GBA Editor
    Martin Holladay | | #5

    The article to guide your installation is this one: Insulating Low-Slope Residential Roofs. As the table in that article clearly shows, the minimum R-value for the rigid foam layer in your climate zone (Zone 4A) is R-15, not R-10. (R-10 is an optimistic estimate of how 2 inches of polyiso performs at cold temperatures.)

    If you choose to use polyiso for the rigid foam layer, it's wise to de-rate the R-value of the polyiso to R-4 per inch or R-5 per inch, because it has performance problems in cold weather. So you need 3 or 4 inches of polyiso, at a minimum. You are aiming for a roof assembly with a total R-value of R-49. So if you include R-15 of foam, you'll need an additional R-34 of some other type of insulation under the roof sheathing.

    The EPDM rubber is a vapor barrier. You don't need another vapor barrier in this type of assembly.

  6. JTuretsky | | #6

    Hey guys, thank you to all for the great response. I was not aware of the degradation effect needed to be accounted for. Today I will be purchasing another layer of 2 inch polyiso. This will bring my total R value including degradation to R-16 or R-20 (4-5 per in.) whatever internal insulation I go for will be R-29-33.

    As for the recycled foam I am interested in this, I wont be able to do it for this immediate use but have other work coming up and would like to find a more local supply. Although, knowing Brooklyn, they might just call the foam "vintage" and charge double.

    Thanks for the answer on the vapor barrier too.

  7. Jon_Lawrence | | #7

    "Although, knowing Brooklyn, they might just call the foam "vintage" and charge double."

    And those Brooklyn hipsters will buy it!

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