Flat roof assembly question
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,
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