Hello, I'm building a new garage that will have a portion to the side that is conditioned space. The entire area will be under a flat roof. The roofing contractors have specced a 1/2" fiberboard layer over the roof deck followed by the EPDM membrane in the section that will be conditioned and a ballast system over the garage proper. These are both non-vented roofs. My initial plan is insulating the underside of the deck cavity with insulation levels required for my climate zone. Including a layer of ccsf insulation followed by cellulose fill. My question is whether there is a potential for the "sandwich" of decay. Or as long as the seal on the underside of the deck is complete is this possibility negated? I have read but I just wanted to be sure. Is another possible strategy to fill the joist spaces with cellulose followed by a layer of polyiso continuous over the bottom of the joists on the inward side? The first would guarantee a good seal, the second would remove thermal bridging on the interior/conditioned side and possibly allow for some moisture loading in the event that the membrane fails? Don't really want to vent the flat roof, have no room above to heavily insulate on the exterior (existing windows). Should I stick with #1 for ease of implementation and not worry about the thermal bridging? THanks, Garret

Hot, unvented roofs often have an internal layer of CCSF with cellulose under that. One difference with your roof is the use of fiberboard instead of an Advantech type sheathing which has somewhat more moisture resistance. So I don't know, but it would make me concerned.
Another issue is - don't forget to thermally isolate your concrete floor from the exterior and from the garage.

It's common with flat roofs to put rigid foam insulation between the roof deck & EPDM sufficient to allow fiber insulation between the rafters with only a class-III vapor retarder on the interior to allow the roof deck to dry toward the interior. With foam above the roof deck you get an excellent thermal break. You say it'll interfere with windows, but how much room do you have to work with (and what climate zone?)

Hi,

Thanks for the responses. I'm in Region 6 (Minneapolis). I would like to go with a layer of rigid above the roof deck but below the EPDM, however, this is a renovation (replace roof and transform garage to living space) and I have an existing bank of windows that is only 4" above finished roof (epdm) level. This is already an issue with blowing snow and rain. The joists are 2x8 with tapered strips to provide the pitch. The overall headroom will be 9'6" or so from floor to joist. Since I have the limit above, I was looking at options below, I am ok with eating up some of that headroom.

I thought I could place strips of rigid on the joists and then mount drywall, not ideal but maybe another option for reduction of bridging.

With an inch or two of ccSPF on the underside of the roof deck and fiber cavity fill you can get away with a lot, even with OSB decking. (see: http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems/view ) You'll still want the interior rigid foam to be somewhat permeable though. An inch of XPS is in the 1-1.3 perm range, and at 2" it's roughly the same permeance as a kraft facer- a class-II vapor retarder so you probably don't want to take it that far.

But you'd be fine with up to 3" of unfaced type-II EPS, which would run ~R12.5 @ ~1-perm