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Polyiso foam board under rafters when using closed-cell: Deciding between two approaches

matt2021 | Posted in Energy Efficiency and Durability on

The ceiling for my future four-season room will have to be spray foamed.  (I would have greatly preferred a different approach, but it gets too complicated with my town, not to mention the problem of finding a contractor willing to follow the alternative approach.) So, I will have 5.5″ of foam between the 4×6 rafters, which are 24″ O.C.  I would still like to mitigate the thermal bridging through the rafters by applying a polyiso board to them.  I am debating between the following two approaches:

(A) Continuous foam board and 5/8″ sheetrock fastened to the rafters with longer screws.  (At least some of the contractors I have been talking to seem to think that no furring stripping be needed.)

(B) Strips of insulation nailed to the rafters only, leaving the space between the rafters free.  (Dana Dorsett was suggesting a similar solution in a ceiling filled with open cell: https://www.greenbuildingadvisor.com/question/rigid-under-rafters)

Here’s what I like about the second option:

– Maybe, in a real-world application, not having to worry about the layer (foam, or for those who do not consider thermal bridging, sheetrock) that will be running across the rafters means that one can tell the spray foam installer to truly fill cavity by the full 5.5″, as protrusions will just be occupying part of the gap.

– Extra room for the installation of (wafer-style) recessed lights.

– Cheaper, as I won’t have to buy boards sufficient to cover the whole ceiling area, in a situation in which maybe (MAYBE) I don’t have much to gain from the added insulation (see above about having a 5.5″ cavity that truly is filled with closed-cell foam.

I wonder what folks think:

– Better have the full layer of foam, as for (A)?  If so, really furring is not needed?  
– Can (B) work without furring?
– Especially if I need to use furring strips, I would prefer using 3/4″ foam boards (or strips of foam boards), as I have little height at one end of the room, and I don’t want to lower the ceiling too much. 

All things considered, which of these options is the best, in terms of energy efficiency as well as of the soundness of the resulting ceiling? (I am not crazy, of course, about the rafters being 24″ O.C. Without furring strips, I’d be using 5/8” drywall, with furring strips 1/2” one.)

Thanks!

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