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1. Expert Member
   Akos|May 15, 2020 09:42am|#1

   Working with foam over 3" is a pain, as the screws get longer, it gets difficult to hit the rafters bellow. Very fussy work especially if you want to keep the structure exposed. Long screws also tend to be pricey and all that foam also costs a fair bit, batts are cheap.

   I recently did a studio roof with exterior insulation only, but used 2.5". There, it was easier to insulate on the outside and keep the old wood ceiling exposed then to insulate on the inside and have to drywall.

   P.S. when going with either all foam or hybrid approach, you are better with complying on U factor basis for insulation. This works out to around R38, with any foam insulation, the ROI for going up to R49 is practically never.

1. 
   Jason Hoetger|May 15, 2020 04:17pm|#2

   Thanks, your comment about "fussiness" really put things into focus for me. I definitely want to avoid a high level of fussiness. Complicated + novel is a recipe for mistakes.

   If I do more than 3" on top, I was thinking of laying down 2x4s halfway through, as suggested in a few other Q&As, to reduce some of that fussiness. But reflecting more on your comment, that would still be a pretty complicated installation; I would probably have to carefully walk the installers through it.

   2" of polyiso (hopefully reclaimed, so I'm derating to 5.5) plus R30 batts (9.5" cavities) should meet the U-factor alternative and reduce thermal bridging. If cost/benefit analysis shows a few more R would be beneficial, I can add polyiso or switch to dense-pack cellulose or open cell foam on the interior. Gives me some additional flexibility and seems to be a cost-effective solution.

   Thanks--your comment was just what I needed!

2. 
   Quinn Sievewright|May 17, 2020 05:37am|#3

   What are your interior ceiling finishes and what will be your roofing material? I've done a number of these types of roofs and might be able to offer some advice.

1. 
   Jason Hoetger|May 18, 2020 01:57pm|#4

   Metal roof, drywall on the interior. Thanks, any advice/insight would be great!

3. 
   Quinn Sievewright|May 21, 2020 04:16am|#5

   Hi Jason
   In which case I would go hybrid. I've done fully exterior, cavity only and hybrid. As Akos touched on because you won't have an exposed timber interior ceiling it makes things easier.
   I am not a building scientist expert by any means but my reading on here suggests this is safe, and as somone who's built a number of these the process and practicalities seam to work.
   Layers from interior to exterior:
   1) Drywall airtight, minimize can/pot lights unless duly sealed to vapour retarder
   2) Smart vapour retarder like Membrain
   3) XX" of rockwool (depth depends on you maintaining the required ratios). A note, I have found flat roofs for only exterior rigid foam to be rather loud in our thunderous west coast rain, Rockwall is an excellent sound cushion.
   4) Ply roof sheathing
   5) Fully adhered vapour barrier (used frequently on commercial flat roofs). I think (someone please correct me) this isn't strictly needed from a building science point of view as you can tape the foam insulation seams and/or the plywood sheathing. But practically speaking, it can be applied pretty quickly and fully waterproofs your building until the overlying layers are completed. Or get your insulation down quickly and tape the seams, that will be pretty water tight. Or buy good quality flashing tape like 3M, G Tape, or the European brands and tape your plywood seams. But if it rains you need to wait for the ply to dry thoroughly before applying the insulation.
   6) 2 x thin (1-3") layers of rigid foam insulation or one thicker layer (again depending on complying with ratios and target R value. Can be attached with commercial insulation washers, or tile baker board washers with screws. I don't think you need to worry about hitting your joists here.
   7) At that low slope your metal roof is likely to require a fully adhered underlayment, see their spec. Flashing this well at the eves
   8) 2X4 spaced according to your metal roof spec and spaced to so screws hit the underlying ceiling joists. If you only have a few inches of foam insulation this is pretty easy. Yes you'll miss some. I use a mix of standard 6" treated screws (to save money) and large headed type structural screws like GRK and Fastenmaster. Make sure the head is countersunk.
   8b option) I have used 1x4 instead of 2x4 as strapping as well. I found it a bit uneven and had to play with screw tightness and shim to get flat. Then overlaid with 1/2" ply then the underlayment, this approach allows the metal roofing to sit directly on the ply and roofing screws to be applied anywhere.
   Hopefully somewhat helpful!