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The context of this question is new construction for a small (28' x 32') outbuilding that will be used for commercial purposes. All work will be permitted and inspected (governed by these codes). I am using a general contractor that will complete the work up to the point of an enclosed shell; I will be managing the completion of the interior of the building (e.g., wiring, plumbing, insulation, drywall, etc.); there are good reasons for this that are not relevant to this question (he is a neighbor that is doing me a favor).

Aside from some aesthetic choices (that I am not allowed to question), we are trying to keep the budget as small as possible.

The plan for finishing the interior of the building includes leaving the trusses exposed (no drywall on the ceiling), for aesthetic reasons that I am not allowed to question. The bottom chords of the trusses and everything above will be spray painted black.

It is my understanding that this plan pretty much requires spray foam between the top chords of the trusses (on the bottom of the roof surface). The building is located in New Hampshire and is subject to lots of winter.

The roofing plan is low-end rib steel panels (ordered to length) in one of the basic colors (that I am not allowed to question).

The contractor tells me that his plan is to place 2x4 strapping 2'OC directly to the trusses, with the metal attached directly to the strapping. No sheathing.

The insulation plan would be to spray the several inches of foam on the bottom of the metal and strapping. There would be no venting. Foam will be continuous from top plate to top plate.

Questions:

  1. Is this a reasonable/acceptable plan? Meets code? Energy efficient? Other?
  2. Does this mean the insulation needs to be replaced if we ever need to re-roof? In other words, if we ever remove the metal, does the insulation come with it? Is it possible to re-roof over the metal without removing it?
  3. Do we need to worry about issues with moisture/rot (e.g., is the strapping going to rot if moisture gets around a screw after the metal has expanded/contracted thousands of times)? The screws have a gasket on them, but I have seen big oval screw holes on old roofing that I acquired for a chicken coop.
  4. Are there reasonable alternatives that we should consider?

One final concern relates to the actual installation of the foam insulation (which will be done by a insulation contractor, not DIY). The building will not be ready for insulation until the middle of December, which means that exterior temperatures will be solidly below freezing. It is my understanding that spray foam must be applied in warmer temperatures. I expect that as soon as a heater is turned on inside the building, we will see significant condensation on the bottom of the metal roofing and I doubt the roofing itself will get very warm, unless the sun is shining on it (the peak of the roof runs basically east/west).

Can foam be applied to cold metal? Or will the metal warm up enough that only the moisture is a concern? Is moisture a problem for the foam during application? Do the insulation guys have magic ways to mitigate any/all of these issues?

  • Have you considered a panelized compact metal roofing system? (They're like Structural Insulated Panels, but for metal roofing -- a bit more expensive in parts, but the labor savings of not having to do on-site spray foaming and whatnot might be worth it?) – ThreePhaseEel Nov 5 '19 at 3:47
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Questions:

Q1: Is this a reasonable/acceptable plan? Yes & no...more below.

Meets code? You'll need to run all of this by your local/city inspectors/building code enforcement folks to get the answer.

Energy efficient? Roofing-wise, I believe it is. Several inches of foam will be quite efficient--but that may be undone by what your other choices are for walls/floor and the open ceiling 'choice'.

Q2: Does this mean the insulation needs to be replaced if we ever need to re-roof? YES. In other words, if we ever remove the metal, does the insulation come with it? YES. Is it possible to re-roof over the metal without removing it? Possible I guess, but I have never seen this done in my locale. It could be problematic and costly.

Q3: Do we need to worry about issues with moisture/rot (e.g., is the strapping going to rot if moisture gets around a screw after the metal has expanded/contracted thousands of times)? YES..for the exact reason you mention.Those gaskets do wear out and relatively quickly.

Q4: Are there reasonable alternatives that we should consider? One thing for sure I would do differently--to mitigate future loss of the insulation investment--use plywood decking over the strapping onto which the metal can be fastened. OR you might consider using a standing seam metal roof with no exposed fasteners. This would be very costly, but would mitigate some of the concerns you mention (and would look GREAT). I assume your use of 'reasonable' is mostly from an economic point of view. SS metal is most likely a bridge too far, but plywood decking is not in my opinion-- about $2200 material to protect probably $6-12K in insulation investment (depending on actual thickness and type). Exterior walls material and insulation choice will also determine the overall energy efficiency of the building. Personally, if I could not afford SS metal, I would use 3/4" plywood roof sheathing--not OSB. I hate OSB because of sagging potential and it rarely ever snows in my locale. I am certain that is a big concern in your neck of the woods. You should ask your insulation contractor about the install temperatures/etc. You may have to wait for spring or a very sunny stretch this winter. Good luck!

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    Thanks for the answer! – Rob Nov 5 '19 at 3:00

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