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.
- Is this a reasonable/acceptable plan? Meets code? Energy efficient? Other?
- 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?
- 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.
- 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?