Insulation for flat roof over minimally-heated garage – GreenBuildingAdvisor

I’m building a shop-on-top-of-garage in a 6B/5B climate zone (my county was changed in the last decade from 5B to 6B, but my particular location is probably closer to 6B with 30/15F average high/low in the coldest months, and hot arid summers). Due to a few design constraints, I will have a low-slope (.5:12) roof over a high-ceiling section of the garage terminating at a head wall transition where it meets the shop wall. Excepting from some major excavation and foundation work (out of the budget), eliminating the design constraints would reduce functionality considerably.

We’ll have a mechanically-seamed metal roof rated for the low slope. The roof will have snow sitting on it ~3 months a year. I’ve done enough reading to know this is a minefield of problems, with moisture potentially coming from below and above and barriers to drying on both sides.  I’m still bouncing between a hot roof and a ventilated roof, but I’m leaning more towards ventilated due to the possibility/likelihood that the metal roof will leak at least somewhat, and it needs a path to dry. I hope to mitigate moisture problems in the following ways:

• The garage won’t be heated to 68F. While the garage is an “unheated” space, 70% of the walls of the garage are below-grade (it’s basically a daylight basement), and 70% of the ceiling of the garage will be the (insulated) floor of the shop. I’ll be using insulated garage doors, putting insulation in the ceiling, etc, so it will inevitably be warmer than the ambient air. I’m aiming for 50F.

• I’ll use peel-and-stick ice/water shield under the roofing and up the wall of the head wall transition, and hire a roofing contractor experienced with the product.

• I’ll undersize the ceiling insulation (e.g. 7-1/4” batts in the 2×10 cavities) to allow more ventilation space.

• I have an HRV in my house that I’m going to replace with an ERV, so I will have a “free” HRV I can use to dehumidify the garage (I expect the garage to get relatively humid after parking a snow-covered vehicle in it).

• I’ll use a “smart” vapor barrier like intello plus, and make sure it’s well sealed over the entire ceiling.

• I’m considering mechanical ventilation, but I haven’t read much positive information about mechanical ventilation and I’m having trouble imagining a effective implementation short of ventilating each cavity independently.

The main draw of a hot roof or SIPs is that we live in an area with great wildfire risk. I will be using ember-resistant vents everywhere, as well as mineral wool insulation, but eliminating vents entirely is safer in that regard.

My main question is whether I’m taking the right approach, what should I consider doing instead/differently, if I’m overthinking this, or whether there’s likely to be an issue with either approach and I really need to bend on design constraints. There’s not a lot of information about design for minimally-heated spaces.

The roof over the (heated) shop will be a more reasonable 2.3:12 pitch, still mechanically seamed. With more potential for vapor condensation from the heated shop and less potential for a leak in the higher-pitched roof, I’m planning on using a well-sealed impermeable poly barrier on the ceiling of the shop (under R-38 batts in 2×12 cavities), and relying on the improved natural ventilation through the higher roof pitch. Maybe I should spring for the intello there as well?