Roof insulation for office shed
I’m in zone 5 and am building a 10×12 office shed anchored to a 12x14x4″ insulated concrete slab. The unit will have a small mini-split for heating/cooling. There will be zip sheathing on the walls and the outer layer of the roof. I’m having difficulty deciding how to insulate the roof. I’m thinking a hybrid solution of foam insulation above the sheathing and rockwool below the sheathing. There’s also R-15 comfortbatts between the studs (16″ oc) and R-6 comfortboard covering the exterior wall sheathing.
It has a 2.5:12 pitched roof. It’s a 14x14ft roof with 1′ overhang on each side except front. 2′ overhang on the front.
My plan is for an unvented roof (air tight, no recessed lights) with the following layers:
- Metal Roof
- 7/16″ zip sheathing
- 1-1/2″ EPS foam (staggered & taped seams)
- 2″ high-density EPS foam (taped seams)
- 1/2″ OSB
- 2×6 rafters spaced 16″ oc
- 2-layer R-15 Comfortbatt (tight to ceiling) – No vapor barrier
- Sheetrock
Is this sufficient to keep the roof sheathing at the proper temp and dry or should I add more/thicker foam boards?
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If IRC is code where you're at, you won't quite meet it prescriptively. For an unvented assembly like this you would need R20 rigid foam and R29 or 30 in the rafters. The mineral wool batts that fit in 2x6 rafters would be R23. Two layers of R15 compressed doesn't seem viable.
That said, the assembly should work safely, and may meet a performance rather than prescriptive R value path. Consider carefully the air barrier transition from wall to roof. To be moisture-safe, you want your air barrier down on the structural sheathing layer rather than all the way up at the metal roofing.
"Consider carefully the air barrier transition from wall to roof. To be moisture-safe, you want your air barrier down on the structural sheathing layer rather than all the way up at the metal roofing"
Thanks Jason. Can you explain this in a bit more detail? I'm hearing you say the transition from wall to roof (or sheathing to sheathing) must be air tight. Is that in the neighborhood of what you're saying?
Right. I opt not to rely on taped foam to be a long-term air barrier. To keep interior moisture from reaching colder surfaces, the air barrier is optimally located at the structural sheathing (given exterior insulation), where it's warm enough in winter to alleviate condensation and not subject to as much heat/moisture cycling and movement.
Roof to wall intersections are notorious for air leakage, i.e less moisture-safe. I would make the zip sheathing your structural sheathing directly on the rafters so it can be taped to wall sheathing. Doesn't have to be, but that would be my preference.
Shifting the zip sheathing to structural, I would need a moisture barrier between the OSB and metal correct?
I was wondering is there any need to use battens between the upper OSB and the metal. Or is direct contact perfectly fine. The metal roofing manufacturer approves direct contact, but I'm not sure if I'm complicating things.
You would need underlayment(s) that meet both code and roofing mfr. spec, yes. If you go the battens route you could potentially omit the upper layer of OSB. Like 2x4s 'on the flat'. Lots of ways to skin the cat. Specifics matter for given roof panels and design slope. Check product data and reach out to reps & installers.
Regarding batt cavities, 2x furring works, as would 2x8 rafters (quicker).
Regarding the R15 compressed; would a better alternative be to add 2x2's to the rafters and use R30 mineral wool rather than the 2xR15? Even with the 2x2's, I would still have plenty of ceiling space left.