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I have a detached garage which I am working on turning into a workshop. As part of the transformation, I would like to insulate it, finish the walls/ceiling and add a mini split for cooling and heating. Most of the time, the space will be unconditioned as I would only be running the HVAC when I am working on a project. I am located in PA, climate zone 5.

  • The garage has a "shed style" single slope roof.

  • I don't know the exact slope, but short side is 8ft, tall side is 12ft.

  • It has no attic, just an open sloped ceiling following the roof line.

  • The roof rafters are 2x8s.

  • There are vented soffits across the entire length of the roof on both the top and bottom, but no ridge vent.

My Plan

  • Install 1" spacers along the rafters at the corners where the rafters meet roof sheathing.

  • Attach 1" foam board to spacers, air sealing all joints with spray foam, creating a 1" channel across the entire width of every rafter bay to ventilate under roof sheathing.

  • Add 1" foam board to seal between top plates and foam board in rafter bay (spray foam sealed around perimeter) - to completely air seal inside garage space from roof.

  • Install 2x6 R23 Rockwool insulation to fill the rest of the rafter bay.

My Questions/Concerns

  • Do I need a ridge vent for my type of roof? Despite the top vented soffit not being the highest point of the roof - it's only about 8 inches than the highest point. Is that sufficient to vent?

  • Will 1" channels spanning each rafter bay be sufficient to vent the roof?

The roof:
Roof

The vented soffits:
Vented Soffits

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  • 1" is sufficient. See IRC R806.3. Avoid a Class I vapor barrier down low for your air sealing if you're using something beyond Kraft paper. It would create a moisture trap between the upper foam boards and the lower air seal. See Table R702.7(1) under IRC R702.7 for vapor barrier classes.
    – popham
    Feb 18 at 21:18
  • And your upper soffit vents are effectively a ridge vent. See IRC R806.2 for vent area requirements. You definitely fall under the 1/300 category. You can find your climate zone by searching Chapter 11 of the IRC for your county's name (Philadelphia is 4A).
    – popham
    Feb 18 at 21:22
  • @popham I had planned on using rockwool insulation under the foam boards and a drop ceiling to finish it off.
    – Yev
    Feb 18 at 22:08

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As long as you have substantial venting at both top and bottom (or near them) this should work fine. No, you don't need additional ridge vent--enough scavenging should occur in that small section.

Yes, 1" should be adequate for such short runs. Most insulation baffles leave barely that after some flex anyway.

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