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Roof assembly for exterior polyiso and metal roof – garden studio

pceverha | Posted in General Questions on

I’m building a modern studio shed that I’d like to air condition on an irregular basis. I’ll probably outfit with a mini-split and keep it no colder than 55 and no warmer than 85, and then 20ish hours a week at room temperature. For multiple reasons – including learning the building science for a future house build – I want there to be a reasonable amount of insulation, and I’m drawn to the exterior polyiso shell idea. The size of this structure is smaller than required for building codes/permits. I’m right on the edge of Zone 2/Zone 3. The rafters/studs will be exposed on the inside, and this will be a single-plane (lean-to) type roof.

I’ve read quite a bit on GBA, but the part that keeps confusing me is the membranes/underlayment need. This is what I’m thinking to do:

Bottom up:
Exposed rafters
1/2 plywood deck
Self-adhered membrane on entire deck
1/2 plywood 
Synthetic underlayment
ribbed metal roof panels
Did I get this right? Feel free to shoot holes in this if I didn’t build the right stack.

Same basic question on the walls – what’s the correct stack with panel siding, polyiso, plywood sheathing.

Thanks for any help you can provide.

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  1. Expert Member
    Malcolm Taylor | | #1


    - You don't need a self-adhered membrane over the ceiling or wall plywood (although it does no harm), just tape the seams so it acts as an effective air-barrier.
    - As a substrate for the roof panels you can use either plywood or strapping. For the wall panels use horizontal strapping to create a rain-screen.

  2. Expert Member
    Akos | | #2

    Most roofing polyiso is fiber faced so it can't be exposed to water. If you are using roofing polyiso for the walls (tends to be the cheapest insulation around me), you need to have a WRB over the foam.

    In your mild climate, you don't need to go crazy with insulation. For a small space, a couple of inches is good enough. A bit more on the roof to reduce cooling load. Say 2" on walls 3" on the roof.

    So for your wall your stackup would be:
    -sheathing with taped seams
    -WRB (heavy duty house wrap)
    -horizontal 1x4 strapping

  3. Darren Williams | | #3

    hello Akos, curious about fiber faced polyiso needing a wrb, i recently had my house resided and at that time we added 4 inches of polyiso, all recycled material, some was foil face and some was fiber faced, my wrb is at the sheathing plane and all windows etc were flashed to that. the foam was taped at all seams and strapped with true 1 inch material for the rainscreen gap, the process took a while and the foam was exposed to rain and i did not notice any deterioration in the fiber facing. should i be worried? how much water will actually get past the siding and how long will it actually be there with a large airgap? i live in the north east and i get more snow than rain... thanks

    1. Expert Member
      Akos | | #4

      I've left fiber faced iso out exposed over winter. Was expecting it to be a wet mess but it looks like it held up just fine.

      Ideally, you want to protect the iso with a WRB, but not as critical as I would have though.

      As long as not a lot of bulk water makes it through your siding and you haven't done something silly like open cladding, I think it will hold up. The rain screen gap makes a big difference with drying as well which should keep you out of trouble.

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