Replies

1. GBA Editor
   Martin Holladay | Sep 18, 2016 04:39pm | #1

   Kevin,
   The use of mineral wool insulation on the exterior side of roof sheathing for a sloped roof assembly is an experimental, thinking-outside-the-box approach. I've heard that it can work, but don't be surprised to discover that you are the first person on your block (or conceivably, in your state or province) to try this approach.

   If you have a ventilation channel above the ComfortBoard, you'll get some windwashing. According to most reports, the effect of windwashing on mineral wool insulation (especially denser types of mineral wool) is less of a problem than the effect of windwashing on fiberglass insulation.

   Are you the roofer? If you are hiring a roofing contractor, verify that the contractor is OK about your plan to install the roofing over purlins instead of solid sheathing.

2. 
   Jon_R | Sep 18, 2016 05:17pm | #2

   I wonder what a purlin over Comforboard is rated at in terms of PSI. Walls see less horizontal pressure while roofs gets walked on, snow loads, etc.

3. Expert Member
   MALCOLM TAYLOR | Sep 18, 2016 06:58pm | #3

   Kevin,
   Just a couple of minor things:
   - You might think about omitting the H clips. They aren't required by code and as your sheathing will be separated by the insulation from the purlins, it doesn't experience any point loading between the roof framing.
   - How are the ventilation channels parallel to the ridge ventilated, or are they really more there as a capillary break?

4. Expert Member
   Michael Maines | Sep 18, 2016 07:12pm | #4

   Comfortboard 80 has compressive strength of 439 psf at 10% compression and 1065 psf at 25% compression. The relationship appears to be linear. 50 psf is a reasonable design load; with 3 1/2" furring 16" o.c., that's about 1.2 psi, which translates to about 4% compression. With your 6" roof insulation, that's about 1/4" deflection. I probably wouldn't want to risk that on a client's house, but if it was an outbuilding at my own house I would give it a try.

5. 
   user-5453257 | Sep 18, 2016 07:43pm | #5

   Martin,

   I'm leaning towards mineral wool over foam because of wildfire concerns.
   If it works for the walls, I don't see why it wouldn't work for the roof too.

   I plan on doing the roofing myself, which is one of the reasons I'm doing a snap-lock standing seam instead of a mechanically seamed one.
   The "snap clad" panel I linked to can be installed over purlins.

   Jon + Michael,

   The roof loads should be transferred to purlins, which then transfer them to the sheathing via screws --- in theory there shouldn't be much load on the insulation whatsoever.
   That's the theory, anyway.

   Do you think a second layer of sheathing on top of the insulation is needed simply to provide a flat surface for roofing? It doesn't seem to me like it'd necessarily be any easier to flatten plywood than purlins.

   Malcolm,

   Good point about the h-clips, I'll bring that up with the structural engineer.
   TBD on ventilation details --- probably just screened openings at the sides of the roof.
   If you're wondering whether the air needs channels that go from the lower side to higher side (rather than parallel to the ridge), I'm relying on Martin's answer to a previous question that parallel-to-ridge channels are sufficient ( https://www.greenbuildingadvisor.com/community/forum/green-building-techniques/31981/exterior-roof-sheathing-ventilation)

6. Expert Member
   MALCOLM TAYLOR | Sep 18, 2016 10:04pm | #6

   Kevin,
   The screws securing the purlins don't carry any load, except for a some shear to prevent the roof from sliding down and off onto your backyard, and occasional uplift due to wind. So Michael's calculation of 1.2 psi on the mineral wool is pretty accurate. Given that he suggests you could be looking at a 1/4" compression, which would leave the screw heads poking up into your metal snap-lack panels, it might be an idea to countersink them 1/2" as a precaution.

7. Expert Member
   Michael Maines | Sep 19, 2016 09:14am | #7

   Kevin, your roofing manufacturer may or may not require continuous sheathing as underlayment. Again, on my own house I probably wouldn't worry about it, but if you want the manufacturer's warranty to be valid, you need to follow their instructions. I've used both double-lock standing seam and rib-panel "agricultural" metal roofing on skip sheathing but I don't know what the snap-lock companies require.

8. GBA Editor
   Martin Holladay | Sep 19, 2016 09:21am | #8

   Kevin,
   One more point: building codes require the use of roofing underlayment (for example, asphalt felt or synthetic roofing underlayment) under roofing. I'm not sure whether your local code enforcement official would care whether the roofing underlayment was directly under the roofing -- but it's possible that the official might.

9. 
   user-5453257 | Oct 03, 2016 11:58am | #9

   Thanks for the help everyone!

   I just realized that since the workshop isn't going to be heated, the chances of ice daming are low.

   I can alleviate the wildfire concerns by simply not venting, which would allow me to use cheaper (possibly recycled) rigid foam.

   In this case, the stackup is the same as second option described here: https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling

   My only question is that, with a stackup of:

   plywood sheathing, 5+ inches of rigid foam, plywood, underlayment, metal roof

   isn't the outer layer of plywood unable to dry in either direction?

10. GBA Editor
    Martin Holladay | Oct 03, 2016 12:27pm | #10

    Kevin,
    Q. "With a stackup of plywood sheathing, 5+ inches of rigid foam, plywood, underlayment, metal roof, isn't the outer layer of plywood unable to dry in either direction?"

    A. Yes. The traditional phrase to describe this type of roof is a "hot roof." The idea is to install the roofing underlayment and roofing on a sunny day when the plywood is dry.

11. 
    user-5453257 | Oct 03, 2016 02:15pm | #11

    Got it, thanks Martin!