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Mounting solar panels above roof with exterior foam?

Steve Mackay | Posted in General Questions on

After the epic sanity check on unvented cathedral ceiling: 

https://www.greenbuildingadvisor.com/question/sanity-check-on-unvented-cathedral-ceiling-assembly

I think I’ve manage to convince my builder that the exterior foam approach might be the way to go to get me over the steel I beam insulation hurdle.  I found a local source for reclaimed polyiso.  I was going to do a hybrid approach with 8″ exterior Polyiso (perhaps a sandwich with EPS to the outside) then on the inside of the roof deck do some ocSPF.

There are still a few issues to figure out with this approach, one of which is how to mount the solar panels.  Anybody got advice on mounting solar panels above foam?  My contractor recommended using 2×8 on edge for structure to mount the panels to. This is obviously a thermal bridge (not near as bad as my steel I beam).

Any details or ideas appreciated?

Steve

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    How would the 2 x 8s be structurally anchored? (Also they're only 7.25" deep, only good for 7" of foam board, not 8".)

    Don't bother using an EPS sandwich- the difference between that an cold-derated polyiso is in the noise from a peak loss point of view, and from an average loss point of view the polyiso will usually outperform it. At 7" roofing polyiso is enough to get you to code min even with out the interior side ocSPF.

    If the foam board is strapped in place with 2x furring or a 5/8" nailer deck through-screwed to the structural roof deck with the appropriate pancake head timber screws on an appropriate spacing is sufficient to manage the PV load. You may need to pay an engineer to specify the screws and spacing, or the minimum width of the furring (2x4s should work if 16" o.c., but for wider spacing it may need to be 2x6) but this is not a rocket science project to analyze.

    With 8" exterior foam and 1.5" thick furring the 11" screw lengths become awkward. Nailer deck solutions under steel roofing are common in the commercial construction. But dropping back to 6" polyiso (R34-ish labeld, R30 derated) and going with 8" interior side ocSPF would still be considerably better than code, has sufficient dew point control at the structural roof deck, and is easier to assemble, whether using furring or a nailer deck.

    http://www.atlasrwi.com/resources/roof/General%20Resources/Guides/ACFoam-Fastening_Pattern_Guide.pdf

    https://www.hunterpanels.com/docman-categories/technical-documents-roof/fastening-pattern-guides/1370-h-shield-nb-steep-slope-wood-framing-16-or-32-oc-concrete-steel/file

    https://www.hunterpanels.com/product-documents/hpanels/speciality-products/98-cool-vent-fasteners/file

  2. Expert Member
    Deleted | | #2

    Deleted

  3. Expert Member
  4. Steve Mackay | | #4

    Dana,

    Is it an either or between the nailer deck and the furring strips? Don't you still need a nailer deck if you use furring strips too? the advantage of the furring strips is now you have an air gap above the polyiso and your nailer?

    Steve

    1. Expert Member
      Dana Dorsett | | #10

      Installing the nailer deck on top of the furring allows the nailer deck to be vented, which also reduces ice damming potential. It's more expensive than putting the nailer tight to the foam, but preferable in high snow load locations.

      With standing seam steel roofing the furring can usually do double duty as purlins for mounting the roofing, no nailer deck required.

  5. GBA Editor
    Brian Pontolilo | | #5

    Hey Steve,

    Have you read this: https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing

    It doesn't address your PV load, but as Dana said, that should not be an issue, though you could have an engineer take a look at your assembly once you dial it in. Otherwise, the article should answer the rest of your questions.

  6. GBA Editor
    Peter Yost | | #6

    Tough to comment on this without climate zone and roof pitch info, correct?

    Peter

    1. Steve Mackay | | #9

      Sorry. Climate zone 6B. 2.5/12 roof pitch.

  7. Expert Member
    Akos | | #7

    If you go with the 2x furring over the foam, you will need an engineer to spec the width. Doing the rough math with 20 psi foam, you will need more than 2x3 width to support the roof load in snow country.

    Overall I think the direct deck mount systems are much quicker to install though. This saves the installer hunting for the wood studs to lag into.

    I know it is easy to feature creep with house designs, but there are a number of direct mount solar systems for standing seam metal roofs. This saves a fair bit of cost in racking. Still more expensive the shingles+rack but now you don't have the issues with low slope shingled roof.

  8. Jon R | | #8

    When it comes to ice dams in a high snow load area, you should read the BuildingScience.com articles. One example: https://www.buildingscience.com/documents/insights/bsi-046-dam-ice-dam

  9. Tim R | | #11

    You could use a fiberglass structural shape to extend thru the foam to mount the panels to. Perhaps an I beam shape.
    See https://www.strongwell.com/products/structural-shapes-and-plate/

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