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Q&A Spotlight

Installing Solar Panels Over Rigid Foam Insulation

Solar installers steer clear of this Passive House retrofit and the owner figures out why

This drawing shows a roof assembly with exterior rigid foam, which has been enough to discourage solar installers from attaching panels. Illustration courtesy Kaster.

Kaster has retrofitted his Bronx, New York, home to Passive House standards, a project that included the installation of 6 in. of rigid foam insulation on top of his roof deck. The upgrades will certainly mean lower energy bills, but it’s also left him with a problem: he can’t find a solar installer who will mount panels on the new roof.

“I’m now seeking to install solar panels, but with all my engagements with local PV installers they don’t seem to have the ability or confidence to find a way to install/anchor the panels to my roof,” Kaster writes in this recent Q&A post. “The PV installers seem to all agree they need to anchor the brackets to the rafters, but how can they find it on one go without making my roof into Swiss cheese is the concern.”

Exterior roof deck insulation is frequently recommended at GBA when the aim is to turn an attic into a conditioned space. There are a number of performance advantages. But as rooftop solar installations become more common, does that mean others face the same dilemma as  Kaster? The question has come up before at GBA, including this post from 2019 and this one from 2016.

Is there a workable solution? That’s the topic for this Q&A Spotlight.

Locating rafters through the foam

Solar panels are typically mounted on metal racks, which are in turn anchored to the roof surface. The installers that Kaster has spoken with are apparently insisting that fasteners attaching the rack-mounting points to the roof go all the way through the sheathing and into a rafter.

But how to find them?

Mark Nagel suggests a couple of approaches. One is to take exacting measurements from reference points that are accessible from above…

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  1. PAUL KUENN | | #1

    I've installed solar for over 30 years and boy has it changed. All the new mounting is congruous with almost any type of roofing material. If the 2xs are screwed through the foam to the rafters as should be every 16" that should be no problem. Most of the mounting systems come with their own lags and 1/2" sheathing over 1.5" pine should bury most or all the threads. Heck, half the jobs we have to fix (many large companies, - some from out of state - come in and do a slop shot job and never return) we'll look it over first and find almost a third of the lags partially missed the rafters and have somehow stayed on the roof after tornados.

  2. edubin | | #2

    If installers clip solar panels to the vertical seams of a standing seam metal roof, are there special provisions for how that roof should be installed prior to solar coming in?

    1. dgsmills | | #3

      I'd like to know this too, as we will be using a similar system on our build.

    2. Expert Member
      MALCOLM TAYLOR | | #4

      Elizabeth and dgsmills,

      That's something I've wondered about too. The fastener schedule for the metal roofs I install do not assume there will be additional loading or wind uplift from solar panels, and there are no provisions for them in their installation manuals.

      I suspect this is one of those situations where if things failed the roofing manufacturer would say it's the panels fault, and the panel manufacturer would say as long as their clips didn't fail their system wasn't at fault. I'd be interested to hear what others who have used these clip systems have done to ensure someone takes responsibility for the install.

  3. neutral_grey | | #5

    Very interested in the same question, as we consider roof options for a new 2nd story addition. I have a positive impression of standing seam from past houses I've been involved in, but see this as a significant gap.

    Some simplistic testing done by another user on their standing seam install ( seems to indicate the strength of the metal panel itself could be the limiting factor (panel tears before clips).

    Since I hope to install these myself (and am keeping the roof shape deliberately simple), I wonder if carefully ensuring the roofing CLIPS are fastened everywhere the RAIL clamps will later be attached, to address the concern raised in the post I linked. The would probably result in significantly more roofing clips used - but it's just under the (pre-planned) solar array footprint, so this should be a non-issue.

    Anyone see that as having a positive effect on strength, or have any better ideas for a "clean slate" design. We have have some leeway on roof slope, but it should be 6-12 or steeper.

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