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Q&A Spotlight

Will This Roof Design Have Problems?

A homeowner questions his architect’s plans for a standing-seam metal roof

Plans call for standing-seam metal roofing over plywood roof sheathing and 2x12 rafters. Insulation would be in the form of closed-cell spray foam. The homeowner wonders whether thermal bridging will lead to problems in the roofing, and possibly in the framing itself.
Image Credit: dunktanktechnician/ CC BY 2.0 / Flickr

Planning a new house in Climate Zone 6, Chad Kotlarz is reviewing his architect’s plans for the roof — and discovers he has a few misgivings.

The unvented roof will be framed with 2×12 rafters, sheathed with plywood and capped with standing-seam metal roofing. Closed-cell spray foam will insulate the rafter bays, and the interior of the cathedral ceiling will be finished with gypsum drywall. An exposed truss with a collar tie provides structural support.

“The main concern is with the thermal bridging from the 2x12s,” Kotlarz writes in Q&A post at Green Building Advisor. “The roof is a 10/12 pitch, with a dormer and two valleys. I was told that the thermal bridging created from the 2x12s would cause hot/cold spots in the roof and would lead to problems with the metal roof. I’m assuming this is from expansion/contraction or condensation, but I’m not certain.”

For reasons he doesn’t explain, Kotlarz doesn’t want to use rigid foam on top of the roof sheathing or on the interior, so those approaches to thermal bridge reduction won’t work.

Will thermal bridging through the rafters cause problems over time, such as rippling in the metal roofing or condensation in the roof framing?

Those are the questions for this Q&A Spotlight.

Consider using scissor trusses

Stephen Sheehy, who built a well-insulated house with a standing-seam roof in Maine, suggests that Kotlarz look at a different way of framing the roof. Sheehy used raised-heel scissor trusses that were deep enough to allow about 20 inches of cellulose plus a ventilation space.

Raised-heel, or energy, trusses are extra deep at the building perimeter to provide more room for insulation than a conventional truss. “A nice benefit to trusses,” Sheehy adds, “is the ability to put…

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5 Comments

  1. C. B. | | #1

    Solutions for "Lack of drying potential"
    How about using:
    http://www.cosella-dorken.com/bvf-ca-en/products/roof/underlayments_metal/products/trela.php
    or:
    http://www.roofaquaguard.com/synthetic-underlayments/dry-tech/
    between the roof sheathing and the standing-seam metal roofing?

    These will provide an airspace under the standing seam roofing for ventilation and drying.

  2. Mark Heizer | | #2

    Peter's venting space is critical
    Peter stressed the important point: allow the roof to dry. Venting under the metal roof is needed. You can get condensation on the underside of the metal roof further slowing the summer drying (night sky effect). And don't muck up the deal by sealing the whole roof deck with ice & water shield.

    Recommend you research solutions applied to SIP roofs in Alaska at the roof peak. You're constructing a site-built SIP. Consider the venting solutions that came out of those studies.

  3. Josh Wimpey | | #3

    I recently had a local metal roofer offer something like C.B
    We have a roof that has closed cell foam insulation applied to the underside of the roof deck and need to replace the old concrete roof shingles. The roofer had a styrofoam product about 1.5" thick with ~1/2" channels on both sides that he combines with a ridge cap to achieve soffit to ridge ventilation above the roof deck and a thermal break. Not sure anyone has experience with this or something similar. We are still considering options including just adding 2" of rigid foam to the roof deck and installing standing seam over top without ventilation.

  4. Expert Member
    Kohta Ueno | | #4

    BSC Resources
    As discussed above, the main issue with spray foam in the rafter cavities without continuous insulation is the thermal bridging through the wood rafters. The consequence of it is mostly embarrassment due to differential frost lines forming on your roof:

    BSI-063: Over-roofing—Don't Do Stupid Things
    http://buildingscience.com/documents/insights/bsi-063-over-roofing

    In went some skylights on the back-side (Photograph 1) and the rafters got sprayed full of 2 lb/ft3 density spray polyurethane foam (SPF) (Photograph 2). The attic was turned into living space. Got two bedrooms and a bathroom, a place to store old ASHRAE Journals and a place for mechanicals. Got ourselves a kinda R-35 roof assembly when you factor in the thermal bridging. In 1995 that seemed pretty good. Installed new shingles over a fully adhered membrane and the house looked pretty darn good (Photograph 3). I felt pretty clever. Life was good.

    Then the first winter came (Photograph 4). Ouch. What to do? Easy. Just don’t tell anyone and don’t invite anyone who knows anything about this stuff over during the day during the winter. That didn’t work. I was a marked man. Folks knew where I lived and wondered what kinda house the self proclaimed building guru lived in. Double ouch. It got worse.

    I concur with Peter Yost that metal panels buckling due to thermal bridging sounds highly unlikely--these systems are designed to handle movement.

    As for drying potentials and spray foams, one excerpt:

    PA-1401: Are You Doing Something Stupid?
    http://buildingscience.com/documents/published-articles/pa-are-you-doing-somethig-stupid/view

    Ready for this… here it comes… roofs leak. They always have. They always will. Sometime you see the leak, sometimes you don’t. This is true regardless of what system you use. There is no greater risk of roof damage with SPF, regardless of type, under a roof deck when exposed to a leak than the risk compared to structural insulated panels (SIPs) or typical commercial compact flat roofs. We can build with multiple layers of insulation and membranes and decking, and somehow we manage to deal with roof leaks. It is not the end of the world. SPF is not a greater risk under a roof deck than the risk associated with commercial compact flat roofs with multiple layers of insulation sandwiched between roof membranes and metal and wood structural decks. SPF is not a greater risk under a roof deck than the risk associated with SIPs.

    As further background, a research report from 2013 on a field survey of spray foam roofs:

    BA-1312: Application of Spray Foam Insulation Under Plywood and OSB Roof Sheathing
    http://buildingscience.com/documents/bareports/ba-1312-application-of-spray-foam-insulation-under-plywood-and-osb-roof-sheathing/view

  5. Expert Member
    Malcolm Taylor | | #5

    Risk
    The excerpt kohta posted:

    "There is no greater risk of roof damage with SPF, regardless of type, under a roof deck when exposed to a leak than the risk compared to structural insulated panels (SIPs) or typical commercial compact flat roofs."

    Somehow seems like cold comfort to me. I've always though of SIPS and commercial flat roofs as fairly risky assemblies and not ones I'd look to emulate in terms of durability or resilience.

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