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Should standing seam metal roofing be vented over a roof insulated with exterior rigid foam?

Jodi Gunderson | Posted in Energy Efficiency and Durability on

Hello,

This question is related to another question (“Is it advisable to use nail base insulation panels over a roof insulated with closed cell spray foam?”) that I asked over a year ago, but for some reason when I try to post to the original thread I get blocked……

The re-roof project got put on hold for quite a while. I have finally gotten the green light from the engineer to move forward – we were concerned about the weight of the new stack-up. What we are hoping to do is insulate the rafter bays with either dense-packed cellulose or open cell SPF (~5 1/2″), install a layer of synthetic roofing underlayment over the existing board sheathing followed by two 2 3/4″ layers (5 1/2″ total) of eps foam board.

We are in CZ5 and get a fair amount of snow so the plan has been to install 2x4s on the flat to create a vent channel below the 5/8″ CDX plywood sheathing. However, the homeowners (my parents) are now considering a standing seam metal roof – we would like to install solar panels on the south facing slope later down the road. I don’t know much about metal roofs (and won’t be installing it either). But I’m wondering if the vent channel is necessary under this type of roofing – or in this particular scenario. Does it make more sense to keep the vent space or can it be omitted?

Also, what underlayment would be best to put on top of the new plywood sheathing? Is it still wise to install an ice and water membrane at the eaves? Thanks again for all the help. Your comments and suggestions are much appreciated.

Regards,

Jodi

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jodi,
    In general, standing-seam metal roofs usually don't leak, even when the roof develops an ice dam. For that reason, some builders don't pay as much attention to features designed to address ice dams (Ice & Water Shield at the eaves, a vent channel under the roofing) when the roofing is standing-seam metal than when the roofing is asphalt shingles.

    I'll point out that most building codes require the installation of some type of peel-and-stick membrane at the eaves, so you probably want to install it for that reason alone. Moreover, a ventilation channel between the top of the rigid foam and the top layer of roof sheathing will reduce the chance of an ice dam when you have deep snow on your roof. Is it necessary? No. Is it a good feature to have to reduce the chance of ice buildup? Yes.

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