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How should I calculate the appropriate NFA for a roof with exterior EPS insulation?

jedi | Posted in Green Building Techniques on

I am working on a reroof of an old building where we are going to add exterior foam insulation after removing the old roofing materials. I plan to vent the roof with 2x4s on top of the foam running from eave to ridge giving us a 1 1/2″ vent space followed by a new plywood deck and standing seam metal panels. The underside of the roof was recently sprayed with 4-5 inches of cc spray foam. After securing the old roof boards with screws, I plan to put a layer of #30 felt paper down followed by (2) 1 1/2″ layers of EPS foam – the idea being that those two materials will allow some drying potential up into the vent space.

Due to the design of the roof, which has massive overhangs, I plan to vent the intake at the eaves with a space between an upper and lower fascia board. It seems any venting calculations I’ve come across don’t pertain to this type of roof. Originally I had planned to bend stainless steel insect screen around spacers between the two fascia boards – but that sounded like a pain from the start. I’m considering using a plastic vent strip made by Cor-a-Vent that is designed just for this type of vent setup. It is a corrugated strip that is 1 inch wide and has 10 sq. in. NFVA [net free ventilation area] per foot. Any help figuring out if this will be adequate would be greatly appreciated. I’ve attached a photo of the building to show the scale of the roof.



  1. GBA Editor
    Martin Holladay | | #1

    The type of roof you are building is robust. It isn't at risk of moisture problems. You don't have to worry too much about the venting area, as long as you have an air inlet at the bottom, and an air outlet at the ridge.

    If you are creating 1.5-inch-high ventilation channels, the area of the vent channel cross section is 18 square inches per linear foot. The Cor-a-Vent product will provide NFA (net free area) equal to 55% of the cross section of the vent channel. That's plenty.

  2. jedi | | #2

    That's great to know. Thanks again Martin.

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