I have new spray foam insulation between 2x6" rafters, and I would like to cover it directly with plywood (for aesthetic reasons) rather than drywall.

Is that an issue with fire barrier code?

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1 Answer 1


Here is the list of acceptable ignition and thermal barriers for spray foam: http://www.icc-es.org/News/Articles/AY126ThermalBarriersSPF2011-51811.pdf

In a nutshell, if the space will not be used as a living or storage space, you can cover the foam with 1/4" thick or thicker plywood. If the space will eventually be used as a living or storage area, you'll need to use 1/2" thick or thicker drywall, or a product or system certified and rated as equivalent in performance.

Let me also speak to the performance of your insulation. 2x6 rafters allow 5.5" of spray foam. 5.5" of closed-cell foam between the rafters will give that roof a total R-value of about R-35, which is slightly below code minimum in most places, but may be decent if it doesn't get too cold where you live. However 5.5" of open-cell foam between the rafters only gives the assembly barely R-19 which is pathetic, below code minimum everywhere, and will not yield impressive performance, not by a long shot. If the spray foam company used open-cell foam (and I suspect they did), it's worth it to cover the underside of the rafters with additional inexpensive insulation to salvage your attic's performance, especially if the space will not be used for living and you can afford the reduced headroom. Several inches of off-the-shelf EPS, XPS, or Polyiso foam boards can be easily and inexpensively fastened to the underside of the rafters, which will substantially boost the performance. For example just four inches of polyiso over 5.5" of open-cell foam will raise the total R-value to 45, more than doubling it! Then you would cover the foam boards with either plywood or drywall just like before, depending on the intended usage.

  • It is open cell: gaco.com/products/PDS-WF-Gaco-052N.pdf
    – Slobvrflw
    Dec 13, 2015 at 20:42
  • 1
    I had been thinking to add another few inches of insulation to the exterior, under a new metal roof, budget permitting, in a few years.
    – Slobvrflw
    Dec 13, 2015 at 20:45
  • Thank you for the clarification on Thermal vs Ignition barrier. Searching for Thermal barrier equivalents to drywall, I found this: foam-tech.com/theory/firebarriers.htm
    – Slobvrflw
    Dec 13, 2015 at 20:47
  • 15/32" - Wood structural panel sheathing or tongue & groove planking 15/32" - Particle Board, Plywood Along with steel, mineral wool, etcetera. My best option may be 1/2" drywall with a thin veneer of wood.
    – Slobvrflw
    Dec 13, 2015 at 20:52
  • 1
    Putting rigid foam under your new metal roofing down the road is an excellent idea.
    – iLikeDirt
    Dec 13, 2015 at 21:22

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