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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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.