Alternate compliance to the IRC thermal barrier requirement for foam insulation
I am planning on doing unfaced fiberglass batt insulation over spray foam plus continuous polyiso on the exterior of a small new house. The building code (R316.4) requires thermal barriers between the interior useable space and foam insulation and the prescribed way of meeting that is 1/2″ gyp board. I was hoping to do wood interior walls and ceilings – plank, OSB or even metal and not do any gyp board. Does any one know of any test reports that I can give the building inspector that shows that an ignition barrier (OSB etc.) plus the noncombustible fiberglass meets the intent of the building code?
Its not uncommon for a plank ceiling to be installed on a cathedral ceiling and if you have rigid insulation above the sheathing then it does not meet code!
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At 3" or more most spray-applied cellulose meets the spec, but not all fiberglass. IIRC any standard thickness/density rock wool of 3" or greater will makes it. If the product has ASTM E 119 the fire-rating greater than 15 minutes it should qualify but IIRC the letter of the code requires a further Evaluation Service Report indicating it passed a corner test variation on ASTM E 119 before it's fully qualified as an ignition barrier.
Unlike fiberglass, rock wool and cellulose are completely opaque to infra-red radiation, and low density fiberglass can cause it to fail the test. High density fiberglass often does though- JM Spider @ 1.8lbs density installation makes the lowered attic & crawlspace ignition barrier test at 2" thickness or higher, but probably doesn't in a 1lb density application:
Most wood prescriptively meets the ignition barrier spec @ 23/32" thickness or more, and that includeds 3/4" ply/OSB for walls. It would include 1x t&g or shiplap planking- not sure about planking with un-lapped edges. For roof decks even 15/32" and half-inch goods comply according to IBC section 2603.4.1.5 :
If batts, rock wool or cellulose at any density & thickness over 3" should make the grade in walls. But in a cathedral ceiling it may need to be damp-sprayed and have data showing that has passed the tests (the manufacturer should have the cert available if they did the test).