2/3 1/3 insulation rule in roof
Old build with flat wood deck on 2x13s. 4 inches Batt in joist space.(R12) 2 inch air space above batt insulation under wood deck No external vents into air space Adding polyiso on top of deck R 28. Before adding the polyiso can/should we put a separator sheet or vapour barrier on top of the wood as there is more than 2/3 the insulating value above the deck There is a poly vb above the drywall ceiling below e joists.
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