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Cabin has EDPM roof, real 2x4 rafters in the one room. I have slabbed on 2x2 to the rafters in the room, inside, to make a 6 inch cavity. This is an accommodation for poor contractor work shoving 6 inch insulation into the 4 inch cavity. Some rafters have fiberglass. One rafter section has nothing. Going to put foam board in it. What are the rules for vapor barriers with the EDPM on the top of the assembly? Do I fill the cavity with the foam board or just one at the bottom with air between to the roof? Roof underlayment is old wood board.

Intent is to then cover that section with 1/2 foam board foil faced down and then wood wainscot. Eaves are rather leaky... No actual ventilation in the eaves.

Northern US zone.

  • “ how to use 2 inch foam insulation” I used some once to make a toilet seat in an outhouse. I then covered it with a thin fleece material. Warm at 30 below – Alaska Man Oct 20 at 0:49
  • EDPM roof? One standard approach is to put it on top of the membrane, plus a wear/weight layer. More common on flat roof applications - there's a product made that's a sheet of cementboard bonded to a sheet of foam - the cement board goes on top. Just lay it in place. – Ecnerwal Oct 20 at 15:09
  • This is an existing structure with an EDPM roof. One room has the ceiling torn down to the roof sheathing. – House DiY Oct 20 at 22:09

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