I have a bedroom I am renovating with no insulation. So I took down the ceiling and towards the back of the house the roof rafters actually almost line up with the ceiling itself. I thought that rules out any batting and blown in application to achieve the suggested R value in the Philadelphia area. The space between the roof and ceiling is vented in front of the house by 2 small vents.

I was all set to cobble and cut 4 inches of polyiso in between the rafters bays right up against the underside of the roof sheathing. But recently read that this could be a bad idea that moisture can rot the roof sheathing but isn’t that the job of the roof anyway or is it an overstated concern? Again the space between the flat roof and ceiling is minuscule and I suppose vented by the 2 small vent holes 60 feet away in front of the house.

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