Best foam for site-built ventilation baffles?
My question is about site built vents for my non-insulated attic space. If I go with 1 1/2″ space (dimensional lumber spacers) under the roof deck, with 1″ foam board attached to the spacers, and add 3″ closed cell spray foam, then 2 layers of polyiso attached to the face of the rafters, would I be OK as far as moisture within all the layers? Is there a better type of board to use on the underside of the roof deck (XPS, EPS, Polyios / coated or non-coated)? Is there a preferred type of board to use for the foam board between the closed cell and the living space? I’m not sure about all the types, concerning what is better for shrinking, R-value in hot or cold weather, thermal drift, and moisture control. Do I want some amount of absorption in my vent, or none? With the closed cell spray foam, would it matter? Would a 2″ space with 1/2″ board under roof deck be better, or not really much difference (more air gap, but less R-value in available space)? This is all in zone 5, and the roof is gambrel (12/12 and 4/12), with 20″ cellulose on the attic floor. Thanks.