The roof currently has 4 of medium density open cell spray under the deck giving us an r value of just under 22.
Open cell foam roof deck.
Open cell spray foam is not a moisture vapor barrier and will allow moisture to pass.
It would come with 3 1 2 or 4 of polyiso adhered to a 7 16 deck.
Open cell spray foam can also be used to create an unvented attic in cold climate zones provided code requirements for a vapor retarder are followed.
Some spray foam manufactures have tested primer coating products that result in the formulation of a class ii vapor retarder directly on the surface of the open cell spray foam.
Common sense tell s me if this is valid marketing then what stops water vapor from moving through the foam under pressure.
Like thicker layers of hfc blown closed cell foam at r30 most open cell foam is a fire hazard during the curing period right after installation.
To do 8 safely it has to be done in two lifts with a cooling curing period between lifts.
Open cell spray polyurethane foam or closed cell spray polyurethane foam ccspf insulation can be sprayed along the underside of the roof sheathing to provide a conditioned and insulated attic space that can be durable and efficient in all climate zones see figure 1.
Unvented roofs with spray foam applied to the underside of the roof deck have been used since the mid 1990s.
Open cell foam is just that open cell.
If there s a roof leak water will pass through the foam and you will quickly be able to find your problem.
Even though it s air tight open cell foam is still pretty vapor open at r30.