So if you re considering spraying foam to the roof deck of your new or existing home you want open cell.
Open cell foam roof deck.
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 is just that open cell.
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.
It would come with 3 1 2 or 4 of polyiso adhered to a 7 16 deck.
Even though it s air tight open cell foam is still pretty vapor open at r30.
If there s a roof leak water will pass through the foam and you will quickly be able to find your problem.
Chemical manufacturer s claim if a roof leaks water will migrate through the foam structure therefore not trapping the water.
Common sense tell s me if this is valid marketing then what stops water vapor from moving through the foam under pressure.
To do 8 safely it has to be done in two lifts with a cooling curing period between lifts.
Unvented roofs with spray foam applied to the underside of the roof deck have been used since the mid 1990s.
Open cell spray foam is not a moisture vapor barrier and will allow moisture to pass.
Instead of installing a couple layers of polyiso with staggered joints and then decking he is proposing using a product like the gaf thermacal insulated nailing base.