Open Cell Foam Roof Deck

Unvented roofs with spray foam applied to the underside of the roof deck have been used since the mid 1990s.

Open cell foam roof 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. Even though it s air tight open cell foam is still pretty vapor open at r30. 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.

The roof currently has 4 of medium density open cell spray under the deck giving us an r value of just under 22. So if you re considering spraying foam to the roof deck of your new or existing home you want 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. 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.

Open cell foam is just that open cell. 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. Open cell spray foam is not a moisture vapor barrier and will allow moisture to pass.