Spray Foam Insulation Under Roof Deck

Regarding open cell foam under a roof deck how about under a floor.
Spray foam insulation under roof deck. The spray foam can also serve as the thermal and vapor control layers in both new and retrofit construction. The biggest benefit of spray foam insulation is its air sealing quality so if air is infiltrating into a spray foam attic then the installers missed some spots and need to go back and make it right. But spray foam insulation used at the underside of the roof deck would need to be covered by a thermal barrier and intumescent coatings are commonly used. The code allows for insulation directly to the underside of the structural roof sheathing providing the unvented attic is contained completely within the.
In the northwest where foam has been sprayed directly to the underside of roof decks for over a decade the building officials interpret the code to require removal of existing insulation. A double wide mobile home on blocks with a crawl space and dirt floor. Despite the advantages of using spray foam under the roof deck there are some potential risks. The attic will be a conditioned space and i m planning to apply spray foam insulation to the underside of the roof sheathing.
Should i put any membrane or the like on the underside of the roof sheathing before spraying the foam in. The roofing materials are installed and consist of timberline shingles and full coverage ice and water barrier the entire roof. Sloped roof with cavity spray foam insulation sprayed on underside of roof deck and covered with a sprayed on thermal or ignition barrier coating over the spray polyurethane foam insulation if necessary based on the material properties of the spray polyurethane foam or as required by code source. The attic is unvented for instance no ridge vent exists.
No airsealing hence i want to remove the fiberbatts and spray 3 inches of closed cell spray foam in before putting the fiberbatts back in place. Open or closed cell spray foam is applied to the underside of the roof sheathing and additional fiberglass or cellulose insulation is blown in as a cost saving method for meeting high insulation requirements and filling in the cavity space between the rafters to the ceiling deck source. The primary risks are rainwater leaks condensation from diffusion air leakage and built in construction moisture. Unlike solid insulation products that require careful measurement cutting and fitting spray foam can cover even the most uneven surface instantly.
I have a conditioned attic currently insulated with r19 fiberbatts between the rafters. Closed cell foam has been applied underside of roof decks since the late 60s early. Polyurethane or polystyrene spray foams can provide thorough coverage to roof rafters and attic walls in much less time than is needed for other materials.