Solvent-based polyurethane coatings have excellent adhesion and when used as a base coat over asphalt, EPDM, TPO or PVC, it eliminates the need for a primer. The resulting savings on labor and material more than offsets the higher cost of the product. When top coated with silicone, it gives you the best of both worlds. It's also an excellent gutter and RV coating.
SPF application is fast and effective. Unlike traditional insulation types like fiberglass, which require a separate air-sealing step, spray foam can air seal and insulate a structure in one step, thus keeping the project moving forward. Combining these steps coupled with a spray-on application decreases installation time and delivers the ability to fit into hard-to-reach places, like eaves, odd-sized cavities, and rims. In addition to installation efficiencies, spray foam can provide excellent thermal, acoustical and indoor air quality performance in any climate. 
Spray foam is available in two different types: open-cell spray foam which is usually $0.44 to $0.65 per board foot and closed-cell spray foam which is about $1 to $1.50 per board foot. The average cost to have spray foam professionally installed is about $2,314. This number can vary depending on whether the home being insulated is newly constructed. However, finished homes are not a good candidate for spray foam insulation but great for weatherization or an energy audit.  
They’re the more expensive option than the first one we have discussed above, but they also offer top benefits for the RV owners who want to repair or maintain their RV roof and protect it from UV rays and harsh weather elements. Based on many reviews, these types can offer more durability than the silicone and acrylic types. They can also offer an excellent impact resistance from twigs, leaves and other outer elements.
A. You can use closed-cell spray foam that way if you want, although you should realize that the spray foam will prevent the roof sheathing from drying inward. This approach should only be used if the roof sheathing is bone dry, because it will never dry when sandwiched in this manner. Most building scientists would prefer to see fiberglass rather than closed-cell spray foam under the roof sheathing in this type of assembly.

Roof Coating Seal

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