Spray foam insulation, when properly installed is both an air barrier and when closed cell foam is used at the proper depth a vapor barrier!  Before hiring any contractor you should establish the brand of foam being used and verify that with the named manufacture.  Please ask for ESR reports and MSDS sheets on any foam being installed in your home. A couple areas where you might consider spray foam insulation include:
Roof coatings are easy to install. Smaller roofs in good, serviceable condition can be a weekend do-it-yourself (DIY) project. Larger roofs with leaks and ponding water issues should be coated by a qualified professional. Additional roof coating products, such as foam or coatings designed for seams, may be required underneath the main coating layers. Supplement coating products are used to reinforce damaged or leak-prone problem areas.[3]
Thoroughly clean the roof surface. Roofs collect oils from asphalt, chimneys, and cars plus lots of dirt and dust. Coatings don’t stick well to any of these. We recommend cleaning with an inexpensive solution of TSP or TSP Substitute in a bucket of water (follow label instructions). Use a broom to scrub ponding areas and areas of peeling coatings. Work a section at a time and hose off the dirty water. Sweep away puddles to promote drying. Do not allow wash water to dry on walls, furniture, windows, and visible surfaces. 
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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