I don't know if it's common or uncommon, but installing a layer of rigid foam above your roof sheathing in order to interrupt thermal bridging is always a good idea, and is preferable in all respects to a roof assembly without any rigid foam above the sheathing. If you decide to install rigid foam above the roof sheathing, you shouldn't use closed-cell spray foam under the sheathing; either use open-cell spray foam (which is vapor-permeable) or a fluffy insulation like cellulose, mineral wool, or fiberglass (any of which will allow the roof sheathing to dry to the interior). http://www.youtube.com/watch?feature=youtube.be&v=ggLAUsiuI_o

These coatings contain various types of acrylic polymer, and are typically formulated to help extend the coating’s life and improve durability. Acrylic coatings are available in a number of colors but are most commonly used as white reflective coatings. Thanks to its durability and low cost, many contractors and facility managers believe this to be the best roof coating.
For maximum durability and leak resistance, we recommend that two coats of topcoat be applied at 100 sq. ft. per gallon, or a minimum of 20 mils dry coating. If two complete topcoats are not applied, always apply two thick coats to problem areas such as scuppers, drain areas, ponding areas, seams, and repaired areas. Avoid using Elastek Solar Magic™ or The Shield™ on ponding roofs. Clean skin and tools promptly with water.
25 years of field study and thousands of satisfied customers have made Liquid EPDM Rubber the roof repair solution for many applications. Liquid EPDM Rubber is so versatile that it can easily be applied to metal, rubber, fiberglass, concrete, wood, fabric, foam and many other surfaces. Liquid EPDM Rubber Coating has proven to outlast acrylic, urethane and elastomerics by 4 times. There may be lower-cost products on the market but none that have the long-term durability, waterproofing ability and tolerance to extreme temperatures like Liquid EPDM. Other roofing coatings require two and three coats so not only does this triple your material cost but also triples your labor time. If you are looking to add less weight to your roof with a one coat system that has proven itself over the years look no further. Here is a sample of some of the companies during that time who have come back time and time again!
It also has a great waterproofing ability that can shield the roof’s membrane from water and moisture damage. You can also count on the liquid roof coating for its outstanding performance for a wide range of applications and uses. This product can be used at tears and seams as well as in sealing air conditioners and venting systems and anywhere else where rubberized coating is needed.

Spray Foam Roof Insulation

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