Perhaps you have heard or read about shingle manufacturers not warrantying their products if the attic roof deck is spray foamed. This was started by some roofing companies that believed if the attic wasn't vented the shingles would degrade and their lifetime shortened. They felt that the shingles would suffer from extreme temperatures and break down faster than those on a vented roof deck. Studies have shown this is not the case. Research compared homes with vented attics and homes with unvented conditioned attics. They found only a two degree difference in homes that had conventional insulation on the ceiling with vented attics and homes with unvented attics and insulation on the roof deck. The study also noted that the roofs location and the shingles color had a greater affect on shingle temperature.
Another thing to think about is there are several shingle manufacturers that also make spray foam. Look for those manufactures to guide towards using their complete roofing systems, which include spray foam insulation at the roofline.
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