Spray polyurethan foam does effectly prevent all six of these heat transfer methods from occurring. These should be the mechanisms by which you judge insulation, in addition to their R-Value. If you're uncertain about installing spray foam insulation, Energy Star has a map that recommends levels of insulation by R-value and cost-effectiveness in the United States.
“It’s been scientifically proven that the temp of the shingles rises only a couple of degrees when there is an unvented attic compared to a vented attic,” said Garry Barlow, estimator at RetroFoam of Michigan. “If we lived in a southern state it has been proven that the longevity of the shingle did diminish by only one year. In the northern states there was no difference in the life span of the shingles.”
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Installations present a number of considerations and recommendations in order to provide a safe environment for application and to avoid overexposure through inhalation. Appropriate ventilation and protective gear is required when applying insulating foam sealant. Installers should wear protective gear, often referred to as “personal protective equipment” (PPE), such as protective clothing, gloves, and eye protection. Direct skin contact should be avoided, and recommendations by the product manufacturer should be followed.
R-value is the term given to thermal resistance to heat flow. The higher the R-value of an insulation product, the more effective the insulation properties. Spray polyurethane foam comes in a range of densities and cel structure. Low density foams are referred to as Open Cel SPF while higher density foams are referred to as Closed Cel foam. 1.8-2 pound polyurethane foam has the highest R-value of readily available spray foam insulation used in homes and buildings.[4]

Roof Spray Foam Insulation