In the early days of insulating attics, people believed the insulation was a heat source. That’s not the case, as the insulation is actually a barrier between two areas of different temperatures. As for the “hot roof” theory, the unvented attic system could make the temperature of the roof go up a little, but this increase won’t cause damage to your shingles.
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.
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One of the key differentiators between traditional insulation materials and spray foam insulation is the latter’s ability to insulate and air seal. Foam insulation provides an air barrier to wherever it is applied to help mitigate air leakage from the building. Air sealing the building envelope with sprayed-in foam insulation also helps address moisture ingress to reduce the risk of mold and mildew growth as well as the formation of ice dams in colder climate zones during the winter months. When you compare foam insulation with traditional fiberglass insulation and cellulose insulation, sprayed insulation minimizes air infiltration, it assists in limiting moisture vapor from entering and escaping the home, which in turn reduces the load on heating and cooling systems. Below is a video that compares fiberglass, cellulose and open-cell spray foam in terms of insulating and air sealing value.
My other question was gonna be this. We ripped all the drywall out after Hurricane Harvey and we found some latent termite damage from some time back. One of the common studs in one corner is pretty well eaten to shreds. I was gonna brace it but then I read about more modern framing practices and I read how each stud is a thermal bridge. So now I'm thinking that I won't bother with it because the house hasn't fallen down and the foam might help a little. Unless you say to brace the stud. Then I'll do it, Martin! :)