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. 1.8-2 pound polyurethane foam has the highest R-value of readily available insulation used in homes and buildings.
Polyurethane is a closed-cell foam insulation material that contains a low-conductivity gas in its cells. As a result of the high thermal resistance of the gas, spray polyurethane insulation typically has an R-value around R-5 to R-6 per inch. In comparison, blown fiberglass typically has an R-Value of only R-2 to R-4 per inch.
Foam Insulation blocks all 3 forms of heat transfer.
Conductive Heat Transfer
The flow of thermal energy through a substance from a higher to a lower temperature region. Foam thermoset plastics block conductive heat transfer due in part to having very loose molecular bonds; In addition the cells of the installed spray foam are either filled with air in the case of open cell foam or 245fa in closed cell foam. 245fa gas found in closed cell insulation is very similar in molecular structure to argon gas, used in high end window systems to block radiant heat from the sun in the cooling months as well as helping to keep radiant heat in the house during the heating months.
Radiant Heat Transfer
The process by which energy in the form of heat is emitted by a warm surface, traveling at the speed of light and does not require a medium to carry it. Thermoset foams, such as spray foam insulation is the ability to reflect radiant heat. This is due to the reflective surface within the cell structure of the foam molecules themselves.
Convective Heat Transfer
Heat which is created elsewhere that is transported by means of a fluid, such as water or in our case air.Spray foam insulation's most important attribute is the ability to air seal creating a custom airtight envelope within the building structure. The added benefit to air sealing is the ability to block convective heat transfer from interior to exterior during heating months and vice versa during cooling months, as the heat cannot escape through gaps in the buildings envelope without the aid of air movement from infiltration as a means of transport.
Read more about this topic: Spray Foams (insulation)
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