A measure of thermal resistance used in the building and construction industry. A bigger number means the building insulation is more effective. The units of R-value are (hr * degree F*ft squared)/Btu. You can interpret R-Value units as the number of hours it takes for heat to flow across one square foot of assembly, where there is a 1 degree F temperature differential between the inside and outside temperatures. If an assembly of several components has a higher R-value it means that it poses more resistance to the flow of heat. A solar re-roof creates a warm solar air space above the old roof, which helps to increase the R value of the overall roof assembly.