Insulation can be one of the most important factors in improving energy efficiency in a building. It slows the flow of heat through a building envelope. Insulation not only saves money by reducing heating and cooling loads but also is a key factor in achieving comfortable living and working spaces.

Insulation ratings

All insulation is rated according to its resistance to heat flow, measured in units of R-value in the United States. The inverse of thermal resistance is conductance, referred to as the U-value (U = 1/R), which in the United States is measured in units of Btu/square foot-°Fahrenheit-hour.

Whereas R-value is intuitively easier to understand (the higher the R-value, the better the insulating properties), U-value is more useful in calculations, because it describes the actual amount of heat that will move through the material for each degree Fahrenheit difference in temperature from one side of the material to the other. R-values of different components can be added (all the different layers of a wall, for example); U-values cannot be directly added (Figure 1).

Figure 1: Addition of R-values
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