It makes good sense to properly insulate new walls. Efficient use of insulation keeps a building warmer in winter and cooler in summer, thereby reducing both heating and air-conditioning costs. Many building regulations now specify minimum R-values for insulation products, but when you need to find the overall thermal conductance of a wall or building, you need to calculate the U-factor. Calculating the U-factor is straightforward when you know the R-values of the materials you have used.
Determine the R-values of all the materials you have used, and then add them together. For example, a 2-by-4-inch stud wall has an R-value of 5, and a layer of 3-inch batt insulation has an R-value of 11. The stud wall plus the insulation has an R-value of 16 — 5 + 11 = 16.
Calculate the U-factor by using this formula: U-factor = 1/Sum of the R-values. Using the previous example with an R-value of 16, the U-factor is one-sixteenth, or 0.0625.
Check your calculations by reversing the calculation and determining 1/U-factor. If the result is not the original R-value, then there was an error in your calculations. Repeat them until the correct result is obtained.