As you can see from the graph to the below, the house with the spray foam insulation preformed almost 40% more efficiently than the house with Fiberglass insulation. But how can this be, considering House 1 had almost twice the R-value on the building envelope? The answer is simple. The house with the spray foam envelope was eliminating air infiltration, as well as stopping the heat transfer at the point of entry. In summary, we have learned that it is more important to have air-impermeable insulation on the building envelope than to have air-permeable products with higher R-values. Applying spray foam insulation to the underside of roof decking significantly adds to the efficiency of the home. Yes, the initial cost of spray foam insulation is greater than other methods, but with the high amount of energy savings you will achieve, it will more than pay for itself in a short amount of time. In this example it payed for it self in just three years. Just think, you can apply your energy savings directly to your monthly mortgage payment. On a thirty-year mortgage that is a lot of savings.