Table 1 indicates the fuel use per square foot and the simple payback resulting from adding rigid insulation in one inch increments to a roof assembly.
|Table 1. Payback to increase R-value of rigid insulation on roof|
to add 1"(yr)
Note that the difference in energy savings between 4" rigid (R-20) and 12" (R-38) fiberglass is also only about $115/yr. for a 5000 square foot roof assuming the fiberglass works perfectly, which of course, it doesn't. In reality, 4" of rigid insulation will probably perform better than 12" of fiberglass because there will be much less infiltration.