Please refer to the attached Law of Diminishing returns chart. As you can see, from the R-20 point on the graph, you already have 96% conductive heat flow resistance. By adding additional R-value to get to a prescriptive level of R-38(typically for batt insulation) for Zone 2, you are only gaining less than 2% of additional heat flow resistance. So you are not getting much for the money invested in the increased R-value. With Icynene open cell spray foam insulation, you have the advantages of the air barrier component coupled with the conductive heat flow resistance, ultimately making the home more energy efficient, so there is no need to increase the R-value beyond R-20.
There are several ways to meet building code when it comes to residential insulation requirements. One of the more popular ways is the Performance method in which, rather than simply supplying the prescriptive insulation values, an energy analysis is done on the home.
Spray foam insulation, being an excellent air barrier material, helps create a tighter home that does not leak interior conditioned air uncontrolled. This allows for the home to perform better and for a lesser R-value of insulation and still comply with building code.These advantages have allowed homes all over Florida utilizing spray foam insulation at an R-value of R-20 and still perform better than traditional insulating materials.
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