I spent a lot of time last October and November researching appropriate insulation options. In the post “Insulation – how much is needed?“, I described the SPF (spray polyurethane foam) phenomenon of diminishing returns.
I somewhat understood the concept of this effect, but had no information on the actual process or cause. So I asked whether anyone had more information on this.
Lo and behold, I received a comment on the post from a gentleman called R. Tom:
“… a report that illustrates a scenario that uses Fourier’s steady-state heat flow equation to evaluate the performance of a typical wall area with a prescribed R performance value. The results are quite profound… it indicates, basically, that the first inch of [SPF] insulation represents 80% of the heat flow reduction in the system, the next inch 9%, the next inch 3%, the next inch 2%, the next two inches 1% each and the next four inches only 1%! So in the first 2″ you are getting 89% of the total performance realized in your [SPF] insulation assembly.”
The report R. Tom mentions is a publication by Icynene Inc., describing the testing process, providing the math and plotting the results in various graphs.
My first thought was: “Is this for real?” I forwarded the report to a mechanical engineer I respect and asked for his opinion. He agreed with the rationales and results of the report.
Here is what my sleepy little brain cells retained. There are three types of heat flow:
- Conductive heat flow
- Convective heat flow
- Radiant heat flow
The R-value is a measure of the conductive heat flow resistance through a material, but ignores the influence of convective and radiant heat flow. SPF insulation has an R-value of 3.6 per inch thickness (as per the report). If properly applied, SPF can eliminate air infiltration (or convective heat flow), and thus delivers up to 89% of the total performance in the first 2 inches. I think I sort of get this.
My second thought was: “How does this impact our project? Do I need to rethink our insulation strategy?” The answer is yes. Now that I understand the effectiveness of the first two inches of SPF and the decline thereafter, I would like to look again at various insulation materials and their performance, cost and environmental footprint.
PS: I found a great website that explains SPF in plain language: