Comparing Fiberglass Batt Insulation R-Values
Hello GBA Folks,
I am grappling with the differences between R19 and R21 fiberglass batt insulation.
R19 is 6.5″ thick whereas R21 is 5.5″ thick. Thinner by one inch and yet 2 points higher in R value. The common explanation as to why this is so, is that the R21 insulation is said to be “more dense”.
But I have always thought of fiberglass batt (any insulation really) as just being a means of trapping air and keeping that air from moving around (due to natural convection). Air is a much better insulator than glass after all.
So how does R21 batt, which is thinner, and has a greater amount of glass per inch, achieve better insulation?
I am either missing something obvious, or this is a subtle thing.
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