Insulation and R-value go together like hard work and sweat. R-value is, of course, the resistance to heat flow. We’ve been taught to think more is better, which is true to a point. I’m changing the insulation in the wall of my home from R-7, originally installed in 1952, to R-15, and I’m excited to see how much more comfortable my home becomes. There is an argument that at a certain level, more insulation will cost dollars to save pennies. At what point do we reach diminishing returns?
Insulating products have different R-values per inch, closed cell spray foam has an R-value of around R-6.5-7 per inch. Rockwool’s Comfortbatt is around R-4.2 per inch and XPS foam insulation is closer to R-5 per inch. No matter which insulation we choose to use, the resistance to heat flow will be the same as long as the installed R-values match the different insulation types. R-20 equals R-20 regardless of if we choose to use closed cell spray foam or dense packed cellulose. But here is the question, at what point is enough R-value enough?
Is stopping 75% of all conductive heat flow enough? (That’s R-4 by the way.) How about 98% of all heat flow? (Rounding has the 98% reduction between R-41 and R-66.) So, how did I calculate those values? We do it by converting R-value to U-factor. U-factor is the rate of heat transfer and is calculated by using the following equation: 1/R-value=U-factor. So, let’s take that R-4 insulation and turn it into the U-factor value. 1/4=0.25, so R-4 is equal to U-0.25. This means that 25% of the available energy is passing through the R-4 insulation, or in other words, we have stopped 75% of the heat flow.
Now, let’s move up to R-13, a typical 2×4 wall assembly using standard fiberglass batts as the insulation. 1/13=0.077. Only 7.7% of the available energy is making its way through the fiberglass insulation, we have stopped 92.3% of the energy flow. R-20 stops 95%, R-30 stops 96.7% and R-40 stops 97.5%. We’ve doubled the insulation level from R-20 to R-40 with only a small increase in heat flow reduction.
To be clear, this only counts towards conductive heat flow at the center of the insulation, convective and radiation heat movement is not taken into account. We are also not including other framing and building components, just the insulation. There is a very good argument to used R-20 cavity insulation with continuous exterior insulation. Another argument is these small reductions spread out over the life of the home do add up, it’s just the monetary reasons are spread out over a long period of time.
When is enough, enough? The answer to that question is the same as so many other building science questions, it depends…