Once upon a time, house insulation meant an extra sweater and stop your damn complaining. Men were men, women were women, and cats and dogs were cats and dogs, I assume. There were heat retention and heating features built into houses, things like double back plaster walls or central chimneys. But until the 20th century, insulation barely existed in any formal sense.
There were some tentative initial steps. My house, built in 1939 contained Kimsul insulation. Kimsul was one of the first commercially available insulations, sold in Sears & Roebuck catalogs with their kit house plans. Kimsul was one inch of cellulose sandwiched between two sheets of Kraft paper. Most of it had disintegrated or did so upon touch. Not much but, hey, you’ve got to start somewhere.
But its from these humble initial efforts that R-value and heat retention entered the national conversation. The question today is that in the increasingly complex world of building science, how meaningful is R-value and can we do better?
The Strikes Against R-value
The challenge, simply put, is that the R-value listed on the packaging is not the same as a wall insulated with the material. If you install R-19 batts in a wall cavity, can you say it is an R-19 wall? No, not really.
There’s thermal bridging where heat travels more easily through the wall studs and past the insulation. There’s holes and gapping where insulation sags at the top, pulls away from the sides, and is forced around electrical outlets and pipes. Air moves both across the wall cavity and within it.
Then there’s the question of what are we measuring: the insulation’s performance in the wall cavity or the performance of the entire wall. The R-value on the insulation bag is the tested heat retention ability of the insulation. It’s the wall we care about.
R-value Is Just One Measure
The fact is that we are moving to a world where building science is of greater complexity and prominence. Most housing in my neck of the woods were uninsulated (or barely insulated) when first constructed. Adding any insulation produced such large improvements in comfort that a crude measure like R-value sufficed. If a frigid house went from R-2 to R-10 and it was measurably more comfortable then R-value worked.
The window industry went through a similar evolution. The end result is the now ubiquitous window labels which detail all their characteristics. The window labels list U-value (insulation) of the whole window, SHCG (solar heat gain coefficient) and emissivity, all vital measures for picking a window. Live in the frigid north? Pick one with good U-value. Live in the sunny south? Pick a low-E window to keep out excess sunlight. The measure of insulation will likely evolve in the same way.
The Future of R-value
R-value could be evaluated as a whole wall value rather than just that of the insulation material. How air permeable is it and does it need to be paired with an air barrier. How vapor permeable is it and is it rain resistant? For example, EPS is somewhat vapor permeable but rain resistant.
So is R-value dead? No, but it needs to be understood as part of the picture of what building material does. A car’s fuel efficiency may be measured with just one number, miles per gallon. Insulation, like windows, has several important characteristics beyond R-value. It is a single measure of insulation performance but not the only one.