Is R-Value Dead as a Dodo?
The R-value of your insulation is just one number — and it’s not enough to tell you everything you need to know about your wall’s thermal performance
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. Houses included features to produce and retain heat, of course — things like double back-plaster walls and 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 1 inch of cellulose sandwiched between two sheets of Kraft paper. Most of the Kimsul in my house had disintegrated or did so upon touch.
Not much — but, hey, you’ve got to start somewhere.
It's from these humble initial efforts that R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. and heat retention entered the national conversation. The question today is: in the increasingly complex world of building science, how meaningful is R-value? Can we do better?
The strikes against R-value
The challenge, simply put, is that the R-value listed on the insulation package is not the same as the R-value of 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.
BLOGS BY ERIC NORTH
There’s thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. at the studs, where heat travels more easily through the framing and past the insulation. There are 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. But it’s the wall we care about.
R-value is just one measure
The fact is that we are moving to a world in which building science, in all of its complexity, assumes greater prominence. Most homes 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 noticeably 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 performance characteristics. The window labels list the U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. (the inverse of R-value) of the whole window and its SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1. (solar heat gain coefficient(SHGC) The fraction of solar gain admitted through a window, expressed as a number between 0 and 1.) — vital measures for picking a window. Live in the frigid north? Pick one with good U-factor. Live in the sunny south? Pick a window with a low SHGC to keep out excess sunlight.
The metrics we use to measure the performance of insulation will likely evolve in the same way.
The future of R-value
It's possible to measure whole-wall R-value instead of the R-value of the insulation material. We can also discuss whether a wall is air-permeable, and whether the wall would benefit from an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both.. How vapor-permeable are the various layers? Is the wall rain-resistant? For example, EPS is somewhat vapor-permeable and rain-resistant.
So is R-value dead? No, but it needs to be understood as part of the picture. It's one measure of a building material. A car’s fuel efficiency may be measured with just one number (miles per gallon). But insulation, like a window, has several important characteristics beyond R-value.
R-value is a single measure of insulation performance; it's not the only one.
- Photo credit: Giles Douglas
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