Equivalent R-Value

This is one of the most confusing things about energy efficiency, but you're going to hear about it and you'll probably have to explain it to buyers at some point. So you'll have to learn it. You can do it now or skip this section for a week and come back when you're feeling up to it.

Heres the basic story of equivalent R-Value. Suppose I have a frame house and an icf house of the same design. The ICF house uses less energy for heating and cooling. The owner of the frame house might say, "That's because its walls have a higher r-value. There about an R-20 and mine are only R-10". So He might come up with a plan to get the same level of savings as his neighbor. He might tear down his house (ok this is only a story) and rebuild it with thicker walls that have more insulation. And he might make sure the work is done well.  And by the time he's done he might truly and honestly have frame walls that are R-20, the same as the ICF walls.

So, will he now have the same energy bills as his neighbor in the ICF house? Well, no actually he won't. The higher R-value walls should cut conduction to be the same, but the ICF walls still get some additional savings because of their lower air infiltration and their high thermal mass. So now someone selling ICF's says to himself, "You know, when people go out shopping for houses and they compare energy efficiency, they always look at R-value and that's about all they look at. But that's not fair. Our walls rate about twice the r-value of frame walls, but even if the frame walls were built so that you doubled their R-value, our houses would still use less energy because we have lower air infiltration and higher thermal mass. So when they look at R-value they only get part of the picture, and I can't get them to look at anything else."

But the ICF salesman could have a bright idea. He could say to himself, "You know, I wonder how high you'd have to jack up the R-value of the frame house to get down to the same energy bill as the ICF house?" Then he could pay the owner to rebuild the frame house again, making the walls thicker and adding more insulation until he got the fuel bill down to the exact same amount as the ICF house. Because the frame house never gets the same savings from lower air infiltration and from thermal mass, you have to keep stuffing insulation into the walls to try to compensate.

Now the ICF salesman goes out and tells people, “An ICF wall has an equivalent R-value of R40. By that I mean, to get the same energy bill as an ICF house you would have to build your frame walls so that they were R-40.. So building with ICF walls is equivalent to building with R-40 frame walls."

Now the ICF salesman has an R-value number to hand out that gives people some kind of idea of the total energy savings from ICF walls. Equivalent R-values this high are not unusual. A few years ago, engineers at Construction Technology Laboratories did energy consumption estimates for ICF and frame houses that showed that a house with ICF walls would still have a lower energy bill than even an R-38 frame wall constructed with 2 x 12 studs, So that means that their equivalent R-value would be somewhere over 38. And this result holds for different climates across the United States. The report of this study is titled Energy Use Of Single-Family Houses with Various Exterior Wall Systems. Home people refer to it and if you want a copy you can order it on the www.concretehmes.com web site or at PCA Publications (800-868-6733).

A lot of people find equivalent R-value to be a useful way to summarize the energy efficiency of ICFs. And whether you do or not, other people will use it so you might as well know what it is. But there are a few important cautions. First, equivalent R-value is not the same as the conventional R-value and you shouldn't pretend it is. ICF walls are not R-40. They still allow heat to conduct through like R-20 walls because that's what they are. It's just that they have other, different ways, to save some energy, Equivalent R-value is a way to boil all the energy savings down and summarize them in one number, But it's a different type of number.
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Second, there's no good way to figure out total fuel bills from the equivalent R-value. If a frame wall has an R-value of 10 and an ICF wall has an equivalent R-value of 40, that does not mean that the ICF building will have one-quarter of the fuel bill. Without an engineering degree, all you can really say is that the higher the equivalent R-value, the greater the energy savings.

Third, the equivalent R-value depends as much on location as it does on the walls. So a certain ICF wall doesn't really have a set equivalent R-value. The equivalent R-value of the same wall could be 50 in St. Louis and 40 in Miami and 30 in Edmonton. The thermal mass savings are lower in extreme climates, and so the equivalent R-value is lower.