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Diminishing Returns of Added Insulation

jollygreenshortguy | Posted in Energy Efficiency and Durability on

Journal of Light Construction ( published an article a few days back that presents interesting information on the diminishing returns of adding insulation. Some of you may find it interesting.
“Misleading R-Value and the Need to Reframe Insulation Scales”

The authors are also proposing a different labeling scheme for insulation, doors and windows, in an attempt to communicate more accurately and more clearly to consumers what they’re getting. I’m not convinced their solution communicates more clearly. But they’ve got some points to make.

I’m definitely on board with the diminishing returns idea. At a certain point I think it would be better to put the financial resources into other areas such as more effective air sealing, upgraded windows, and more efficient appliances, maybe even solar energy collection systems.

I suppose climate zone plays a role in where that line is drawn.

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  1. Expert Member
    DCcontrarian | | #1

    I just don't buy his premise: that the way insulation is labeled is confusing, which is causing consumers to make poor choices. Yes, insulation effectiveness is non-linear, and yes, there are diminishing returns, but that's just the way insulation works. In particular, he seems to think that consumers overbuy insulation, which is the exact opposite of my experience. Most people don't care at all about insulation, to the extent they have it in their homes it's because the building codes require it and local officials are willing to enforce the code. Even a lot of people in the construction business consider insulation to be a necessary evil, another intrusion of the nanny state.

    Given that most people are only going to have insulation to the extent that code requires it, the question I would ask is not about how insulation is labeled, but rather about how it's treated by codes. I'd say that the IRC actually strikes a pretty good balance between effectiveness, simplicity and cost-effectiveness.

  2. Expert Member
    BILL WICHERS | | #2

    There is some merit to the concept of putting more effort into properly installing insulation instead of just buying more of it. I agree that insulating labeling isn't the issue though. If people want to do a good job, they need to learn about R values and how they work -- we can't just redefine everything because someone doesn't want to take the time to learn. A simple diagram can easily show that going from R15 to R30 will lose half the energy lost through R15, but going from R30 to R60 will only be a 25% improvement relative to R15 alone, and will be much less noticeable in absolute terms, and it will be using a LOT more insulating material to achieve.

    I think it would make more sense to put more emphasis on the importance of air sealing and avoiding thermal bridges. Doubling up headers where they aren't needed is pretty common, and doesn't help anything, for example. A few air leaks can cancel out the gain of a whole lot of extra R value too. A little education goes a long way here, and I've long thought that that's the biggest thing GBA contributes to the building community. It's important to allocate resources effectively, since most people have a relatively fixed budget, so how to allocate those finite dollars for the most benefit is important. Just throwing money at the problem is inefficient, and not usually even possible for most people.


  3. Expert Member
    DCcontrarian | | #3

    He proposes an alternate rating for insulation, effectiveness, which he defines as 100*(1-U) or 100*(1-1/R). He gives the example of R-16 being equivalent to 1-1/16 or 93.75. I see two problems with this scale, one minor and one major.

    The minor problem is where is zero on this scale? It's at R-1. What is R-0.5? It's -100. In the world of insulation, what does zero effectiveness mean? What does negative effectiveness mean?

    The bigger problem comes when you compare, say, R-16 to R-32. R-16 is 93.75, let's round it off to 94. R-32 rounds off to 97. It's easy to look at 94 and 97 and say, OK, those are close -- both are A's -- not much difference. But an assembly with R-16 is going to use TWICE as much energy as one with R-32. This rating system is exactly the opposite of the way people should be thinking about insulation!

    The reality is there are a lot of situations where adding more insulation costs very little, and it keeps having impact. We should be encouraging people to use lots of insulation in those situations.

    1. Expert Member
      BILL WICHERS | | #4

      I think his proposed scale is more confusing than the R value system. A more appropriate way to go would be the decibel scale used for things like signal strengths and sound pressure levels, but people get confused with that too, since it can go above and below zero, with zero being a defined reference unit (often 1 milliwatt). What dB does though is make a log scale look linear, which is what insulation does too. This means 3dB is twice as much, 6dB is four times as much, etc. If you define R15 as your reference, that is zero, and we'll call the scale dBR, for decibels relative to R value. R30 is now +3dBR. R60 is +6dBR. If you go the "other way", down grading to R7.5 would be -3dBR

      It works, but I don't think it really helps to make things any less confusing -- it just makes the numbers look different. If we were to change the rating, I would use the dB scale since it's already defined and commonly using in the scientific and technical fields, and it does what we want here. The dB scale in this case also has the added advantage of making the difference between insulation values not look so huge, so people thinking "Wow! R60 is *so much more* than R30" will see "hmm, 6 isn't that much more than 3". The trick then becomes setting a good reference point so that people don't skimp, thinking it doesn't matter.


      1. Expert Member
        DCcontrarian | | #5

        The difference is that human perception of loudness is logarithmic. Energy consumption -- at least if you believe the prevailing model -- is linear to R-value.

        1. Expert Member
          BILL WICHERS | | #6

          Energy though is the same thing. The reason the dB scale is used for signal strengths in wires and cables is because of the way losses add. If you lose half your signal in 100 feet of cable, then you lose half of that half in the next hundred feet, for 75% loss in 200 feet of cable. That's -3dB for the first cable, another -3dB for the second, so -6dB for the system overall. Insulation works the exact same way, with the first R15 batt cutting your energy loss in half, the second R15 batt layer cutting that half by half, so R30 is losing only 1/4 as much as you started with.

          The way dB is usually used for people's perception of sound is that for people to perceive something as "half as loud", they need to actually hear 1/4 of the sound energy, so -6dB (which is 4 times less energy) is perceived has half as loud. Slightly different for that application.


    2. jollygreenshortguy | | #7

      "The reality is there are a lot of situations where adding more insulation costs very little, and it keeps having impact. We should be encouraging people to use lots of insulation in those situations."
      Agreed! It's not always the case, but there is a situation that arises very often in house construction. Once you're committed to something, like having a window (most houses do) then you've already committed to a substantial labor cost. You can use that labor cost to install a junk window that's inefficient and will fail in 5 years, or you can use that same labor cost to install a high quality window that will save energy and last 25 years. Yes, the better window will cost more, but the labor will remain roughly the same. So you get a lot more for your money when you put the labor into quality products.
      The same with insulation. If you're going to hire an outfit to drive across town, set up their rig, and blow insulation into your attic, you might as well have them blow twice as much. The labor and time will only be a fraction more.

      1. Expert Member
        BILL WICHERS | | #8

        Except that there are some issues that can come up. Going from R60 to R100, for example, is unlikely to ever make a noticeable difference, or pay for itself. That means that money would have been better spent elsewhere, such as better air sealing. With very heavy insulation loads, you can also risk sagging ceiling drywall, so that's another concern when getting into really high levels of insulation. While it's true that "more insulation will save energy", at some point you get into such miniscule savings that it's not worth it, and you also get into the potential for new problems due to the weight of the insulation.


  4. Expert Member
    Michael Maines | | #9

    Not many people really understand logarithmic scales. I don't know how many people I've confused, trying to explain decibel ratings or how doubling R-value cuts heat loss in half, regardless of the actual R-values, but it's a lot. This is not a wise mission, in my opinion.

    If you want the public to understand the basics better, just start using R-value for windows. People seem to intuitively understand the idea of diminishing returns; adding yet another measurement scale to the mix would just add confusion.

  5. maine_tyler | | #10

    His points on diminishing returns are fine, but I agree his percentage scale is actually more confusing. Actually it's worse than confusing, it fails to address most of the problems he claims R value has. It does nothing to indicate that whole house r value (lack of bridges) is what matters, or the other losses, like convective, that he talks about. The ONLY thing it does is set some relative bar by which to make judgements. Since r value is mathematically sound as he says, there is little issue other than misunderstanding application.

    Also this:
    " if we go from standard cavity insulation in a 2x4 wall (R-11) to a 2x6 wall (R-19), we do gain 42% more R-value (an increase of R-8). But many, even those who work in the building industry, assume that we get 42% greater resistance to heat flow, which is false. The 2x6 wall slows heat flow by only 4% more than the 2x4 wall."

    Is strange speak. You might even say incorrect unless he qualifies how hes using those percentages. Hes certainly wrong to call out others as wrong. As we all know, doubling r value cuts heat loss in half, which to me (and science) means a 50% reduction in heat flow from case 1 to case 2. But he's trying to redefine the whole system to a benchmark. I get why but it's clunky.
    42% greater resistance? Huh? we get 1.72 times the resistance.

    1. Expert Member
      DCcontrarian | | #11

      I can't shake the feeling that he's a guy who feels that current codes have gone overboard in what they require in terms of insulation. That attitude is really common in the business. And he's looking for a way to prove it.

      But his math is atrocious.

      1. Expert Member
        BILL WICHERS | | #12

        It might be some of the same stuff the spray foam guys push, but in a different way. You know the "our spray foam R value is better than that other product's R value, because spray foam air seals too!", which isn't really correct, because air sealing is a seperate issue. Sometimes I see that done by accident (often by installers), other times on purpose (usually by manufacturers). I think the guy in the video is more on the unintentional side.

        We probably are up in the point of diminishing returns for R value, but it would make more sense to just establish some benchmark, which is effectively done in the code already with the minimum required R values for each climate zone, then build on that. I don't see that the goofy method in the video does anything to help here, it seems more confusing to me, but maybe that's because I'm already used to working with the current R value system?


      2. maine_tyler | | #13

        Yeah you could be right DC. My sense is that he learned about diminishing returns and was so intrigued by it, felt there needed to be a system that conveyed the concept to consumers (or just wanted to write a piece about such).

        But what does his system really convey to a consumer. What does 80%, or 87% or 94% resistance to heat loss mean to decision making? Is 80% good enough for some people? Maybe even 75? And thats still just cavity value if between studs. R value has a similar issue in that its just a number, but it's a useful number for figuring out how the building will manage conductive heat flow (whole wall or whole house r value being ideal, and being that which actually addresses most of his concerns).

        1. Expert Member
          DCcontrarian | | #14

          If I got all C's in school, should I be happy with 75% resistance to heat loss?

        2. Expert Member
          BILL WICHERS | | #15

          This reminds me of a story I heard years ago about one of the fast food chains coming out with a 1/3 pound (might have been 1/2 pound) burger, and having issues selling it because people thought the 1/4 pound burger was a better deal -- 4 is bigger than 3 (or 2), right?

          To accurately describe anything somewhat complex requires some complexity. It might be better to just go with something like the Energy Star program's "this will cost you about this much $ per year" thing. Regardless of the way it's shown though, many people aren't going to understand or pay attention. Remember that extra insulation doesn't usually add value to a home at sale time, unfortunately, which is another issue that has been discussed on GBA in the past.


  6. matt2021 | | #16

    I've been following this conversation with great interest. As a NON-expert (by ANY measure), but having heard, for instance, so much about the importance of considering the efficiency of windows, I would:

    - Vote in favor of standardly formulating U-factor requirements in terms of R-value -- a small but helpful simplification for consumers, I think, and one likely to help people put in perspective the energy efficiency of their walls as a whole.

    - I would also would like to see the IECC offer recommendations about wall assemblies' (walls and windows combined) R value (unless the IECC does that already, and I am not aware of it.)

    Overall, I suspect that MORE, not less, complexity might be desirable, and not always be incompatible with effective communication to users.

    Somewhat unrelatedly, I myself have been presented with claims, by spray foam installers, that "The whole R value system is based on fiberglass; it should be revised for foam," or that "Two inches of closed cell in a ceiling are enough": the confusion between insulation and air sealing indeed seems to be pretty rampant in the spray foam world.

    1. Expert Member
      Michael Maines | | #17

      To be fair, R-value was indeed originally devised by and for the fiberglass insulation industry. But to say that means the test methods won't work for other insulation types is faulty logic.

      1. matt2021 | | #18

        Thanks for the clarification, Michael, and for the link to Martin's article!

    2. Expert Member
      BILL WICHERS | | #19

      U value is the inverse of R value. That means a window with a U value of 0.22 is 1/0.22 = R4.55. It's easy to convert between the two, an R15 batt, for example, becomes 1/15 = U 0.067

      R value is a measure of the resistance of heat flow through a material. It is measured the same way for any material, fiberglass, mineral wool, spray foam, even a piece of framing lumber. The material itself doesn't matter. R value is defined for the purposes of measuring insulation for a certain set of conditions (temperatures), then you just measure the heat flowing through the insulation under those conditions to calculate the R value. It's a standardized system now under ASTM (American Society for Testing and Materials, which is a professional organization).

      The spray foam line "spray foam is better than regular R value!" is based on spray foam doing a good job of air sealing. Air sealing though is entirely different from R value, and they don't affect each other for testing purposes. R value is R value. If you were to build a box and insulate it with spray foam, then build the same box again and insulate it with fiberglass, the R value would measure the same for both boxes assuming you air sealed the box first. It's true that air sealing as a step in the construction process is more important when insulating with an air permeable insulating material like a fiberglass batt, but that has nothing to with the R value of the material. If you were to do a crummy job of spray foaming a structure so that there were still lots of air leaks, spray foam would perform poorly just like any other insulating material.


      1. matt2021 | | #20

        Bill, it could have not been explained better!

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