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Q&A Spotlight

Finding the Insulation Sweet Spot

A builder in Montana wonders whether new tests should be conducted to find the point of diminishing returns for insulation

An experimental wall in Montana built by Anders Lewendal includes 1 inch of rigid foam nailed to the outside of a 2x6 framed wall, 2 inches of spray polyurethane foam behind it, and then 3 1/2 inches of cellulose, for a total R-value of about 32.
Image Credit: Anders Lewendal

Anders Lewendal, a builder in Bozeman, Montana, is wrestling with a familiar dilemma: What’s the right amount of insulation to put in a house?

“Our theory,” he writes in Q&A post at GreenBuildingAdvisor, “is that too little insulation wastes energy and equally, too much insulation wastes energy. Where is the sweet spot in each climate zone?”

To that end, Lewendal is proposing more performance testing.

“We are interested in knowing if GBA has conducted any performance testing that makes diminishing returns conclusions,” writes Lewendal. “If not, we are hoping GBA might give us some advice that makes our experiment productive.”

[Coincidentally, Lewendal is the founder of a “build American” campaign promoting the use of U.S. building materials. GBA has published two articles on his efforts: One Builder’s Buy-American Strategy and A 100-Year-Old Energy Star Home.]

The work has already been done

Lewendal could save himself the trouble of a new study, replies GBA senior editor Martin Holladay, because the work has already been done.

“What you call a ‘theory’ is a truism enshrined in our building codes,” Holladay says. “The entire reason that the minimum insulation values in U.S. building codes are higher in Minnesota than in Florida is the well-understood calculation that you call a ‘theory.’”

If you double the R-value of your insulation, the rate of heat loss is cut in half, Holladay continues. “The only questions are (a) whether more insulation is a good investment, and (b) whether the embodied energy of the insulation materials exceeds the energy that is likely to be saved over the lifespan of the insulation,” he says.

Lewendal proposes the construction of three identical houses and studying the effects of adding more insulation. Although it’s possible this exact test has not been performed before, Holladay says, it really doesn’t matter.

“Michael Blasnik (among other researchers) has assembled energy use data on hundreds of thousands of U.S. homes,” he says. “Energy researchers have developed sophisticated models that have been repeatedly validated by comparing modeled results to test home performance. In short, we know exactly what happens when we add R-20 of cellulose to an attic with R-38 cellulose. Of course, different families operate their houses differently. But we have all the data we need to do the calculations that you apparently think have never been made.”

Parts of the equation we don’t know

Calculating the “sweet spot” of exactly the right amount of insulation with any precision requires two bits of information, adds Ron Keagle, the cost of energy and the cost of money over time. “It also depends on individual perception of thermal comfort and their willingness to pay for it,” he says, “although I suppose you could average that across all homeowners.”

Another wild card, says James Howison, are the occupants themselves. Suppose they spend $200 a month on heating and cooling in a leaky, poorly insulated house. They stay on budget by adjusting the thermostat — a little cooler in winter, a little warmer in summer. With a better insulated house, they still spend $200 a month on energy but they can afford to be more comfortable, Howison says.

“On one hand one could say that the improvements are yielding more comfort and are therefore efficient, but from an energy perspective it’s problematic,” he writes. “I suspect that this applies with existing housing, perhaps less with new housing. I think that the proposals to include energy costs in budgeting for getting mortgages would really help this, by including this expectation.”

Holladay adds that there’s one more thing to ponder: “Here is a huge factor: should we include the external costs of burning fossil fuels in our fuel cost assumptions? Right now, the U.S. government is unwilling to enact carbon taxes that reflect the true economic cost of global climate change. As a result, every U.S. homeowner pays less for electricity, natural gas, and fuel oil than would be the case if the price of energy included the the true cost to the planet of burning fossil fuels.”

No, we still don’t know the answer

Despite claims to the contrary, Lewendal isn’t convinced enough research has been conducted. He’s done the background reading suggested by Holladay and others, and is familiar with the suggestion that above-grade walls in cold climates be insulated to R-40.

“What if the diminishing returns for insulation here in Bozeman is R-30 and we took your advice and installed R-40 in the next thousand homes and it turns out R-30 is where the curve bends down reducing the marginal improvement in performance?” he says. “The cost of going from R-30 to R-40 is about $3K. What is the opportunity cost for our customers if we overspent $3 million on insulation?”

In fact, Lewendal says he has used two energy modeling programs and can’t conclude R-40 is best for his area. “We have studied models from all over the world and found that countries like Turkey and those in Scandinavia have done a better job of modeling the diminishing returns of insulation than we have,” he says. “Still, we are not convinced that prescriptive modeling matches performance very well. My best example is the PHIUS [Passive House Institute U.S.] model. They think that R-70 plus walls will give homeowners the best value. I am quite sure that a very low [air changes per hour] and modest insulation is more appropriate. The exact number for us here in Bozeman is what I want to determine.”

R-40 may be ideal but consumers aren’t listening

The bottom line, Lewendal adds, is that consumers don’t seem to be responding to the consensus that R-40 walls are close to ideal in a cold climate.

“Most homes get about an R-21 because the government says it is good and our cities enforce that level of insulation,” he writes. “A few homes get about R-70 because they think Wolfgang [Feist of the Passive House Institute] is a smart guy and they will pay almost anything to reduce CO2 even if it means making more CO2 than the opportunity cost of that extra insulation… So, how do we get the average homeowner to ask for what your blogs have suggested, which is a PGH or pretty good house?”

Lewendal thinks there are enough uncertainties to justify his new study.

But to Keagle, his quest to find the insulation sweet spot can be based only in part on objective science.

“The rest is intuitive and subjective,” Keagle writes. “Part of that is simply belief. You can build an example house and prove what it does. That would be convincing to the extent that it confirms part of the objectivity of the sweet spot.

“But communication, information, explanation, and marketing can also be convincing without an example. Or the example can be part of the marketing as a working demonstration. I don’t see any of this as reinventing the wheel. The goal is to sell the public on the idea of higher efficiency.”

Passivhaus targets aren’t based on cost-effectiveness

Holladay agrees with Lewendal that R-70 walls are overkill. Holladay notes, “You’re right; PHPP [Passive House Planning Package software] pays no attention to cost-effectiveness. All PHPP tells you is how to hit 15 kWh per square meter per year.”

He also agrees that R-40 isn’t the right answer for all cold-climate builders. “If you have done the calculations for your housing type, your wall insulation type, your insulation costs, and your payback time frame, and you have come up with R-30, I have absolutely no reason to doubt you. I have consistently said, ‘You have to do the calculations.’”

Our expert’s opinion

GBA technical director Peter Yost added this:

On one level, this sort of discussion drives me crazy. You simply can’t energy-model a single answer to the question of the “right” level of insulation or home energy efficiency. There are just too many variables, including changing wall configuration and systems with greater assembly depths; ever-increasing and unpredictable energy prices; climate change; assembly performance impact on the “right” mechanical system.

And since many energy-modeling program results are either directly or inherently linked to simple payback analysis of the various energy measures, that really makes my head explode. We should not be using term-based payback analysis for long-term durable goods, like houses and their building assemblies. Please see the BuildingGreen blog I wrote on value transfer.

And please also consider a recent GBA Energy Solutions blog by Alex Wilson in which he suggests that the insulation sweet spot can be a function of the PV sweet spot. I like the idea of comparing the opportunity costs for insulation and renewable energy, although to make the comparison really “apples to apples” the two approaches would need to have identical service lives (the PV system would need to last as long as the wall assemblies, or the insulation in them).

In any event, I think the insulation sweet spot is a lot like the literal use of the term sweet, in relation to food: the best flavors are not just sweet, but a combination of flavors. The insulation “sweet spot” is actually a more complicated flavor involving more than just insulation.


  1. User avater
    Dana Dorsett | | #1

    The comfort sweet spot.
    Part of the PassivHaus spec is about the minimum and maximum temperatures of the exterior wall/ceiling surfaces, which is in part a comfort argument. At R40 (whole-wall) the temperature extremes are well-bounded in a MT climate and WAY more comfortable than code-min, but would probably still fall outside the PassivHaus boundary.

    The net-present-value on future energy savings calculations as well as the opportunity cost relative to PV solar can be useful markers, but paying for accurate energy use modeling of the house may be more valuable and cost-effective than blindly spending on ever more R-value (or PV.) But there will never be an single sweet spot, by any metric.

  2. User avater
    Mike Eliason | | #2

    there are very few PH
    there are very few PH projects w/ R-70 walls. chris corson's in 7400 HDD maine isn't even R-60. thorsten's on sunrise was R-75 (14000 HDDs). based on the number of projects i've looked over (a lot), i'd estimate under 2%. R-70 is overkill for PH in scandinavia, most of the projects in sweden are R-50 to R-60.

    PHPP doesn't tell a user HOW to get to 4.75kBTU/ft2a. only IF. HOW they get there is up to the USER.

    despite what i may think about the lackluster leadership at PHIUS, is there any instance where they've ever claimed R-70 walls give homeowners the best value?!? i've never seen it.

    also, i'd love to see a PH wall where the embodied carbon of the added insulation actually exceeded the CO2 savings. does. not. exist.

  3. User avater
    Dana Dorsett | | #3

    Depends on how and what "carbon" you're calculating.
    Embodied carbon would be a good thing, if it's the sequestered as carbon in cellulose. It's quite a different animal if you're talking about the CO2 equivalent global warming potential of the HCF134a of US-made XPS at high-R, which would exceed the CO2 savings over a code-min fiberglass insulated over the anticipated lifespan of the building if XPS were only insulation used to meet code-min.

    Lewendal isn't way far off the mark with R-70 for what isdone to hit PassiveHouse energy use levels in a US climate-zone 6 location. There's a PassiveHouse (5 crow-miles from my house) in zone 5 that was built with ~R58-R66 walls, a location with a winter climate slightly cooler than Stockholm, but a much warmer summer climate:


    Bozeman MT is colder in winter than the bigger population centers of Sweden (Uppsala, Stockholm)- more comparable to Boden, in the far north, but much hotter (needs air conditioning) than any Swedish location in summer.!dashboard;a=USA/MT/Bozeman!dashboard;a=Sweden/Boden

    Most of Scandinavia is fairly temperate, (even in high HDD locations), and cooling energy demands are miniscule. The cooling degree days are enough to actually matter in Bozeman, but not so much in Uppsala. The Baltic moderates seasonal climate extremes & averages quite well.

    Lewendal never made the assertion that PHIUS was offering up R70 as a best value, but it's not a 2% outlier for that climate either, and it's not an outrageous value to throw out there.

  4. Shane Claflin | | #4

    cost of energy
    One problem is the cost of heating/cooling. It's a variable we have no control over. Let's say, this year your calcs determine that R-30 is the breaking point, and next year the electric company suddenly decides to increase your rates. 20-20 hindsight would tell you that R-40 is a hedge against rising energy costs in the future.

  5. User avater
    Mike Eliason | | #5

    even the guy who did


    even the guy who did PHPP for the beaton project admitted it was entirely too much insulation. this is one of the (many) issues i have w/ failing to optimize projects (e.g. PH ends up looking ridiculously difficult to achieve - in majority of instances, it shouldn't be).

    My best example is the PHIUS [Passive House Institute U.S.] model. They think that R-70 plus walls will give homeowners the best value.

  6. Luke Morton | | #6

    We each have different values...
    While this is all thought provoking, it feels like the original proposition asks us to be blind men in the parable of the elephant (go ahead and google that).
    And I feel like you all feel that too...

    What's the right R-value of a wall? I think a better question is what the good (UA) value for a house is. At least that seems to cover the whole envelope and not just one assembly that might have substantially different weightings depending on the massing of the building. Shouldn't we also be talking about the R-value of windows, ceilings, floors, etc.?

    Here's an example-- a literal question that came across the energy modeling forums recently:
    What is the right R-value for a wall in a house that's an A-frame. Code says one thing, "Quality" construction might say something different. I secretly joked to myself that the A-frame should be built Passivhaus-tight and then be nicknamed "Hole in the Wall" Passivhaus. :)

    Given the conversation above, it definitely seems that this is a subjective question. Once we establish a clear metric or metrics (e.g. first cost, Life Cycle Cost, or as my friends here suggest... polar bears), we can then figure out a more formal way of optimizing according to that metric.

    I don't mean to say that once we establish a metric, then everything is solved, but it shifts the focus away from subjective or hedonic-based or non-market services (e.g. comfort), to ones involving more easily attainable heuristics. I think we could all learn about arguments of how much insulation costs in our different local markets. I personally find that very interesting...

    As for PHIUS and their metrics-- perhaps you should read Katrina's blog on it

    And-- the only formal limitation on the R-value of a wall in Passivhaus that I've found is that the interior surface temperature on the peak heating condition must be greater than 60.8 Fahrenheit. This means that if it's -20 degrees outside, we must have assemblies that are better than R-9.04
    I feel like that's attainable. ;)

  7. User avater
    Dana Dorsett | | #7

    I stand corrected! (response to mike eliason & Lucas Morton )
    I didn't back-read Lewendal's original post on that- apparently mike did!

    IIRC Beaton himself was the driver of the higher R, since as a home building contractor himself his costs didn't have the additional cost burden of contractor profit, and he was using it in some ways a demonstration project. That's not say that his double-wall layup wouldn't be reasonable for the retail clients, given that it was all comparatively low cost cellulose, not foam. The last R10 or even R20 just doesn't add much to the overall cost of the assembly. His plug loads are turning out to be WAY over the design numbers (teenagers playing video games on the big-screen?), leading to higher cooling power use, with significant cooling loads even during the winter. (His mult-split is also a bit oversized for the actual loads too.)

    Lucas: I haven't looked at it recently, but I had remembered an interior surface temp higher than the 16C you cited. To be sure, that's not a particularly comfortable wall temp, but one commonly seen in houses in my neighborhood (at temps well above -20F). The additional comfort factor of R40 walls is real.

  8. Jerry Chwang | | #8

    What about the weakest link?
    No, not the game show...

    It seems to me the amount of glazing in the design would also be another variable that makes it difficult to provide a universal answer. If the windows are ~R6, does it make sense to have over 10X that in the walls? Would a more optimal solution not utilize some of those insulation $ to further upgrade the windows? (Sort of like the PV cost comparison argument)

    CAVEAT: I'm not a professional, but a homeowner trying to educate myself so I may be totally off base. This particular question is something I've been struggling with though, so appreciate any insights.

  9. Shane Claflin | | #9

    Deep Pockets
    I think it just comes down to what price bracket you are catering to.

  10. Ed Dunn | | #10

    I have used the amounts of insulation recommended by Oak Ridge National Laboratories since I started building houses. It has worked for me very well. Why re-invent the wheel? The work has been done and the 2012 Energy Code uses these figures.

  11. User avater
    Dana Dorsett | | #11

    uh, Ed...
    The IRC2012 (and even the IRC 2009) minimum R values on the high end of the ranges n the Oak Ridge numbers and now have an air-tightness requirement. The lower end of the Oak Ridge range charte would not meet current code min in many states. The new (and not yet widely adopted) IRC 2012 values are listed in TABLE N1102.1.1 :

  12. User avater
    Jon R | | #12

    payback period
    > and your payback time frame

    This bears repeating with an emphasis on "your". If you sell the house, it's unlikely that you will get much more for the energy improvements. From a $ standpoint, don't do super insulation if you are only going to be there 5 years.

  13. Roger Anthony | | #13

    Insulation -terms of reference.
    Should we not start from the point. How well is this home built! Then, is it made of bricks with a slate roof and we can expect it to last for 400 years - without expensive maintenance. Or is it made of sticks with a life expectancy of 50 years - and will require constant attention.
    Only when the terms of reference are agreed - can we make sense of the question.
    In the last couple of days we have been told that by spending $7,000 on new LED light fittings a company will save $3,000 a year on electricity. This will be repeated every year for the next 35 years, with the savings growing as the price of electricity rises and the money saved grows with interest. Insulation, heating and cooling costs need to be viewed over the expected life time of the building.

  14. Aj Builder, Upstate NY Zone 6a | | #14

    House size Or no
    House size
    Or no House
    No heat
    No AC

    What all you are not stating is your starting point. You all want big comfortable homes with all the TVs and electronic gizmos of the day... But now you want to debate some impossibly easy way to do so for as close to nothing as your irrational minds and pencils and or googlings can attain.


    Absolute nonsense. Easter bunnies are more real.

  15. Ted Kidd | | #15

    Presumptions Are Dangerously Incorrect
    I'm not calling anyone a simpleton (yet), but DANGEROUSLY SIMPLISTIC is looking for the "energy" sweet spot for each zone. Houses need to be systems that work together, not individual components glued together. The whole point of structures it that they work for occupants. And they should last a long time with low maintenance.

    "If you double the R-value of your insulation, the rate of heat loss is cut in half."

    Wrong. That's not the case. It is NOT linear. The first inch does WAY more than the second inch. The first two inches do WAY more than the next four. Incorrect assumptions lead to really bad design thinking.

    Just because the "Energy" benefit of MORE insulation drops off should not be the determining factor of how much insulation to use! Comfort and durability should trump. A really well insulated house that is uncomfortable will use a lot of energy. A house with "sweet spot" insulation may make managing Relative Humidity properly impossible due to dew point surface temperature problems.

    BRILLIANT thinker to follow is ROBERT BEAN -

    People keep looking for magic bullets without any understanding of critical connections required for good design.

  16. User avater GBA Editor
    Martin Holladay | | #16

    Response to Ted Kidd
    You wrote, " 'If you double the R-value of your insulation, the rate of heat loss is cut in half.' Wrong. That's not the case. It is NOT linear."

    I'm sorry, Ted, but the original statement is correct.

    Every time you double the insulation thickness, the heat flow rate is cut in half.

    If you double the insulation thickness from 1 inch to 2 inches, the rate of heat flow is cut in half.
    If you then double the insulation thickness from 2 inch to 4 inches, the rate of heat flow is cut in half.
    If you then double the insulation thickness from 4 inch to 8 inches, the rate of heat flow is cut in half.

    This rule never changes. But of course, each time you double the thickness of insulation, the thickness of insulation needed gets larger (because you are doubling), while the amount of heat (and $) being saved is smaller. So there are diminishing returns.

    You got the diminishing returns part right -- but you you were mistaken when you called labeled the sentence ("If you double the R-value of your insulation, the rate of heat loss is cut in half") wrong. The sentence is correct.

  17. Anders Lewendal | | #17

    diminishing returns on insulation
    Ted: I don't think you understand what I mean by Sweet Spot. As most everyone above agrees, the are many variables. I am assuming a model home that has an ACH of 2 or less with less than 15% glazing and is well designed. In Montana, zone 6, our 2012 code will allow us to insulate to R21. I am guessing most of the writers above think that is on the low side of what is efficient. Most also agree that R-70 is on the high side if you consider opportunity cost. I may have exaggerated the R70 in my comments a couple of years ago.

    I have built over 160 homes in Montana over the years including Energy Star homes, a LEED home and today we poured the footings for a Passive House. The R-value of this new home is R-65. 15.5" of dense packed cellulose at R4.2 per inch. Here's the rub. The vast majority of homes in Montana and the US are built with code insulation that often underperforms due to inferior installation. If we build one million new homes a year in the US that uses just R10 more or less than some reasonable "Sweet Spot" we will waste millions (billions) of dollars and resources (CO2) over the next fifty years.

    If Martin is right and all of the math is correct and all of the models are right on, why are we still building millions of homes with R21 (Montana)? Builders and buyers in American are not buying R40 or R50 homes. OK, a very small percentage of them are. I have done several myself including my home.

    The question is: How do we build a less shitty home? I am interested in the millions of homes built, not the few outliers.

  18. David Kellers | | #18

    Life cycle costs.
    I used to follow the idea of diminishing returns however; when you drop from three HVAC units to one high efficiency unit and then realize that those HVAC units have to be replaced in maybe 15-20 years where the insulation does not, energy costs go up, ect. The life cycle cost over the next 200 years seems to point to more insulation. Unless you are focusing on your one year and done contract.

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