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.