Hello Martin: I have a question about insulation diminishing returns below. Thank you in advance for your thoughts.
anderslewendal
| Posted in Energy Efficiency and Durability on
I am working on a proposal to better understand diminishing returns on insulation. Our theory is that too little insulation wastes energy and equally, too much insulation wastes energy. Where is the sweet spot in each climate zone?
Attached is our proposal. We are interested in knowing if GBA has conducted any performance testing that makes diminishing returns conclusions. If not, we are hoping GBA might give us some advice that makes our experiment productive.
Opps. The file is too large to send. Could you e-mail me at [email protected] and I will forward you the file directly?
Anders Lewendal 406-580-5804
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Replies
Anders,
You wrote, "Our theory is that too little insulation wastes energy and equally, too much insulation wastes energy. Where is the sweet spot in each climate zone?"
What you call a "theory" is a truism enshrined in our building codes. 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."
Every time you double insulation R-value, you cut the rate of heat loss in half. That fact doesn't change, even when you increase insulation from R-100 to R-200. More insulation always reduces the rate of heat flow.
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.
You may want to read this blog: Can Foam Insulation Be Too Thick?
Anders,
There are two things to consider; one is the principle of diminishing returns, and the other is the “sweet spot” that you mentioned. I assume that “sweet spot” refers to the proper amount of insulation. It is an obvious question because we know that it is possible to have too little insulation and too much insulation. So it follows that there must be some point where the amount of insulation is just right.
The problem is that the calculation for that sweet spot needs to factor the future cost of energy and money over time. It also depends on individual perception of thermal comfort and their willingness to pay for it, although I suppose you could average that across all homeowners.
I also suspect that people have an energy budget that they are used to spending on home energy (heating or cooling). Call it $200 just for neatness. With leaky envelope/less insulation they spend $200 by adjusting their set-points (cooler in winter, warmer in summer), with better envelope/ more insulation they continue to spend $200 by making their set-point closer to what they'd prefer (warmer in winter/cooler in summer).
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. 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.
The price-elasticity curves of comfort here are going to be non-linear. ie the first x% of benefit from energy reduction might result in spending less, but as the total energy cost falls, I think the homeowners adjust the set-points more. After all if your energy bill is $20/month why shouldn't you be comfortable?
Martin, you are saying that this is embodied in codes, but there is no notion of embodied energy in the codes, AFAIK, just R values. And different types of insulation have very different R-values (your point b, but that's not at all in the codes).
Ron,
You wrote, "The problem is that the calculation for that sweet spot needs to factor the future cost of energy and money over time. It also depends on individual perception of thermal comfort and their willingness to pay for it, although I suppose you could average that across all homeowners."
You're right. A determination of the "sweet spot" depends on one's assumptions. 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.
I listed many of the factors that can affect one's assumptions when making these calculations in my article, Payback Calculations for Energy-Efficiency Improvements.
Anders,
Thanks for e-mailing me your document. In that document, you wrote, "To the best of our knowledge, no one has satisfactorily performance tested the diminishing returns [of different insulation thicknesses] on identical homes."
While it is possible that your statement is literally true, I don't think it is particularly relevant. Michael Blasnik (among other researchers) has assembled energy use data on hundreds of thousands of U.S. homes. 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.
What we don't have is a consensus on the variables that we have already listed: estimates of future energy prices, for example, or estimates of the cost of borrowing money over the next 50 years.
Martin: I sent our proposal to you yesterday and look forward to your input. Maybe you can share the highlights here. You said earlier that going from R-100 to R-200 will cut the rate of heat loss in half. That is true, but when the heat loss at R-100 at .6 ACH is almost zero, then cutting that heat loss in half has a very high opportunity cost to the consumer. Just as a starting point, I figure we should reallocate our energy efficiency efforts somewhere else if we cannot beat the ROI on a ten year T-bill. You are also right that there are many other considerations for buyers. Still, if the insulation addition has a very low ROI, then the homeowner could use those extra funds to buy a lot just a little closer to their work or buy a bike. There are many ways to globally reduce CO2 production.
Anders,
No one on the GBA site, including me, is advocating R-200 insulation for walls or ceilings. Your thoughts about determining the optimal insulation thickness for various building components aren't particularly original.
It sounds as if you are just beginning to think about residential energy use issues. I'm happy to report that your observations, while elementary, are on the right track.
Martin: No doubt, the are many prescriptive studies out there that try to evaluate the performance of insulation. I have ready many of them and used many of their ideas on homes over the last 25 years. When I see the IECC recommending a 2x6 wall with fiberglass batts and then one inch of foam board over OSB in Bozeman, Montana I wonder what they have been studying. Is that what they consider the sweet spot? If they didn't think that, why are they recommending that?
Let's just say I agree with you. In Bozeman, we use mostly natural gas to heat our homes. I have met with natural gas suppliers who indicate that the cost to consumers will increase very slowly considering the supply of shale gas they expect to find over the next 100 years. With that in mind, what R value should we use in Bozeman for our homes? Our current energy code, 2009 IECC, requires a R-21 fiberglass wall with a ACH 4. Montana adopted 4 instead of the 7 from the 09 IECC. Is that the sweet spot after decades of the research you mentioned? Personally, I think it is closer to a real R-30, but I cannot prove it. What we want to do is to find the insulation ROI on three homes with everything else being equal. That requires having the homes empty for one year. When we are finished with that we can compare heating systems or something else. Performance does not alway match up with prescriptive findings I have found.
Anders,
One of the presentations that building scientist John Straube has been sharing recently is titled “How Much Insulation is Too Much?” He gave that presentation at the NESEA conference in Boston this spring. If you ever get a chance to hear Straube's presentation in the future, you should probably go.
I reported on Straube's presentation in one of my recent blogs, Pearls of Wisdom From Recent Conferences. That reports includes many quotes from Straube's presentation.
Anders,
Clearly, the minimum requirements in our building codes do not represent optimum insulation levels. GBA has been advising readers to install above-code levels of insulation for many years.
Moreover, U.S. building codes have only recently begun paying attention to envelope air leakage.
I will repeat: you don't need to measure energy use in three empty houses for one year to determine optimum insulation levels. The study you propose has already been performed, repeatedly, beginning decades ago. Building a new house in Bozeman and measuring the energy required to heat it and cool it for a year will not yield any new data.
USDA Forest Service General Technical Report PNW-32
1975
I found a handy graph in the above Technical Report. Its old but perhaps the physics hasn't changed. I will try to attach it so everyone can see it. Clearly the first inches are most important. Even knowing of the graph I installed R50 cellulose in my attic.
Cheers, john linck - the toymaker
That attachment worked. I will try attaching the whole publication.
john
Anders , you sound just like me a few years ago, before i got interested ( by my own needs ) , and before i learned here ( on GBA ) .
As sifu Martin pointed out, you are on the right path ... but you are not bringing anything new to the table.
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Anders,
No one on the GBA site, including me, is advocating R-200 insulation for walls or ceilings. Your thoughts about determining the optimal insulation thickness for various building components aren't particularly original.
It sounds as if you are just beginning to think about residential energy use issues. I'm happy to report that your observations, while elementary, are on the right track.
Answered by Martin Holladay, GBA Advisor
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ahahhaah , i like to read you Martin :p
Anders, most of what you discuss can be easily ( could also be very complex depending on the time you have to invest ) calculated. It's all fairly logical.
Very hard to settle on the period you will use for your calculus that will represent " returns " .
first owner, house expected life, insulation life, embodied energy, 10-20-50-100 years ..
future costs and technologies ...
you need to decide on your basic values to be able to work something out
But then again, how are you justifying a "study" on that matter ?
If you cannot "proove" that R30 is closer to what is required, why do you assume it is ??
Also, i would suggest you learn how to work with basic formulas for heatloss,
heating and cooling degree days, solar heat gains ..etc..etc...
have fun :)
Thanks for the input. Since that sweet spot is resolved for insulation diminishing returns, could you please inform myself and all of the builders here in Bozeman, Montana what that perfect insulation level is using todays energy costs. I am looking for that level of insulation where adding one inch more will result in a ROI of less than say 4 or 5% with everything else being equal.
Anders: why don't you try and find out the formulas by yourself using the values you want/need ??
That way you would grow with knowledge.
Do you understand the complexity involved in calculating ROI ??
Would you accept a +-25% answer??
It needs to be calculated for each specific cases.
There costs and labors depends on the design and local material prices...
Orientation and SHG, ventilation ....
The only thing that can be calculated with ease for any situation,
is a standard heat loss through wall R/U value VS R/U value cost ( labor design and materials ) VS location HDD or CDD, and then u end up with a maximum theoritical energy saving per year
( or day/hours/months if you have used something other than HDD data )
and then u need to keep in mind that some newer heating/cooling solutions cut that in 2 to 5
(COP of heat pumps )
Again keep in mind that this is maximum potential ...it will always be lower
( internal gains, SHG, mass effect etc. )
have fun!
Anders,
Q. "Could you please inform myself and all of the builders here in Bozeman, Montana, what that perfect insulation level is using today's energy costs?"
A. There are two answers to your question: the short answer and the long answer.
The short answer -- it's as good as any -- is the one devised for cold climate builders by the Building Science Corporation, which recommends that you insulate to 5-10-20-40-60. This means R-5 windows, R-10 horizontal insulation under your basement slab, R-20 insulation on your basement walls, R-40 insulation on your above-grade walls, and R-60 insulation for your ceiling or roof.
The long answer is: only you can do the calculations. The answer depends on the local cost of insulation, the local labor costs, the familiarity of local insulation contractors with different insulation systems, the prices provided by your local subcontractors, the local availability of heating fuels, your local cost for electricity and heating fuels, your assumptions about the likely lifespan of your building and the insulation within it, and your borrowing costs (interest rates).
It sounds like you want to ignore the question of energy cost inflation, but that element introduces tremendous variations in your answer.
All of these variables, and more, are described in detail in some of the articles that I recommended that you should read:
Payback Calculations for Energy-Efficiency Improvements
Can Foam Insulation Be Too Thick?
Pearls of Wisdom From Recent Conferences
You original question assumed that the trickiest part of the calculation was figuring out how much energy it takes to heat a house with certain specifications. But that part of the calculation is trivial, and is easily performed with energy modeling software. The hard part of the equation concerns construction costs and energy costs.
If you are unfamiliar with energy modeling software, heating load calculations, and cooling load calculations, I suggest that you read the following articles:
How to Perform a Heat-Loss Calculation — Part 1
How to Perform a Heat-Loss Calculation — Part 2
Calculating Cooling Loads
Energy Modeling Isn’t Very Accurate
Thank you Martin. I have read most of the articles you suggested and I do understand heat load calculations. And yes, energy costs and construction costs play a large factor in the ROI on insulation. It would be easy to use a generic model as you suggest above and put in R-40 in the walls.
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 that R-30 is where the curve bends down reducing the marginal improvement in performance? 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?
We have run all kinds of modeling software from HERS to PHPP and cannot conclude that R-40 is the most efficient. We cannot predict the future cost of energy here in Bozeman, but I was invited to speak at an event that was sponsored by a state natural gas association. They told me that gas prices will increase with inflation for some time to come. Our installers and suppliers are competent using blown in cellulose, sprayed closed cell foam, and using insulation and outsulation.
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. Still, we are not convinced that prescriptive modeling matches performance very well. My best example is the PHIUS model. They think that R-70 plus walls will give homeowners the best value. I am quite sure that a very low ACH and modest insulation is more appropriate. The exact number for us here in Bozeman is what I want to determine.
I do appreciate your input.
Anders,
You're right; PHPP pays no attention to cost-effectiveness. All PHPP tells you is how to hit 15 kWh per square meter per year.
If you know the cost to go from R-30 walls to R-40 walls, you can do your own calculations. You can compute how much energy the insulation upgrade saves per year and convert that amount of energy into dollars. If your insulation upgrade saves you $30 (for example) in energy per year, it's up to you to decide whether that is a good use of your $3,000. (The energy savings will depend on your energy prices, your climate, the configuration of your house, and how many windows the house has, among many other factors.) Some homeowners will think it's a good investment, while others will think it's a terrible investment. There is never going to be just one answer to the question. But it sounds like you have all of the information you need to make your own calculation.
Anders : you need to start from basic ... " diminishing returns" needs a time value to it.
Anders: " I am looking for that level of insulation where adding one inch more will result in a ROI of less than say 4 or 5% with everything else being equal."
In most cases, if heating with natural gas at current pricing, looking at a retail contracted for installed price for blown fiber insulation that level will be less than code-min. If heating with 25 cent electricity in baseboards or ducted electric furnace maybe not. But if insulating with sprayed closed cell polyurethane it would still be way less than code-min, even with the resistance-heating option.
You really have to do the math on both the insulation costs and energy costs to get to the bottom line, but insisting on a 4% ROI is a bit silly given what safer bonds are paying, and most people would also still need to do the math on true cost of the tax deductible mortgage payments on the insulation upgrades as well as the after-tax energy payment savings to come up with reasonable financial break-even models, even is you assumed zero energy cost inflation/deflation.
Do you really think you're going to get an after-tax return of 4-5% or better at a lower non-volatile risk performance of insulation? Is there decades long hedge against energy price inflation that can be purchased on the cheap?
Yes, there's a number, and but code-minimums are really the very low end of "reasonable" financially from a building lifecycle point of view, but necessary to avoid the issue of home-buyers not willing to pay more a simple-return break-even analysis 10 years or less, or however long/short their perceived dwelling period will be.
The correct financial balance point will clearly not be the same for everyone- not all home buyers in Bozeman are in the same tax bracket, nor do they all have the same source-energy costs, or the same HVAC efficiencies, but the code-minimums can usually be beaten considerably even with 50cent/therm gas when insulating using optimal price/performance methods, when you use more sophisticated financial models, assuming you're looking at the present value of the savings over 25+ years. But the real number will vary by quite a lot- the energy cost, insulation cost, lending costs, buyers tax bracket, & buyer's risk tolerance details all matter.
The other thing to factor in is the current and future financial & environmental costs of photovoltaic solar vs. the financial & environmental costs of the insulation. At 1.5x code-min in whole-assembly R (with all thermal bridging factored in) with optimal shading factors many homes might be better off with (currently in my neighborhood) $4/watt grid-attached PV rather than higher-R, especially when taking federal/state/local subsidies. (I'm not sure how rich those are in Bozeman.)
So, if you thought there's a simple answer to that question, I hope I've disabused you of that notion by now. It's never really going to be simple, and there are multiple moving factors in the equations that will yield wildly differing results. There is no such thing as " The exact number for us here in Bozeman...".
But I s'pose could have just punted, and read you the numbers for zone 6 in table 2, page 10 of this document :-) :
http://www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones
Read the whole first chapter- the values in that table is a starting point (and they're "whole-assembly" numbers, not center-cavity on stick built numbers), but your actual labor, energy and interest costs will vary.
You can even thinker, nobody knows, but we might found a way to manufacturer cheap VIP ( vacuum panels ) with super long service life, or servicable connections etc...that would revent the hole insulation industry on cost and performance ...all of our numbers here would be good for garbage ..
Folks, I am quite sure we are on the same side. I have spent hours reading your posts on GBA and have learned plenty over the years. Math and prescriptive analysis may get close to real life, but if the market is not responding to the consensus that an R-40 wall is close to ideal as Martin suggests, then we are not making an impact on the built environment.
Most homes get about an R-21 because the government says it is good and our cities enforce that level of insulation. A few homes get about R-70 because they think Wolfgang 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. A good bicycle comes to mind. So, how do we get the average homeowner to ask for what your blogs have suggested, which is a PGH or pretty good house? One of our builders calls it a less shitty house.
By building three identical homes with three insulation levels and monitoring the total cost of heating energy for one year in a residential neighborhood, we might get the average homeowner to understand where the cost benefit curve bend down. Real basic economics might help.
I will give you another example: We continue to lose US manufacturing overseas often because US consumers think we just don't make anything in this country anymore. We produce 21% of everything on the planet. To prove to the average person that buying US made products is easy and a small 5% change in our purchasing habits could create 3 million new American manufacturing jobs, I built a new LEED home using only American made products. There is not one foreign screw, nail, light bulb, appliance or anything else in the home. Many millions have heard the story and that simple exercise might have an impact on our behavior and economy.
I am not trying to reinvent the wheel, just show the average person the value of finding the sweet spot in our lives starting with insulation. Martin, you might be right about R-40 walls, but very few buyers are paying attention. We hope to change that. Thanks for listening.
Anders,
In your original post, you wrote, "I am working on a proposal to better understand diminishing returns on insulation. Our theory is that too little insulation wastes energy and equally, too much insulation wastes energy. Where is the sweet spot in each climate zone?"
In response, I suggested that your theory is a well-known truism. I also suggested that finding the "sweet spot" is a matter of mathematics, not energy monitoring. Furthermore, I suggested that the results of these mathematical calculations are all over the map, due to the wide variations in assumptions that must be brought to the table before you can make the calculations. Someone who wants the payback period to be 12 years will get a completely different answer from someone who wants the payback period to be 100 years, and someone who assumes that energy costs will increase at a 7% annual rate will get a completely different answer from someone who assumes that energy costs will be flat.
In spite of these explanations, you just wrote, "By building three identical homes with three insulation levels and monitoring the total cost of heating energy for one year in a residential neighborhood, we might get the average homeowner to understand where the cost benefit curve bend down."
You might. But using a one-year experiment to convince homeowners to accept lower insulation levels is also deceptive, because your one-year experiment makes no allowances for the possibility that insulation costs will rise. And since your experiment has been repeatedly performed by energy researchers in a wide number of climates for decades, you won't learn anything new from your proposed experiment. That's why I am mystified by your statement, "I am not trying to reinvent the wheel." If you don't want to try to reinvent the wheel, why are your proposing an extremely expensive experiment to do exactly that?
Martin: if you are right, the market would have adjusted to something close to R-40 wall insulation a long time ago. They have not. A local building scientist, Neal Ganser, who created the CORBOND corporation tells me that the sweet spot is closer to R-30. If you are so convincing, why is the 2009 IECC calling for R-21? Why did we just read forty or so studies with names like "Optimum insulation thickness for external walls for energy savings."?
Sorry, we are not convinced. Neither is the market.
Anders: " A local building scientist, Neal Ganser, who created the CORBOND corporation tells me that the sweet spot is closer to R-30."
OK, so show us his math, and his cost (of money, energy and insulation) assumptions!
Anders,
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."
Evidently you have -- so no one is arguing. Every person has to figure what works for their needs, using their cost inputs, climate, and other variables.
I will ask him and get back to you.
Anders,
I know Neal Ganser. He's a smart guy. As he knows -- and as you probably also know -- closed-cell spray foam insulation (like Corbond) is very expensive per R-value, so it's a certainty that you reach the point of diminishing returns much faster with Corbond insulation than with a less expensive insulation like cellulose.
So, if you choose cellulose, you'll get more for your money -- and you will likely find that your calculations justify higher R-values than can be justified if you use Corbond.
The sweet spot is only partly objective science. The rest is intuitive and subjective. 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.
The main reason to provide a working model is to prove the objective scientific premise, because the interested, but uninformed public seems to approach the idea with the expectation that this is only a matter of cost/benefit with a simple answer, and they want that answer.
But since the answer is not just objective science, it might be worthwhile to get the public to change their quest for just the simple answer of cost/benefit, as opposed to trying to give them that simple answer.
Okay, I am new at this and have some questions about a project I am panning for my home. First, for my own edification, where can I find a chart of basic R-values for thicknesses of different materials? the home U just bought is very, very interesting and fun, however it was built in 1943 out of concrete block, with poured concrete ceilings/roofs. The design included seven exterior doors, four of which are single pane glass. All windows have been upgraded with replacement thermals, except for one very large and two smaller designer-type windows. The house is built on a slab and the roofs are 6-inch thick.
Because of the concrete floors, there is minimal duct work, with just one duct in each of the two 20 x 12 upstairs bedrooms, and one in each of the other rooms downstairs. Not very effective in winter or summer.
I am sizing a new 14 SEER AC unit presently as the 3.5 Ton unit cannot keep up with the temp changes.
My plans are over the next year and a half, are to completely cover the exterior of the house with 3-inch foam and stucco, keeping the thermal mass inside. The roofs, which are flat, will be a 1:4 pitch solid foam, hot wired out of block, with osb and metal covering, and drainage built in on all sides.
I was a contractor for 35 years, and have many of the skills, tools and tech knowledge to do much of the work, however I am laying this out for you guys to dissect. I also am a writer, and have written for Journal of Light Construction, and research is no stranger to me.
Please...: Questions, Comments, suggestions. I am considering closing off some of the doors, digging out and insulating the foundations down to 18 inches, but I want to keep the overall aesthetics of the place as much as possible.
Wanna hear the real clincher? I am a youngish 65 years old and on a budget, so I can't just throw money at the house, but since the advent of the green movement, many of us want things to be both environmentally and economically practical.
Thanks,
Jim
I'm surprised that anybody could make an argument for even R30 whole-wall R using Corbond as the primary insulation.
Or is he calling ~5" of Corbond in a 2x6 studwall "R30", despite a whole-wall R between R16 -R17 (including the gypsum, sheathing & siding) after thermal bridging is factored in? If that's the case, we're not even talking the same LANGUAGE!
Even at R-infinity/inch you can't hit R30 whole-wall at a typical 25% framing fraction- best you can do is about R25, in 5" of cavity fill. R27.5 with a full 5.5" (assuming R1.2/inch framing timbers) unless using a species of wood with much better than the typical R1/inch for Doug-fir, or R1.2/inch for hemlock.
For peops like me and (probabaly Martin) a representative R30 wall would be a 2x6 studwall with R20 fiber (~R14, with gypsum, sheathing & siding included), with 4" of rigid rock wool or EPS insulating sheathing (or 2.5" of polyiso) slipped in between the sheathing & siding, which comes in at around R30 for whole-wall performance. Even with the additional installation labor and scrap rates it will usually come in cheaper than 5" of Corbond in the stud bays, and deliver nearly 2x the performance- a true R30, not R16.
And until/unless they move over to one of the new low global warming potential blowing agents, 5" of Corbond is something of a net global warming disaster, even if you were heating the place with a coal-stove, the GWP of the offset energy use would never equal that of the blowing agent itself, even at a code-min R.
Dana: We are not proposing to use only closed cell foam. Neal and I have used several combinations. Attached is a picture of one of our experiments. It has one inch of closed cell Corbond board on the outside nailed against the 2x6 studs with two inches of sprayed Corbond on the inside sealing the cavity and then three and a half inches of cellulose sprayed on the inside. Any shearwall required was installed on the inside. That is something close to R-32.
I am now doing a project with 9" of cellulose mainly because a 2x6 exterior wall with a 2x4 interior wall attached to the exterior from the inside makes an easy application. That is about R-36. Maybe a little over done but inexpensive with standard materials and building techniques.
We are also hoping to influence the code council to approve an R-value that works with most applications for the average homeowner in each zone. Are we going to get it perfect? No. But we do think R-21 misses the mark and R-70 is overkill and wasteful. How you get to a reasonable R value depends a lot on local techniques, material costs, and design. You already know this.
We are just trying to live in the sweet spot. GBA has influenced my projects in a good way over the years. Keep it up.
Jim,
Q. "Where can I find a chart of basic R-values for thicknesses of different materials?"
A. Here at GBA: Insulation Overview
Elsewhere on the web -- lots of places, including:
http://www.mnshi.umn.edu/kb/scale/insulation.html
http://en.wikipedia.org/wiki/R-value_%28insulation%29
Anders: "Attached is a picture of one of our experiments. It has one inch of closed cell Corbond board on the outside nailed against the 2x6 studs with two inches of sprayed Corbond on the inside sealing the cavity and then three and a half inches of cellulose sprayed on the inside. Any shearwall required was installed on the inside. That is something close to R-32"
With only 1" (~R6) Corbond for thermal break, at a 25% framing fraction that comes out to only about R21 whole-wall. If it's 24" o.c. framing you might get that up to R24 using advanced-framing techniques, but the 2" of Corbond thermally bridged by the framing is largely wasted. (At a 20% framing fraction you're looking at only R23 whole-wall, giving R1 for the combined siding/sheathing/gypsum.) You'd do better with less foam and less money using 5.5" of cellulose in the cavities and 2" of Corbond on the exterior, which would deliver about R26-R27 whole-wall, and would have sufficient exterior-R for dew point control at the sheathing, eliminating the need for stiffer vapor retardency than class-III goods (standard latex paint) on the interior.
Anders has tried to invoke efficient market theory in the insulation issue: "Martin, if you are right, the market would have adjusted to something close to R-40 wall insulation a long time ago. "
I also believe that the market itself is smarter than all of us, and eventually comes to the correct price.
Unfortunately, in the insulation market, there is a disconnect between supply and demand that is delaying the adjustment to R-40.
Homebuyers don't know that R-40 is about where they should be. They don't know it because most builders tell them it's overkill. Builders keep this myth alive for two reasons:
1. They don't believe they can recoup the extra cost of R40 in the selling price of the house. (They're right)
2. The builder doesn't pay the utility bill.
So energy codes are implemented as a consumer protection, but energy codes always lag behind the true sweet spot.
Kevin,
Regarding your comment as follows:
"Homebuyers don't know that R-40 is about where they should be. They don't know it because most builders tell them it's overkill. Builders keep this myth alive for two reasons:
1. They don't believe they can recoup the extra cost of R40 in the selling price of the house. (They're right)
2. The builder doesn't pay the utility bill."
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I think that is absolutely correct. I sense that prospective home buyers are becoming more curious and receptive to building to a higher standard of energy efficiency. But there is an undercurrent of resistance in the construction industry that tells people that more than average insulation is not cost effective in terms of payback.
Apparently the industry has this attitude because building to a higher standard make the process more complex and takes it into less familiar areas. So even though it has the potential to yield a higher profit, it comes with a risk of making less than would be the case if they just stay with the status quo.
I would say that we have arrived at a point in time where there is great opportunity for any builder to break out of the pack and market to this growing interest among the public.
R40 whole-wall makes some sense in US climate zone 6 even with natural gas at current prices, but not so much in San Diego or Seattle. The tables cooked up by BSC by climate zone really IS a good starting point, but it's only a starting point. BSC specifies R35 whole wall for climate zone 6 in table 2, p10 of this document:
http://www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones
But the true financial/lifecycle sweet spot will vary a lot depending on what your insulation & energy costs are. Using 1" exterior Corbond with 2" spray Corbond + 3.5" of celluose as cavitiy fill is on the very-expensive side for insulation costs, but may be close to the sweet-spot for an R21 whole-wall at that price in an MT climate & market. The fact that you can do about 20% better than that on whole-wall R for the same or less money (as outlined in my prior post) would be a better use of the (lower volume of) foam, and a much sweeter financial spot to hit. It would also yield a more resilient assembly, since the drying capacity at the sheathing would be much-improved.