When it comes to the topic of energy efficiency, this matter of diminishing returns on insulation is problematic. On one hand, it guarantees that there is a point where added insulation makes no sense at all. And, furthermore, mathematics is capable of determining that point. But the problem is that there are many factors that need to be measured in order to reach the mathematical conclusion. And many of those factors can only be estimated. So, the mathematical conclusion can only be an estimate.
Suppose you determine that six inches of closed cell foam is all you need. Seven inches would be one inch too much. And say you are basing this conclusion on a payback cycle of 50 years. But what if you would be slightly more comfortable in the winter with 6-1/2” of foam than with 6”, even though the extra ½” would not pay in terms of money? How much is 50 years of being slightly more comfortable worth?
What about the next person who owns the house after your 50-year payback cycle? Perhaps the payback cycle should be the life of the house rather than an arbitrary number. How do you predict the life of the house?
What if your six inches of foam is slightly defective and performs like five inches? What if it shrinks and loses R-value? What if it does not cure properly and never achieves its proper R-value?
So, while the point of diminishing returns sounds like a no-brainer, it is impossible to apply it in a practical manner. If you are building your dream house, and an exhaustive heat calc shows that the walls should have five inches of closed cell foam, will you install exactly five inches of foam? Or will you fill the 5-1/2” cavity full because it is easier to control the thickness that way?
Some have mentioned that the determination of the cutoff point for diminishing returns depends on the preference on the individual making the determination. But if it is an individual preference, how can it be calculated mathematically?
I would say that diminishing returns is generally an argument to limit insulation only as a way to limit cost, even though it is advanced as a pretext of being based on cost versus payback. I would speculate that most of the time, the citation of diminishing returns is generally used to justify using less insulation than the amount that would be economically justified over time.
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What if ... you made a post and no one bothered to read it.
Exceeding the 2 x 6 stud cavity
I would say that the issue of diminishing returns comes into play most accutely when deciding whether or not to exceed the insulation cavity thickness of a 2 x 6 stud wall. And secondarily, it comes into play when deciding whether to use solid rafters in a vaulted ceiling or to revert to scissors trusses.
But particularly with the 2 x 6 wall, the difficulty of exceeding that thickness needs a justification, and the conclusion that it does not pay back is often a large part of that justification.
((As a side note, I can never read the last post in this forum once more than a few posts are made. To be able to see the last post, somebody has to add a post to make the last post the second-to-the-last post.))
To the topic: I am just making the observation that diminishing returns are most often cited (without comprehensive math) for the purpose of justifying less insulation than might be advantageous over time.
This tends to correlate with the fact that, before the advent of spray foam, nearly everybody agreed that exceeding 5-1/2” of fiberglass did not pay back the investment.
I've thought this many times when reading your posts..........
but this time I really truly have to ask.
What the heck are you talking about?
"((As a side note, I can never read the last post in this forum once more than a few posts are made. To be able to see the last post, somebody has to add a post to make the last post the second-to-the-last post.))"
Explanation
calvin wrote:
but this time I really truly have to ask.
What the heck are you talking about?
"((As a side note, I can never read the last post in this forum once more than a few posts are made. To be able to see the last post, somebody has to add a post to make the last post the second-to-the-last post.))"
At the bottom of the forum page, there is a big list of Taunton magazine publications. Just above that section is a block with a little notation on the lower right side that says “Ad choices.” In the main part of the block, it is advertising a Free DYI Insulation Guide. There is also this identity: Jmhomoeowner.com/ProjectGuide. That is an active link.
After about 3-5 posts are made, the last post falls under that block of advertising. I can’t get rid of the block, and I can’t drag the post out from under it. Sometimes I can read the first sentence of that post that is covered by the block. In any case, I cannot reply to that post because the reply and control buttons are all hidden under that block. I can read and reply to any post above that last post.
If somebody else were to reply to the last post by Dick Russell, then that new post would be hidden and Dick Russell’s post would become visible so I could read it and reply to it. There is one other wrinkle to this that I have not gotten a handle on. Sometimes when I reply to a post in the middle of the page where the buttons and the post is visible, as soon as I enter anything into the text composition box, the control buttons for that post become hidden under that same ad box, thus making it impossible to post. But that behavior does not seem consistent.
This began all of a sudden about 3-4 weeks ago.
I just noticed that while the phrase “Ad choices” does not act like an active link, if I right click on it and select “Open Link” it opens to something explaining that it is part of Google. There is a lot of explanation about it.
You should be using Firefox and Ad Block. I see no ads at all on these pages.
I rarely agree with Florida, but in this case I concur (sorta) -- Firefox and FlashBlock.
The other option is to select "Newest first" for the sorting option.
That all depends on energy costs
Twenty years ago, when power was 2 or 3 cents/kwh, electric heating with R12 walls made perfect sense in the Pacific Northwest, or anywhere else using the Bonneville Power Administration grid.
With power at 6 to 10 cents/kwh, the whole equation has changed.
And, if you assume that the rates on the Bonneville grid are gong to get closer to what the rest of the nation pays as the inerconnection between the various power grids increases, it changes again.
Economic analysis of insualtion values, and time of return are pretty much guess work, because we don't really know what the future costs will be. Last winter propane was at $3.25/gal, last week the manager at one fo the local propane vendors said the prices are the lowest he remembers, at $1.60/gal, and might go down.
We got a notice in the mail a few months back casually mentioning one of our utility rates (either water or electric) is going up 25%.
All the math is really telling you is at what point the cost of installed insulation exceeds future energy savings. It's never going to be an absolute because we can't know future energy costs except to say that they will probably be higher. Once you've reached that magic point, in your example 6 inches, any more insulation is going to make such an insignificant change I doubt anyone could feel the difference. But that's why some people drive Kias and other people drive Cadillacs. They will both get you where you're going but the Caddy will get you there in comfort and style, but at a much higher upfront and future cost. .
I think the point is that the math shows you where you won't feel any more comfy with the extra inch. Whether you can pay to get up to that flat part of the line is the only variable.
Put it this way, if you can't feel the difference between 6" of foam, and 4' of foam - you aren't going to feel the difference between 6" of foam and 7" of foam either.
Another point of view
I prefer to think that if adding significantly to the R value of the insulation incurs a cost that really has you thinking about "diminishing returns" then perhaps you are using the wrong insulation.
This subject almost always comes up in connection with spray foam insulation, and the usual response is always the same, that the argument about X inches is enough and saves "95% of the heat" is put forth by the spray foam vendors, who can't easily compete with dense packed cellulose or BIBS for a really good high R wall.
QUESTION:
What is the cost comparison for a given R-value created by the following types on insulation?
1) Fiberglass batts of normal density
2) BIBS
3) Dense blown cellulous
4) Closed cell spray foam
What is the cost per square foot for R-1 of each of these types of insulation?
If it's math you want
Diminishing returns is just that, it really just turns into a cost benefit analysis after all regardless of R-values. How much do you value the comfort of proportional heat/cold distribution, the reduction of sound infiltration, sealed air benefits if you use certain products? If you can put your preferences into an equation F(h) of valuation, then map that over the cost for an extra inch of insulation you might have something, not a lot of people have their preferences figured out mathmatically though haha. Take the derivative of the valuation equation which derives the growth formula for your perceived hapiness for every inch of insulation you have.
F(h) = D(x)^2 + S(x) + A <---- diminishing returns
Where D is hapiness gained from reduced heat/cold distribtion for every inch (x), S is hapiness gained from reduced sound infiltration for every inch and A is the hapiness gained from reduced air flow for each inch (doesn't matter so not dependent on x).
F'(h) = 2D(x) + S
There's your hapiness growth as valued by the amount of money it costs for x inches of insulation.
For the sake of keeping things easy, lets just say the cost for an inch of insulation for your house is $100
C(x) = 100(x) cost equation
C'(x) = 100 growth of cost for each new inch of insulation
Set the cost for an inch of insulation to equal the benefit growth and you have
100 = 2D(x) + S
Solve for x which would be the amount (in inches) of insulation you want as depending on your hapiness gained... Haven't flexed those muscles in a while haha.
Hey, look at the post below OC boy!
See how Rob added information disclosing his interests? I doesn't make him less credible, it makes him more credible.
You've written alot so far, but you don't have any credibility, other thn being known a an OC corporate shill.
diminishing returns is actually pretty simple. I have an R25 wall here in maine, no thermal bridging, supertight. In the winter, you cannot differentiate between the inside surface temp of that wall and my R60 ceiling with any accuracy... just nice, uniform temps. comfort is off the table for the wall insulation at that point... windows and leakage are the factors left at that point.
now, you could do AUST calculations if you really want to nail this down, but really, I think insulation has become pretty simply described in the following way:
1. You hit premium economy in most cases around R20-25 in a heating climate.
2. You can basically maximize your efficiency by R40-50, but you'll lose the economical test by a little, unless energy spikes or the cost of your insulation is very low.
anything over that is vanity, you need huge amounts of insulation to make any difference, and the difference is very small.
this assumes equality of tightness, which is not true of all comparisons and is a part of the reason why fiberglass loses to foam or cellulose. So this is really comparing all "tight" methods, and you have to take thermal bridging into account too, though that typically only shaves a few R off and so in MOST cases should not be a big factor in this comparison.
and again it doesn't address windows, where every R means a lot more because you're starting with a small R to begin with.
And it is easily scewed.
Like a lot of things the end result of the calculation is infuenced greatly by the initial assumptions.
If you assume the comfortable temperature is 76, or 66. If you assume the outside mean temp is ten degrees hotter or colder. If you include the change in temperature loss a persistent 30 mile an hour wind makes. If you assume energy costs will remain stable, or double, or triple.
All of those change the result.