In the Fine Homebuilding article (No. 204) on spay foam insulation, they mention that foam insulation has diminishing returns. This term is often used in reference to the economic pay-back of insulation; however, the reference in the article was to a diminishing return of R-value per inch, as the insulation layer gets thicker. I don’t get that.
Insulation is rated at R-value per inch. How can it be rated that way if the R-value per inch varies according to how many inches thick the insulation is?
Replies
They were mistaken, they misstated, or you misunderstood them. R values are strictly additive (so long as you ignore issues like air infiltration) so 4 inches of something (other than air/water/vacuum) has twice the R value of 2 inches.
But the savings in energy/$ per each additional R is a diminishing curve.
What the article says:
I always thought that R-values were additive, as you say. The magazine discusses this in detail. One thing they say is, "you would think that an R-40 wall full of spray foam would perform twice as well as a wall sprayed to R-20, but that's not the case." They go on to explain that close cell foam reaches diminishing returns at around 3-4 inches, and that each additional inch of foam yields little performance.
They say that as the thickness of the foam increases, the insulating value diminishes drastically.
They say that when you go from an R-20 foam wall to R-40, it only reduces the conductive heat flow by an additional 2%.
They say that when you go from an R-20 foam wall to R-40, it only reduces the conductive heat flow by an additional 2%.
That's the point -- they're just saying it poorly (and the writer may have misunderstand what he was told). When you work through the arithmetic for a typical case you'll see that the first inch or so of insulation may cut your heat loss 40%, the next inch an additional 20% (just picking numbers out of the air), the next inch an additional 10%, etc. By the time you get to R20 or so you've reduced heat loss so much that there simply isn't that much more to be reduced. And this is before you take into account the "real life" issue that windows can never be much better than about R5, so their presence becomes the major leakage point, swamping leakage through the walls.
Pondering this
I understand what diminishing returns looks like, but I am not sure I understand it in scientific terms. In reading a bunch of material on the subject, every explanation seems to get really muddled.
The FHB article says that open cell foam at R-3.5 per inch reaches diminishing returns around 5 inches, which equals R-17.5. It says the closed cell foam at R-6 per inch reaches diminishing returns at 3-4 inches, which equals R18-24. These are the only diminishing return values I have ever seen for any insulation. Since the values are different for open and closed cell foam, I assume that they may be different for other types of insulation. But in both open and closed cell foam, the diminishing returns kicks in at about R-20. And this is quite an abrupt plateau where the return falls off. Rather than just diminish, the return instantly goes to near zero for added R-value.
I assume this factor of diminishing return with an insulation type is something that can be measured in a laboratory with just a sample of the insulation. But how do you make this calculation for a house where some of the area has considerably reduced R-value due to doors and windows? How do you account for the fact that hot air rises, so there is more temperature differential between inside and outside in the upper areas of the house envelope, and therefore a faster rate of heat transfer through the insulation in those higher areas.
I can see diminishing returns coming into play when all of the heat loss is nearly stopped. But R-20 is nowhere near that point. I would guess that R-20 is less than 25% of the way to zero heat loss, if that. Isn’t there some way to cut the heat loss a lot more?
Is it possibly the case that if the house had no doors and windows and no heat stratification, then R-20 in the envelope would come very close to stopping all heat loss?
But even if that were the case, houses have doors and windows, and in the case of those houses, why insulate above R-20? Going up to R-40 won’t help offset the losses of the doors and windows because there is so very little added insulation benefit above R-20.
Diminishing Returns
I called Owens Corning and Johns Manville today and asked them if they could provide specifications on the diminishing returns of their fiberglass insulation. Neither one could do so.
There's a reason for that
I called Owens Corning and Johns Manville today and asked them if they could provide specifications on the diminishing returns of their fiberglass insulation. Neither one could do so.
6" is just as useless as 3", they're not gonna want to fess up to that.
Joe H
UNRAVELING THE MYSTERY
There is a great mystery surrounding the specification of R-value and diminishing returns. I was told by the technical contact at Johns Manville that my question about their diminishing returns is the hardest question to answer. I would say that it is also a hard question to ask.
Look at it this way: Say we have a closed metal container and we are generating heat inside. That heat is conducting though the walls and dissipating to the outside. So heat is being lost, the gas meter is running, and it is totaling up the bill over time.
Now add one inch of fiberglass insulation to the walls of the container, and the heat loss slows down. Correspondingly, the gas meter slows down, and so the bill will be cheaper over time.
To make this simple, let’s just apply a pure numerical scale to represent the speed of heat loss. Say the fastest speed in the un-insulated condition is 100. Totally stopping heat loss is zero. The one-inch of fiberglass slowed from 100 down to a smaller number, but I do not know what that number is. Maybe it is 95 or thereabouts, but that is just a guess.
Here is what I want to see: A chart that shows the heat loss number starting at 100, and then what that number changes to when I add one-inch of fiberglass, and then what the number changes to every time I add another inch of fiberglass.
I want to see this chart for every type of insulation.
That's all theory
What about every cut or squeeze every time there's romex or a box in the cavity? Every pipe or whatever your fiberglass goes to R-1 or less. 3", 6" makes no difference, FG is better than nothing but not by much,
Joe H
MEASURING INSULATION PERFORMANCE
Joe,
Yes I understand what you are saying about the complications in a building that compromise the fiberglass performance. In a good design and installation, these compromises need to be eliminated. But, I want to set all of that aside and just focus on the thermal performance of the insulation alone.
There is theory behind this performance, but the performance itself can be measured as a practical effect. That is what I am looking for. I can calculate the cost of insulation. I want to know what the insulation does for me. I can’t believe people are insulating buildings without this knowledge.
Advertising?
You've seen the pink panther advertising FG, have you ever seen a TV ad for cellulose or foam? I haven't.
People are insulating building based on that advertising and that's it available at every big box and yard. And it looks like any idiot can do it so why not?
My choice best job would be flash of foam and dense pack cells. Dense pack cells second choice.
Guess a lot depends on budget, location, local contractors, yada yada yada.
Here no cells unless you like paying home depot prices of $0.50 pound unless I buy by the truckload.
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Quote from a couple months ago --
Stabilized Standard, 30 # bags, floor loaded, approximately 1125 bags/53' trailer, is $7.22 per bag.
Not a bad deal but where the hell do I put it? That's the delivered to me price.
Joe H
guess all that HTML gibberish is part of this wonderful Jason Revson web wonderfulness. Ignore it.
Conductive heat transfer: q= k A dT / s. Where q is the rate of heat transfer, k is the coefficient of thermal conduction of the specific material one is interested in, A is the area exposed, dT is the difference in temperature between the hot side and the cold side, and s is the thickness of the insulating material.
So, you can see from the equation that your desired chart can only be drawn for one dT at a time. Sort of impractical to make such a chart. But, you can also see that the heat transfer is directly inverse to the thickness of your insulation... 2" will reduce the heat transfer twice as much as 1".
But in a wall, there are many more factors at stake. Conduction is only part of the puzzle. There's also convection at many interfaces within the wall as well as the conductive heat transfer through the walls materials other than the insulating medium. Some like to include radiation... but that's really a waste of time.
To come up with the best rate of return for your situation will require a specific analysis of your wall. You will also need to know what your energy costs are... now and in the future. Want to buy a crystal ball? You'll need one. Or, make a guess. An educated guess might get you closer.
I find it annoying when someone says a certain amount of insulation saves X % of "the heat." Whenever you see the word "percent," ask "percent of what?" If , say, 97% of "the heat" has been saved, the lost 3% still could be a large enough amount to be worth chasing. From some of those curves, it appears that the reference point may be an R1 wall, meaning an open stud wall with no insulation at all.
In designing a new house for a heating climate, it makes sense to do energy modeling, tallying up the predicted heat losses for the various components of the the house's exterior shell. Then you can go after the bigger pieces of the whole, making improvements where you can. Some things are easier to improve on, such as attic insulation, where in most cases you can heap up loose insulation fairly cheaply. Other things are harder or at least more expensive to improve on, such as windows. Retrofits almost always are more expensive than doing it right the first time.
Finally, resist any assertions that "all the heat goes" some particular place, like the ceiling or the windows. Heat is lost through all parts of the shell. It's just a matter of how much here and how much there.
Houses Without Furnaces
I know what you mean about the percentage. All of these terms are thrown around without enough qualifiers to pin down exactly what they mean. Even R-value is meaningless if there are diminishing returns. If every added inch of insulation gives less added benefit than the previous inch, you need to know what that falloff rate is in order to know the cost of heating the building.
Overall, here is what I am getting at: I want to know the practical limit to how much heating energy-efficiency can be achieved for a home. I know the doors, and especially windows will lose heat very fast compared to any typical insulation layer. Also, air leaks, air change, and makeup air lose heat. But what if you do a heat calculation on a house with no air leaks, no windows, and no doors?
If you analyzed it that way, how small could you make the heating cost for a given outside temperature if you kept adding insulation?
Once I know that answer, then I can look at controlling the heat loss of the windows, doors, air ventilation, etc.
I know there are claims of houses being built for cold climates that don’t need a furnace. So, if that is true, it would appear that it is possible to dramatically reduce heat loss by increasing insulation. And if that is true, what is holding people back from building those type of houses?
If you had a box, insulated on 6 sides, with no doors or windows, you could keep adding insulation and reduce heat loss to whatever low level you wanted. There's no theoretical limit, at least not until you get into exotic quantum mechanical "tunneling" or some such.
Dan,
That seems like it would be true except that you probably cannot achieve zero heat loss no matter how much insulation you add. So now all I want to know is how much you can slow the heat loss with each additional inch of fiberglass. Then I could figure out what the payback would be.
According to Fine Homebuilding No. 204, there is barely any additional slowing of heat loss with closed cell foam once you get to a thickness of three inches.
Owens-Corning told me that I have to email them to get their technical specification of the diminishing returns of fiberglass rolls and batts. So I will report back with what they tell me.
No, you cannot achieve zero heat loss. But you can come as close as you want (and can afford, and have room for). There's nothing "mysterious" -- it's in the heat loss formula: When you double the thickness of the insulation you cut heat loss in half.
If you have a box that built of 1 inch foam board (with no openings) and it takes 100 watts to keep the inside at a certain temperature, increase the insulation to 7 inches and it'll take a bit less than 1 watt (+/- a small delta for radiation effects).Need more explanation
Dan,
I do not understand your explanation when you say, "When you double the thickness of the insulation you cut heat loss in half."
If you lose 100 watts at 1" of insulation and you double the insulation to 2", it cuts the loss to 50 watts. Therefore:
1" = 100 watts
2" = 50 watts
4" = 25 watts
8" = 12.5 watts
However, you say that if you start with 1" with 100 watt loss, and increase insulation to 7", it cuts the loss to less than 1 watt.
Would you please explain this further?
There is your answer
Your calculations have given you the answer to the dimishing return question.
Apply a cost factor to each level of insulation, then do your comparison of heat loss reduction as a percentage of cost for each level. with each of those numbers you could also figure out the dollar value of energy for a year and how long it would take you to recoup the intial cost in saved energy cost.
That is what the diminishing return means, not that the R-value falls off as thickness increases.
FHB #204 article on spray foam
K Design,
Unfortunately, Rob Yagid's article in FHB #204 included some errors.
The graph on page 35 is misleading. It purports to teach us about the "efficiency" of open-cell and closed-cell spray polyurethane foam. However, the y-axis of the graph is meaningless -- there is no such thing as the "efficiency" of insulation. The caption is flat-out wrong: "As the thickness of the insulation increases for both open-cell and closed-cell foam, the insulating value of each diminishes drastically." Actually, as the thickness of the insulation increases, the insulating value of the insulation does not diminish -- it INCREASES. The only thing that diminishes is the energy saved per dollar invested in insulation.
Finally, the phenomenon applies to all types of insulation, not just spray polyurethane foam. The only reason this phenomenon is emphasized by spray-foam salespeople is that their product is so expensive that they need a complicated sales pitch to justify the typically low R-values they sell.
As I have often pointed out, the R-value per inch of spray polyurethane foam, like the R-value per inch of all insulation types, does not appreciably change with thickness. If an insulation has an R-value of R-3.5 per inch, then 2 inches of insulation has an R-value of R-7.0, and 10 inches of insulation has an R-value of R-35.
Every time you double the thickness of your insulation layer, you cut the rate of heat loss through the insulation in half.
The FHB article should have noted, "As the thickness of the insulation increases for both open-cell and closed-cell foam, the insulating value also increases."
Diminishing Returns
Martin,
Thanks for that insight into the Fine Homebuilding article on foam. I was indeed wondering what they meant by “efficiency” on the vertical side of that graph. According to their explanation of the diminishing returns, the vertical side of the graph would mean: Reduction of heat loss rate.
But aside from that ambiguity of the graph, clearly, the article says in the text that closed cell foam reaches a point of diminishing returns at a thickness of about 3-4 inches. From there, it concludes that, “Each additional inch of spray foam yields little performance.” And their graph backs that up.
Therefore, I conclude that this information on diminishing returns of the insulation performance is completely false. It also seems like a tremendous disservice to the foam insulation industry. I am not a subscriber, but I wonder if this error and ambiguity on page 35 of issue #204 was mentioned or corrected in subsequent issues. Does anybody know if it was?
You say that “the only thing that diminishes is the energy saved per dollar invested.” So that would be the only so-called diminishing returns relating to insulation thickness if I understand it. In thinking about this, I have developed the following explanation of why the energy saved diminishes with each dollar invested for additional insulation:
When you cut your heat loss in half with each doubling of the insulation:
The amount of heat loss you have cut is less than what was cut by the previous doubling of the insulation because there is less total heat loss remaining after the previous doubling. So half of the lesser amount of heat is less than half of the previous amount.
Yeah, basically when you double from, say, 4" to 8" that second 4" is only worth half as much as the first 4".
Grinding forward
How do I establish the starting point? If I start with zero insulation and double it, it is still zero. Here is one way that I have analysed it:
I start with 1" of insulation and arbritrarily assign a number of 100 to it that represents heat loss. Then I double the insulation to 2", and the heat loss number changes to 50. I keep doubling the insulation thickness and halving the heat loss number. When I get to 32" of insulation, the heat loss number is 3.125. So, 3.125 is about 3% of 100, so I have cut the heating cost by about 97%.
But what confuses me is that if I start with 1/2" of insulation at heat loss value of 100, I get to a heat loss number of 3.125 at 16" of insulation, whereas in the previous example, it took 32" of insulation to get to 3.125.
What am I missing here?
I posed a question to Owens-Corning about diminshing returns, and got a very strange explanation from them, but I will get to that later.
Think of it from the other direction. If you lose 1 watt at 32", you'll double that heat loss each time you cut the insulation in half. In theory you could get thin to the point that you're losing about a megawatt, after halving it 20 times. (If my figuring is right, the insulation would be 0.000030517578125" thick.) Of course, by this time the insulating effects of the air on either side of the wall would have kicked in (imagine a house with walls of thin silver foil -- it would still provide some insulation effectiveness if you had to take refuge in such a structure in sub-zero weather), so you can't really carry a physical experiment to that extreme.
Hopefully this isn't too much scarey math:
Take the equation that sapwood gave earlier, Q= k*A*Tdiff / s, where Q is the heat loss, k is the R value, A is the area, Tdiff, is the temperature differntial, and s is the thickness. I trust it because it I checked and found it in my old heat transfer text book from college.
Then you, (this is the scarey math part),hold k, A, and Tdiff as constants, and call them C.
The equation is now Q=C/s.
If you do the derivitive of Q with respect to thickness you get: Q=C/s^2. Which mathmatically proves that if you double the thcikness of the insulation while holding the R value, area, and temperature differential constant, you have 1/4th the heat loss.
So as the thickness doubles the heat loss is cut by 1/4th, not by 1/2.
If you start with a given heat loss, or Q, at 1-inch of insulation thickness, then as you vary the thickness "s" you get an inverse exponential curve.
Which is shown in the attached graph.
Try imagining it this way: Insulation is a filter... your house is filled with water (conditioned air).
No filter (insulation) allows all of your water to flow right out immediatly. That's 100% loss per hour.
You put up 1" of filter, the water takes longer to drain out... say it takes 1 hour to drain, so now your 1" is 50% efficient... or R1. That 1" of filter costs $30
2" of filter keep the water in for 2 hours... that's R2, and $60
3" keep water in for 4 hours... R3 and $90
R4 is 8 hours and $120
r5 is 16 and $150
r6is 32 and $180
r7 is 64 and $210
r8 is 128 and $240
r9 is 256 and $270
Keep in mind, it costs $20 to refill your house with water. The difference per day between R5 and R1 is $30 vs $480 per day. The difference between R8 and R9 is $.09 a day... so how many days would it take to pay back that extra $30 you spent on the added inch of filter to slow that water flow?
How many years would it take to pay back the difference between R9 and R18?
Also, don't forget that sometimes the water level outside the house is the same as inside (A non- heating/cooling day)... that's a day that DOESN'T count during the payback phase... each day where the water is 25% of the level inside mans the payback value of the insulation for that day is reduced to 75%
The only thing I don’t understand is the how we get from zero insulation to a starting point of 1” of insulation.
Why are we starting at 1-inch? If I start at 1/16-inch, and double it to 1/8-inch, I save half of the total heat loss occurring at 1/16-inch just by adding another 1/16-inch.
But if I start with 1-inch of insulation and add 1/16-inch, the benefit of that 1/16-inch is far less than the 1/16-inch I added in the case where I started with 1/16-inch and doubled it.
So why start at 1” of insulation? Why not ½”or ¼”? It makes a difference in the final result.
holy smokes.
I'm thinking I could heat my house with all the hot air expended in this thread.
So why start at 1” of insulation? Why not ½”or ¼”? It makes a difference in the final result.
It doesn't make a difference in the final result, but once you get much thinner than about 1/2 inch the effect of the insulation is swamped by the limitations of air-to-air heat transfer.
Consider if you had a house made of thin silver foil -- though there would be no insulation, the house would still be able to hold heat to a degree, since the transfer of heat from air to foil to air is relatively slow.
Now It Is Clear.
Okay, I think I see what the problem was with what I was seeing in my earlier post called "Grinding forward." I thought I was getting a different result starting with 1" of insulation versus starting with 1/2". But I had assigned an arbitrary number of 100 to represent the heat loss rate at 1", and also used 100 for the 1/2" start.
So I really can't compare the outcome when using 100 in each case of the two different thicknesses. If I used 100 as the arbitrary heat loss value for 1" of insulation, I would have to use 200 as the arbitrary number for 1/2" of insulation. Then the outcome should be the same for starting with the two different thicknesses.
You're getting lost in the numbers.
Take my example of the house filled with water, kept inside only with some kind of filter medium. You are asking why I start at 1" depth, and not a smaller depth...
OK, Lets start at a smaller depth... 1/16 is too thick... lets go down to one human hair, streatched from floor to ceiling on all 4 sides of my house. It's much more effective than having nothing there - I think it reduces the outward flow of water by .0001 picoliters per hour! Now lets doble that effectivenes by using 2 individual strands of hair - now I've DOUBLED the effectiveness of my filter as in now reduces flow by .0002 picoliters per hour.
I'm still reducing the outflow, but I've started at such a small scale that it's really not noticable at all.
At one end of the outflow scale you are holding back an 8' wall of water with a single human hair... at the other end of the scale you are only allowing that entire 8' tall wall of water flow though a hole the size of a human hair.
Wikipedia says that super insulated houses typically have R40 walls, R60 roofs. So, I guess that's the practical point of no return on house insulation, since they typically do not require a furnace.
SUPERINSULATED
That is what I am thinking about. I built my house in 1984 as superinsulated. It has 24" of fiberglass in the walls and 30" in the scissors trusses of the vaulted ceiling. Right now, I am designing another one that will have 14" in the walls and 30" in the roof trusses. But I have never done a heat calculation on either one of these. The one I built was actually a major remodel with not much left of the original excpet the basement.
The new design is a clean sheet of paper, so I want to eventually do a heat caluculation and see just what the payback would be.
The claim of not needing a furnace is interesting, and I think a bit dubious. With the superinsulated houses that were first built in the early 1980s, they were said to not need a furnace because they would be heated by human occupants, lighting, cooking, etc. But there was often a mention of having a small wood stove for backup. So, depending on how much clothing one wore, and how much wood they burned, a furnace was not needed.
I can state unequivocally that, in MN, with this house with 24" of fiberglass in the walls and 30" in the roof, I could not survive without a furnace. Of course there are other variables in the equation such as windws, doors, and efficiency of the air-to-air heat exchanger. I think it would be possible for houses built to these general insulation levels to cut the heating costs at least 75% from typical average construction.
Well, I suppose a baseboard heater or wood burner is not a furnace, so the claims may be true. The thing is, doors and windows leak enough heat to greatly reduce the R value of the whole house. This can cause serious problems with your calculations. If 10% of your house is, say, R6, the heat loss through that percentage will eclipse the amount lost through the rest of the house to the point that there will be a theoretical limit to the effective practical R value of insulation. In other words, you will have a certain amount of heat loss that cannot be easily reduced, without having a house that is built like a thermos. So, you will have a heating load for that part of the house, and reducing the heat loss in rest of the house by an additional 100% may only reduce the heating load by (pulling a number out of my ass) 5%.
I do believe you can easily reduce your heating load to 25% of the average house, but it may not be a good investment due to the payoff time frame.
I have seen houses in MN that get along OK without a furnace or other major source of heat -- just a little backup resistance unit for when the house is unoccupied.
But the problem with these is that the windows are few and small, and people don't really like to live in such a house.
Another thought...
Insulation technology has progessed a lot sice 1984. Closed cell foam blocks air currents, and provides considerably more R value/inch than fiberglass does. Dense pack cellulose is another option. Then there is the issue of thermal bridging, which can cause a lot of heat loss. there are many ways to avoid thermal bridging, one of the best being double walls with foam sheeting between them. Also, electrical runs can be made more efficient in terms of heat loss.
SUPERINSULATED COST/BENEFIT
It certainly is a complicated issue to decide how far to go to save heat cost. One factor is that we may be on the verge of a large and quick price hike. Maybe not, but I would anticipate that happening now, versus any other time during the last 25 years or so. With super insulation, you not only have the cost of the extra insulation, but also the cost of more framing and labor. However, I think one of the biggest costs for a well designed superinsulated house is the design of the framing.
In my house, I have double stud walls, but it was a complicated remodel, and the framing is more complicated than just double studs. It is very hard to lay out a rational framing plan and have it all come together without lots of little issues to resolve along the way. Getting a perfectly sealed vapor barrier worked into the scheme of things is quite a challenge. I have my wiring completely separted from the insulation cavities.
Fiberglass batts and rolls need to be installed with the kind of care and accuracy that one might use for building a plywood set of shelves, for example. Contrary to what everybody says, some compression of the fiberglass can be beneficial. Window and door tunnels get complicated.
The payback is in the reduced heating costs, but also in a greater comfort. When the heat loss is low, the walls are warmer. In a higher heat loss house, the furnace might keep the air warm enough, but the walls will be cooler. The radiant loss to the cooler walls is often what people describe as a chill. A lot of times, it is assumed to be caused by cold air leaking in, but it is actually heat being pulled away from a person by a cool wall by radiant transfer.
A lot of early super insulated houses may have been more comfortable, but the eclectic styling and lack of windows would have driven me up a wall. Houses are meant to be enjoyed, not just be energy efficient. It sounds like you are pretty much on track to success though.
Limiting the Windows
Mark,
I have not looked too closesly at those first generation superinsulated houses, but I seem to recall that they were sort of plain looking. I will have to do some research on those. I don't recall what they had for windows. There is also a brand new generation of superinsulated houses that I have not really looked into. Actually, I think those are zero carbon footprint or something.
A superinsulated house should miminize the window size and quantity, but there is no hard and fast rule. Some people like a lot of windows, and that conflicts with superinsulated principles, but I would just use somewhat less than the average amount of windows. I would use as high efficiency as possible, and they would all be casements to get the most unobstructed views.
One thing that would really improve the overall heat loss from windows would be to have insulating covers that could be opened and closed. It is usually the coldest at night, and you don't need windows at night. They could also be closed in the daytime to some extent during especially cold days. In MN, you could probably close them for the whole month of November, and it would be less dreary than having them open to see outside.
The only problem is that the hinged covers need to be on the outside. If they were on the inside, they would be handy to manually operate, but they would need to be gasketed to keep moisture from getting past the cover and freezing on the glass and on the extra-cold window frame behind the insualted cover. Covers on the outside would probably have to be motorized, and they would have to remain operable in snow and ice conditions.
While you're at it...........
Figure the gain/loss ratio for a passive solar house in your window equation. How much do you gain in the day v. how much you lose at night. More gain? More loss? Zero out?
Of course, there's a difference between superinsulated and passive solar. With passive solar you need some heat storage scheme -- a Trombe wall or some such.
Yes there's a difference, they're not even close.....
And I may have confused you.
If you don't want windows because they lose heat, yet don't want to live in a cave either...........
What do you gain (and of course not fleeting gain-stored gain) from good glass.
What do you lose.
Is the answer at least zero if done right?
But net zero does no good if you don't have effective heat storage -- you run the AC during the day and the furnace at night.
A lot of the early super insulated houses had no north facing windows, and a "contemporary" style that looks dated now. Also the windows were usually small rectangles that allowed you to look outside, but were more like viewing ports than windows. I always wondered what they were thinking.
Passive Solar / Superinsulated
When I built my house, I was thinking about different building schemes, including passive solar. But since, I was doing a remodel, I decided not to go that route. Passive solar or even active solar seems to work best if the house is designed for it right from the ground up. And I am not really sold on solar overall. Passive solar could really add some interest to a house if you had a solar greenhouse incorporating a big storage mass of masonry or whatever, lots of glass, and an operating nightime closure system.
For either passive or active solar, better than average insualtion would be a natural feature to include. You could even combine passive solar with superinsulated, however, as has been mentioned, they are fundamentally two different approaches. So, while solar should be extra-insulated, superinsulation can stand on its own without needing solar. In the final analysis, I think that superinsulated alone make the most sense, at least for my climate.
But superinsulated can seems sort of mundane and clunky. It needs to be combined with features that enhance the principle and, at the same time, give it some personality. Compact, and space-efficient are principles that compliment superinsulated. I would offset the minimalist space layout with extra high quality materials, and details.
One of the most complex energy-efficient schemes to come down the pike was double-envelope houes.
Please
Answer this.
You insulate a house-you are building from scratch.
Your insulation is done answering that plus minus inches thing you have been talking about.
However, you design to include passive solar elements and of course you incorporate some form of retention technique-passive solar is not just sticking glass in the south side. That's a given and deserves no comment.
You do not include any active solar.
Is there a net gain in either savings or comfort?
I'm not talking about keeping the thermostate at 63 degrees to save money. I don't live with that woman.
Calvin,
I think you can gain in savings and comfort, but there is an extra cost. But even with the extra cost, I think the payback is within a reasonable time if the whole approach is done sensibly. I have worked on a few active solar installations in which the design seemed really ill-conceived. I don't think they were worth it. These were with maufactured solar air heaters. They were big panels about 12-14" thick with some really high quality tempered glass on the face. They were quite costly.
The panels seemed to work well enough if the sun was shining, but you had to get the heat ducted into the house, and probably into some type of mass storage. In one installation, the design called for the panels to be about 60 ft. behind the house and around the corner on a deck. What was really unaccounted for was just how much work would be needed to run well insulated heat and return ducts from the panels to the house. This would have required a well built, and highly insulated chase, and they never built that. So it was kind of a half-hearted attempt that probably contributed something, but not enough to make it worthwile.
Another thing to consider is that a lot of people built energy efficient houses and made all sorts of claims about how well they performed. You have to take all of that with a grain of salt.
I agree about not having the house temperature at 63 degrees. I can't stand going over to visit people when they have their heat set way low in the wintertime. I want at least 72 degrees in the air and on the walls. I have been in way too many buildings that were too cold, even in summer. I have worked in places that had killer air conditioning pounding away continously pouring 65 degree air down my neck. No thanks.
kdesign...
i've been reading this thread...
some of it seems a little esoteric...but
a couple questions... in your 1984 superinsulated house.... have you ever done a blower-door test ?
what does it tell you?
fg insulation is notoriously poor in air leakage...
a lot of the techniques that were state-of-the art in '84 are obviously flawed by today's standards..
i considered our double wall house , built in '85 to be super-insulated... but it is really a giant seive by today' standards
1st thing i would do iffen i was you...is boot the fg....switch to dens-pak cellulose....eliminate all the thermal bridges you can.... look at some of the standards prescribed by programs like Energy Star 3.0
lot of good points in their perscriptions
and... of course.... look at year round living....heating/cooling/open window season
some of the things we did in the '70's were pretty drastic and dreadfull in terms of design esthetics and natural lighting/ventilation
I agree with your observations of some of the things done in the 1970s. Yes fiberglass is impermeable to airflow, but I do not rely on fiberglass to stop air movement. Ultimately, the vapor barrier stops all air infiltration. The vapor barrier is 100% sealed. All seams are sandwiched in the framing members. In this sanwiching, I used silicone RTV to make standing beads that I allowed to cure on one member. That became a gasket that would compress and seal off the polyethylene vapor barrier when the framing member was screwed down to sanwich the vapor barrier. I have never done a blower door test, but I am convinced the envelop is air tight. I intentionally allowed some air infiltration into the fiberglass from the outside for the purpose of scavenging out any vapor that happend to get past the vapor barrier.
It also explains the diminishing return
The deriviative also explains why there is a diminishing return for extra thickness.
Details
Mike,
I have thought about doing the blower door test just out of curiosity about the integrity of the vapor barrier. I am confident that the vapor barrier is air tight, along with the window and door tunnels. But there are some openings in places such as around the air exchanger pipes in and out of the basement. I have a small widow air conditioner mounted in a tunnel through an upper wall. I never make an effort to seal that off thoroughly in the winter.
I have a Vanee heat recovery ventilator, and sometimes I wonder if that is adequate. My general sense is that small air leaks are not an issue to the point where they need to be chased down and sealed. I have had some details somewhat unresolved for periods of time, such as a threshold seal, and even with that being open, it does not seem significant. It probably costs some small money, but there is no sense of discomfort from it.
I have sense that the fiberglass is inadequate here, and no sense that it is somehow outdated and inefficient compared to other insulations. I am working on a design of another superinsulated house, and I have decided to use fiberglass for that one. There are several reasons that I have ruled out foam. My main focus for the new design compared to this house is how to handle the cold-side ventilation for the walls and roof system, and the details of the air infiltration barrier for the walls. For this house, I did not use a barrier like Tyvek.
I have 2 X 4 horizontal nailers on 2’ centers for 1 X 8 redwood T&G vertical siding. There is no sheathing because the nailers were considered to be a structural substitute for the sheathing. I just ran 30# felt vertically on top of the nailers. I overlapped the vertical seams, but did not nail them off since they are not directly backed by any vertical faming. I wanted to let a little air get into the insulation to ventilate it in case any vapor did get past the vapor barrier. But, of course, too much air intrusion into the fiberglass will unnecessarily degrade the R-value. So I am not really sure how that detail is performing. If I were to build again, I would use some type of infiltration barrier material.
Lately, I have been studying the issues of cold-side ventilation. That article by Joe Lstiburek called, A Crash Course in Roof Venting is quite interesting.
Starting at 1/16 inch
K Design,
If you were an advertiser eager to overstate the value of your 1/16-inch-thick insulation product, it would, indeed, make sense to compare the insulation performance of your 1/16-inch-thick product to the performance of a house with, say, 1/64 inch of insulation.
So what? There are just advertising tricks.
If 1 inch = R-3.5 (for example), 1 inch of insulation will always perform the same. And 2 inches of insulation will always result in an assembly with half the heat-flow rate of the 1 inch assembly. The insulation always performs the same. Your hypothetical comparisons are all true, and they are all potential fodder for charlatans who write ad copy, but they don't change the physics of R-3.5 insulation. They are just comparisons.
I think that if any manufacturer wants to brag about an insulation product or a wall assembly, they should always compare its performance to a wall with the code-minimum amount of insulation. After all, that's the worst wall you can build without getting arrested. So when spray-foam contractors compare their product to a wall with 1/2 inch of insulation, I'm not impressed. That wall is illegal and therefore irrelevant.