Home » Public Forums » GMCnet » Re: [GMCnet] Insulating coach
| Re: [GMCnet] Insulating coach [message #229506] |
Tue, 12 November 2013 11:56  |
George DV
Messages: 132
Registered: August 2004
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R value is a number used in a LINEAR equation to calculate total heat transfer in convective energy. The reciprocal of the R value is used however by us sliderule weenies.
So a R value of 10 would be seen as +++++ 1/10 +++ in the calculation
Take it from there with your ratios.
Secondarily. Remember insulation works on the principal of STAGNANT air.
Fibreglas or whatever only traps air and inpedes its movement!
It is NOT magic
And more is not necessarily bettter
George Delvecchio
Beautiful upstate NY
Recouperating
76 Glenbrook
Sent from my Verizon Wireless Device
"A." <markbb1@netzero.com> wrote:
>
>
>SeanKidd wrote on Tue, 12 November 2013 04:21
>> adding insulation (R-7 to R-10) in your coach is a lot of effort for a 2% gain...
>We are going to continue to disagree here.
>
>If you could post a URL to a source that purports that R-1 cuts heat transfer by 50%, I can take a look at it. But for now, I reject your numbers. And there should be no argument that doubling the R-value halves the heat transfer. You CAN reach a point where doubling the insulation only cuts your energy consumption by 2%, but that 2% point is closer to going from R-30 to R-60, not going from R-7 to R-14.
>
>If you approached that point in single digits of R-value, then the ROI on construction insulation would be decades or centuries. And we know that is not the case.
>
>The rest of your post is completely accurate.
>--
>'73 23' Sequoia For Sale
>'73 23' CanyonLands For Sale
>UA (Upper Alabama)
>CanyonLands most likely for a parts coach. Sequoia being restored to service.
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| Re: [GMCnet] Insulating coach [message #229507 is a reply to message #229506] |
Tue, 12 November 2013 12:08  |
Dennis S
Messages: 3046
Registered: November 2005
Karma: 2
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Senior Member |
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| George DV wrote on Tue, 12 November 2013 11:56 |
R value is a number used in a LINEAR equation to calculate total heat transfer in convective energy. The reciprocal of the R value is used however by us sliderule weenies.
So a R value of 10 would be seen as +++++ 1/10 +++ in the calculation
Take it from there with your ratios.
Secondarily. Remember insulation works on the principal of STAGNANT air.
Fibreglas or whatever only traps air and inpedes its movement!
It is NOT magic
And more is not necessarily bettter
George Delvecchio
Beautiful upstate NY
Recouperating
76 Glenbrook
Sent from my Verizon Wireless Device
"A." <markbb1@netzero.com> wrote:
>
>
>SeanKidd wrote on Tue, 12 November 2013 04:21
>> adding insulation (R-7 to R-10) in your coach is a lot of effort for a 2% gain...
>We are going to continue to disagree here.
>
>If you could post a URL to a source that purports that R-1 cuts heat transfer by 50%, I can take a look at it. But for now, I reject your numbers. And there should be no argument that doubling the R-value halves the heat transfer. You CAN reach a point where doubling the insulation only cuts your energy consumption by 2%, but that 2% point is closer to going from R-30 to R-60, not going from R-7 to R-14.
>
>If you approached that point in single digits of R-value, then the ROI on construction insulation would be decades or centuries. And we know that is not the case.
>
>The rest of your post is completely accurate.
>--
>'73 23' Sequoia For Sale
>'73 23' CanyonLands For Sale
>UA (Upper Alabama)
>CanyonLands most likely for a parts coach. Sequoia being restored to service.
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There is this info on R value -- valid (?) --
http://www.insulationsmart.com/what_will_i_save.htm
Dennis
Dennis S
73 Painted Desert 230
Memphis TN Metro
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| Re: [GMCnet] Insulating coach [message #229544 is a reply to message #229533] |
Tue, 12 November 2013 14:39 |
A Hamilto
Messages: 4508
Registered: April 2011
Karma: 39
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Senior Member |
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| Robert Mueller wrote on Tue, 12 November 2013 13:41 |
A,
According to the article the figures are "based on Fourier's Law of Thermodynamics."
Are you saying those laws are incorrect?" 
Regards,
Rob M.
The Pedantic Mechanic
USAussie - Downunder
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No. I am saying the real world throws more variables into the mix than that article takes into account.
The numbers on that table are suspect, because it said an R-value of 8 stopped 90% of heat transfer, but R-16 only blocked 95% ( or was it 97%?). If it was "true", the first R-8 would block 90% of the heat and the second R-8 would block 90% of the heat that got through the first R-8, meaning R-16 would block 99% of heat transfer.
The laws of thermodynamics are not wrong, but that table is.
R-value, as it applies to products on the market, does not agree with the information being presented in that table. If it did, the difference in utility bills for a house with R-19 in the attic would not be significantly different than an identical house with R-30 or R-50 in the attic. I have enough practical experience to reject the applicablilty of those figures to real world application. I can stand in a residential area after it snows where my in-laws live and pick out the houses with R-19 in their attics vs R-30 or more by how fast the snow melts off the roof. It takes a lot of energy to melt snow.
You could do the same thing with a GMC insulated to R-7 in the ceiling and one insulated to R-10 in the ceiling. Get the inside temperature to 70F, dump an inch of snow on them and see how fast the snow melts on each. There is no question in my mind. The one with R-7 will melt the snow about 30% faster than the one with R-10. That's a lot of heat transfer.
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| Re: [GMCnet] Insulating coach [message #229752 is a reply to message #229544] |
Wed, 13 November 2013 23:38 |
A Hamilto
Messages: 4508
Registered: April 2011
Karma: 39
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Senior Member |
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| A Hamilto wrote on Tue, 12 November 2013 14:39 |
The numbers on that table are suspect...
|
In the interest of not letting things go...
I hate to see misinformation like that posted on a site that otherwise could have a lot of credibility.
Ref http://www.insulationsmart.com/what_will_i_save.htm
I don't know where to buy R-8, but I could redo that table with some real world values. First, replace the R-12 with R-13, R-20 with R-19 and R-32 with R-30.
Then Use real values for insulation prices. I just looked stuff up at HD. R-13 is a little less than 30 cents a SF (not 90 cents for R-12), R-19 is a little less than 48 cents a SF (not $1.40), and R-30 is a little less than 37 cents a SF (not $2.00).
I don't know why R-19 is so much more than R-30.
Remember that installation costs are virtually identical for all three. So the only cost difference is the material itself.
So if R-13 blocks 93% of heat transfer, and R-30 blocks 97% of heat transfer, then R-30 should save about 4% per year on the web page's $750 annual heating and cooling bill. That's $30 per year. Cost difference for 4,000 SF of R30 vs R-13 is $280. Payback would thus be 9 years and 4 months. The website says 107 years!
R-19 would save 3% over R13, or $22.50 per year, $720 dollars more for $4,000 SF of R-19 vs R-13 would be a payback of 22 years and 6 months. The website says 71 years.
Both of those payback periods are using THEIR percentages. My experience is that the higher R-values make a bigger difference than that table indicates.
Sorry folks. I just couldn't let it go. That web site is an outright lie. Don't believe everything you read on the internet.
And another moral of the story is to do the math. Payback for R-30 is pretty reasonable, even at only 4% per year.
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| Re: [GMCnet] Insulating coach [message #229782 is a reply to message #229752] |
Thu, 14 November 2013 08:40 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Senior Member |
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A,
Seems to me that it would be interesting if you submitted the information below to the people that published that table to see what
they have to say.
Regards,
Rob M.
The Pedantic Mechanic
-----Original Message-----
From: A.
A Hamilto wrote on Tue, 12 November 2013 14:39
> The numbers on that table are suspect...
In the interest of not letting things go...
I hate to see misinformation like that posted on a site that otherwise could have a lot of credibility.
Ref http://www.insulationsmart.com/what_will_i_save.htm
I don't know where to buy R-8, but I could redo that table with some real world values. First, replace the R-12 with R-13, R-20
with R-19 and R-32 with R-30.
Then Use real values for insulation prices. I just looked stuff up at HD. R-13 is a little less than 30 cents a SF (not 90 cents
for R-12), R-19 is a little less than 48 cents a SF (not $1.40), and R-30 is a little less than 37 cents a SF (not $2.00).
I don't know why R-19 is so much more than R-30.
Remember that installation costs are virtually identical for all three. So the only cost difference is the material itself.
So if R-13 blocks 93% of heat transfer, and R-30 blocks 97% of heat transfer, then R-30 should save about 4% per year on the web
page's $750 annual heating and cooling bill. That's $30 per year. Cost difference for 4,000 SF of R30 vs R-13 is $280. Payback
would thus be 9 years and 4 months. The website says 107 years!
R-19 would save 3% over R13, or $22.50 per year, $720 dollars more for $4,000 SF of R-19 vs R-13 would be a payback of 22 years and
6 months. The website says 71 years.
Both of those payback periods are using THEIR percentages. My experience is that the higher R-values make a bigger difference than
that table indicates.
Sorry folks. I just couldn't let it go. That web site is an outright lie. Don't believe everything you read on the internet.
And another moral of the story is to do the math. Payback for R-30 is pretty reasonable, even at only 4% per year.
--
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Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: [GMCnet] Insulating coach [message #229788 is a reply to message #229782] |
Thu, 14 November 2013 09:18 |
Jp Benson
Messages: 649
Registered: October 2011
Location: Fla
Karma: 2
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Senior Member |
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Under the table in question is the assertion:
"Figures for amount of heat flow reduced are based on Fourier's Law of
Thermodynamics."
Which only means that those figures are quite valid for a precisely
controlled laboratory experiment.
Here's a good essay of Fourier's Law of Conduction:
http://www.me.umn.edu/courses/old_me_course_pages/me3333/essays/essay%203.pdf
Within that essay is the statement:
"While those solution methods possess a degree of elegance, they are of
minimal use for solving the complex heat conduction problems that are
encountered in engineering practice."
The website with the table covers it's butt with the disclaimer:
"Information or data contained on this website is not guaranteed to be
current, accurate or complete and is subject to change, without notice. "
JP
On 11/14/2013 9:40 AM, Rob Mueller wrote:
> A,
>
> Seems to me that it would be interesting if you submitted the information below to the people that published that table to see what
> they have to say.
>
> Regards,
> Rob M.
> The Pedantic Mechanic
>
>
> -----Original Message-----
> From: A.
>
> A Hamilto wrote on Tue, 12 November 2013 14:39
>> The numbers on that table are suspect...
> In the interest of not letting things go...
>
> I hate to see misinformation like that posted on a site that otherwise could have a lot of credibility.
>
> Ref http://www.insulationsmart.com/what_will_i_save.htm
>
> I don't know where to buy R-8, but I could redo that table with some real world values. First, replace the R-12 with R-13, R-20
> with R-19 and R-32 with R-30.
>
> Then Use real values for insulation prices. I just looked stuff up at HD. R-13 is a little less than 30 cents a SF (not 90 cents
> for R-12), R-19 is a little less than 48 cents a SF (not $1.40), and R-30 is a little less than 37 cents a SF (not $2.00).
>
> I don't know why R-19 is so much more than R-30.
>
> Remember that installation costs are virtually identical for all three. So the only cost difference is the material itself.
>
> So if R-13 blocks 93% of heat transfer, and R-30 blocks 97% of heat transfer, then R-30 should save about 4% per year on the web
> page's $750 annual heating and cooling bill. That's $30 per year. Cost difference for 4,000 SF of R30 vs R-13 is $280. Payback
> would thus be 9 years and 4 months. The website says 107 years!
>
> R-19 would save 3% over R13, or $22.50 per year, $720 dollars more for $4,000 SF of R-19 vs R-13 would be a payback of 22 years and
> 6 months. The website says 71 years.
>
> Both of those payback periods are using THEIR percentages. My experience is that the higher R-values make a bigger difference than
> that table indicates.
>
> Sorry folks. I just couldn't let it go. That web site is an outright lie. Don't believe everything you read on the internet.
>
> And another moral of the story is to do the math. Payback for R-30 is pretty reasonable, even at only 4% per year.
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| Re: [GMCnet] Insulating coach [message #229941 is a reply to message #229752] |
Fri, 15 November 2013 16:48 |
A Hamilto
Messages: 4508
Registered: April 2011
Karma: 39
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Senior Member |
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| A Hamilto wrote on Wed, 13 November 2013 23:38 |
...So if R-13 blocks 93% of heat transfer, and R-30 blocks 97% of heat transfer, then R-30 should save about 4% per year on the web page's $750 annual heating and cooling bill. That's $30 per year. Cost difference for 4,000 SF of R30 vs R-13 is $280. Payback would thus be 9 years and 4 months. The website says 107 years!
R-19 would save 3% over R13, or $22.50 per year, $720 dollars more for $4,000 SF of R-19 vs R-13 would be a payback of 22 years and 6 months. The website says 71 years.
Both of those payback periods are using THEIR percentages. My experience is that the higher R-values make a bigger difference than that table indicates.
Sorry folks. I just couldn't let it go. That web site is an outright lie. Don't believe everything you read on the internet.
And another moral of the story is to do the math. Payback for R-30 is pretty reasonable, even at only 4% per year.
|
A light came on and I had time to beat the dead horse, so I thought I would toss out one more thought.
If the average cost for heating and air conditioning for a standard size house is $750, and if the average R-value is R-16 (R-13 in the walls and R19 in the attic), that means that the cost is based on the structure only transmitting 3% of the heat (R-16 reduces heat transfer by 96.875%).
Doubling the R-value to R-32 will cut the heat loss in half, down from 3% to 1.5%. So the $750 will still be reduced by half to $375.
So 2% is a lot more significanct than I first thought. It can still represent hundreds of dollars a year. And it explains why I had a gut feeling that there were more dollars to be saved than the percentages implied.
As it relates to your GMC, that solid foam stuff that comes in 4' X 8' sheets can be acquired in 1/2", 3/4", 1", 1-1/2" and 2" thicknesses.
It is about R-6.8 per inch. Last time I looked, the price difference was about $10 per sheet for each 1/2" of thickness. Given 26 linear feet of ceiling, it would take 6-1/2 sheets to do the ceiling. So if you have to choose between 1" and 1-1/2", it would cost about $65 for the extra half inch in the entire ceiling. Will it ever pay for itself? There are no wrong answers. Myself, for another 32 cents a square foot, I would fill the entire 1-1/2" and get R-10.
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| Re: [GMCnet] Insulating coach [message #230021 is a reply to message #229941] |
Sat, 16 November 2013 10:10 |
jayrabe
Messages: 509
Registered: June 2009
Location: Portland, OR
Karma: 0
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Senior Member |
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I used the solid foam isocyanurate (I think).Yes, it comes it various thicknesses. However I strongly recommend not using 1" or above if you intend to follow the curvature of the ceiling near the side walls. If you use mutiple layers of 1/2" or 3/4", you can put a row of shallow scores and get the stuff to bend, but most of the time the 1" stuff just broke when I tried to bend it that much.
Also, like a lot have pointed out, you win or lose depending on the details. R value can be totally erased by unsealed edges or seams that let air drafts through. I used the spray foam in a can to "glue" the foam panels in and to fill all the cracks and edges.
JWID
Jay 76 PB Portland, OR
> To: gmclist@temp.gmcnet.org
> From: markbb1@netzero.com
> Date: Fri, 15 Nov 2013 16:49:00 -0600
> Subject: Re: [GMCnet] Insulating coach
>
> As it relates to your GMC, that solid foam stuff that comes in 4' X 8' sheets can be acquired in 1/2", 3/4", 1", 1-1/2" and 2" thicknesses.
>
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