| 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:
|
Senior Member |
|
|
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.
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
|
|
|
|
GMCforum
Search
Help
Register
Login
Home
