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US R-values of radiant barriers - Page 4

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Posted by News on July 30, 2006, 11:24 pm
 


I am aware of that and have previously read it. It is quite old now, 1999.
At the time the performance in cold climates from other non-comprehensive
tests in the USA was not too encouraging.


Actis claim the equivalent of 210mm of Rockwool.  I believe they do have an
effect on cooling when pinned to the rafters of a roof.  What is the overall
claim for rockwool equivalent thickness for heating by tests in the US?
There must be some ballpark. No one is going to type in Nicks program, they
read the makers blurb, or test results to confirm the blurb.

I have the impression much of any heat saving is because this stuff is
air-tight. More the draught prevention is making the difference rather than
the reflective qualities of the material itself.  I hope I am wrong and it
does what they say. If so my attic gets done out in it.  Until something
more concrete in realistic more real world testing the jury is still out and
it stays out of my attic. 35C here means I may have to act with the attic by
next summer - but using what has to be determined.


Posted by nicksanspam on July 31, 2006, 4:37 am
 


The second edition (1998) of Pitts & Sissom's Schaum's Outline on Heat
Transfer gives k = 0.023 Btu-ft/h-F-ft^2, ie 0.276 Btu-in-h/F-ft^2, ie
US R3.62 per inch, at a rock wool density of 10 lb/ft^3.


No need to type much. Just save it in a file, remove the headers, and run it.
Or look at the on-line RIMA Handbook and use a calculator, which takes about
5 minutes.


That's assumed to begin with, but foil helps. For instance, the RIMA Handbook
says a horizontal foil with E2 and E3 = 0.03 and 3" airspaces 1 and 2 above
and below the foil with E1 and E4 = 0.8 boundaries and downward heatflow and
110 F above and 80 below has E = 0.0298 for both airspaces and an overall
dT = 110-80 = 30 F. Assuming the foil is 80+dT/2 = 95 F, the mean temp in
airspace 1 is Tm1 = (110+95)/2 = 102.5 F, and Tm2 = (95+80)/2 = 87.5. From
Table 4 on page 25 of the Handbook, hc = 0.075 for both airspaces. Equation 3
on page 22 says hr1 and hr2 = 0.00686((Tm+459.7))/100)^3 = 1.219 and 1.124.
Equation 1 says R1 = 1/(Ehr1+hc) = 8.98 and R2 = 9.22, so R = R1+R2 = 18.2,
and dT1 = 30x8.98/18.2 = 14.8 F and dT2 = 15.2. Close enough. We could
iterate if needed, using these new dTs to find new Tms.


No. If we replace the foil above with another E2 = E3 = 0.8 opaque surface,
then E = 0.8 vs 0.0298 for both airspaces, so R1 = 0.952 and R2 = 1.026 and
the overall R = 1.98 vs 18.2, ie 9 TIMES less. If we replace the foil with
IR-transparent polyethylene film, the difference is even greater, even though
there's still draught prevention. OTOH, if we add more foils or move the foil
up so there's only one airspace, that doesn't help much in this case, given
the same overall airspace dimension.

Rock wool would only add 3.62x6" = US R13.13 vs 18.2, using a lot more stuff.


Au contraire. This has been settled science for over 50 years :-) See

Robinson and F.J. Powell, "The Thermal Insulating Value of Airspaces,"
Housing Research Paper No. 32, National Bureau of Standards Project NE-12,
National Bureau of Standards, Washington DC (1954), and

Yarbrough, "Assessments of Reflective Insulation for Residential and
Commercial Applications," Oak Ridge National Laboratory Report ORNL/TM 8819,
Oak Ridge, TN (1983), and

Yarbrough, "Estimation of the Thermal Resistance of a Series of Reflective
Air Spaces Bounded by Parallel Low Emittance Surfaces," Proceedings of
the Conference on Fire Safety and Thermal Insulation, S.A. Siddiqui,
Editor (1990) pp 214-231, and

Yarbrough, "Thermal Resistance of a Air Ducts with Bubblepack Reflective
Insulation," Journal of Thermal Insulation 15 137-151, (1991).

Nick


Posted by .p.jm on July 31, 2006, 5:08 am
 
    Nick, you make me want a brain-douche.



On 31 Jul 2006 00:37:34 -0400, nicksanspam@ece.villanova.edu wrote:


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Posted by Jeff on July 31, 2006, 1:00 pm
 nicksanspam@ece.villanova.edu wrote:

 From The Passive Solar Energy Handbook, Edward Mazria 1979 we have this
in Appendix E.6 Resistance values of airspaces

Horizontal, Heatflow Down
NR=Non Reflective

Thickness |  Season  |  NR/NR | NR/Aluminum Coated  | NR/Foil
3/4            W         1.02         2.39              3.55
1 1/2          W         1.14         3.21              5.74
4              W         1.23         4.02              8.94
3/4            S         0.84         2.08              3.25
1 1/2          S         0.93         2.76              5.24
4              S         0.99         3.38              8.03


Obviously that's all from observations.

What strikes me for my application at hand, insulating under staple up
radiant, is that 8.94 for a single radiant barrier. It sure makes foil
double bubble look good.

   Jeff



Posted by daestrom on July 31, 2006, 10:11 pm
 

<snip>

One thing though about radiant barriers.  It's well settled that the upper
surface of horizontal installations will not retain its low emissivity.
Unless you fancy wiping and cleaning off the dust every year or so, it will
accumulate and lose its effectiveness.

In attics, it's advised to put the radiant barrier on the rafters overhead
so the radiant surface is on the underside.  For underfloor installations,
the same thing.  The foil goes on the underside to limit the accumulation of
dust that will ruin its effectiveness.

daestrom


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