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Re: [Phys-l] Insulation R-value



There are lots of good references for the definition of R values.
According to Serway or Cutnell & Johnson (and I'm sure just about every
other general physics book),

R = L/k

And has units

(ft^2 F hr) / BTU

One advantage is that R values can be added, so if you add more
insulation to your house, you can find the total R by simple addition.


Tim Folkerts



-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Quist,
Oren
Sent: Friday, February 17, 2006 2:24 PM
To: PHYS-L Maillist
Subject: Re: [Phys-l] Insulation R-value

The logic is correct, but there is some interesting conversion of units.
I think, the R- value is defined to be

Minutes/BTU/square foot/degree F

As I say, the conversion is not scientifically logical, but very user
friendly for the construction trade.
Time in minutes
Energy in BTU's
Area in square foot
And, of course delta T is in degrees F.

Oren Quist, SDSU

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Larry
Woolf
Sent: Friday, February 17, 2006 12:12 PM
To: PHYS-L Maillist
Subject: Re: [Phys-l] Insulation R-value

The thermal resistance R is defined by:
R = delta T/(dQ/dt)
R = r(L/A) = 1/K = (L/A)/k
where
r is the thermal resistivity
k is the thermal conductivity
K is the thermal conductance.

Larry Woolf
General Atomics
www.ga.com
www.sci-ed-ga.org

-----Original Message-----

On Feb 17, 2006, at 11:57 AM, ALVIN BACHMAN wrote:
R is related to the reciprocal of k.
dQ/dt= k A (T1-T0) / L = A (T1-T0) / R

Thanks Al, I was not aware that R is defined differently from k.
Perhaps k should be called thermal konductivity and R should be called
thermal Resistivity. We want house walls to be made from materials that
have large R and small k. Right?
Ludwik Kowalski
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