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13-07-2007, 14:18   #1
10-10-20
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Convert W/mK to R-values

I'm trying to work out how to convert thermal conductivity values to relative values.
I've tried Googling this to no avail.

If a product has a thermal conductivity of 0.037 W/mK, how do I convert this to an R-value?
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13-07-2007, 15:10   #2
Borzoi
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Quote:
Originally Posted by 10-10-20
I'm trying to work out how to convert thermal conductivity values to relative values.
I've tried Googling this to no avail.

If a product has a thermal conductivity of 0.037 W/mK, how do I convert this to an R-value?
The r-value is a thermal resistance,
The k-value is thermal conductivity = 1/r, usually W/mK
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13-07-2007, 15:14   #3
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Ah for jasus sake!!

So, what if one company gives R ratings, and another just gives W/mK, how do I compare products?
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13-07-2007, 16:16   #4
hobie
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10-10-20 .... If you compare two products that look very similar in spec with an identical thickness, can you see one with an an R value of approx 30 (27 to be exact) compared to another with a K value of 0.037 W/mK .....

If you can I have a formula ..... if not then I'm lost ....

Last edited by hobie; 13-07-2007 at 16:25.
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13-07-2007, 16:47   #5
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No, they give one or another!

Examples:

http://www.sheepwoolinsulation.ie/techprods.asp
FIW Measured Thermal Conductivity (lambda): 0.0354 W/mK
Accepted Thermal Conductivity – incorporating 5% tolerance (lambda): 0.0370 W/mK
CE Harmonised Conductivity: 0.0390 W/mK

http://www.leroymerlin.fr/mpng2-fron...ille-render=on
résistance thermique : 1.75 R.

I'm stumped!
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13-07-2007, 17:20   #6
Alun
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W/mK is a measure of thermal conductivity, i.e. conductance per meter measured in Watts /meter * degree Kelvin. Resistivity, the resistance per meter is the inverse of this.

R is a measure of thermal resistance for the thickness of material concerned (for the example you gave 70mm, or 0.07m). Resistance = resistivity * thickness.

So 0.037 W/mK = (1 / 0.037) * 0.07 = 1.89 which is close to the R value quoted for the second example you gave.

So to convert from W/mK to R, take the inverse and multiply by the thickness in metres of the material you're comparing it to.


http://en.wikipedia.org/wiki/R-value_(insulation)
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13-07-2007, 21:40   #7
Reyman
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U value is strictly speaking measured in Watts per metre squared per degree Kelvin gradient. R value is the reverse of this. Alun has worked out an example for you
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