Convert W/mK to R-values

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?

Borzoi

10-10-20 wrote:

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

10-10-20

Ah for jasus sake!!

So, what if one company gives R ratings, and another just gives W/mK, how do I compare products?

hobie

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 ....

10-10-20

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-front/pre?zone=zonecatalogue&idLSPub=1049897268&1121266850.idnode=1121263863161080&1121266850.pos=4&01-comment-choisir-render=on&02-multicritere-render=off&03-zoom-produit-render=off&03b-mise-en-avant-produits-render=off&04-selectionne-pour-vous-render=off&05b-voir-aussi-famille1-render=on&05c-voir-aussi-famille2-render=on&05d-voir-aussi-famille3-render=on&05e-voir-aussi-univers-render=on&05f-voir-aussi-ss-univers-render=on&06-liste-conseils-famille-render=on&07-livres-famille-render=on&08-liste-idees-famille-render=on
résistance thermique : 1.75 R.

I'm stumped!

Alun

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)

Reyman

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