The r-value is a thermal resistance,

The k-value is thermal conductivity = 1/r, usually W/mK

Ah for jasus sake!!

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

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 ....

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!

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)

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

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?