U-value

"R" value is the inverse of "U" value or...."U" value = 1 / "R".

Now.... you CAN'T ADD "R" values! Such as R-13 fibreglass with R-8 iso board doesn't = R-21.

You CAN ADD "U" values though. and then when you're done adding all "U" values, convert to "R".

I suspect "R" value came into being because the average layman and typical consumer/purchaser of insulating materials, could not fathom decimal numbers...

'u' is a Coefficient of Heat TRansfer and exprerssed in Btu/hr/sf/dtF temp difference. You do NOT add 'u' factors to get total 'u'. But you invert the 'u' factor which becomes Resistance, R, then you add the R's for the various materials you are using; THEN you invert R to obtain total 'u'.

Most often, materials are listed with their 'R' values and is an indication of the insulating quality of the materials. Then, by taking the reciprocal of R you obtain 'u' which is used in the equation

Q(Btu/hr) = (u) x (sf) x(dt)F.

Example: 4" batt fiberglass has an average R value of 11, the 'u' value would be 1/11 or 0.091.

Hope this helps.

There is one more component that must be considered-surface resistance at the air to solid interface also acts to slow the flow of heat. If the surface is the exterior of a building, then wind speed will affect the resistance to heat transfer.

You don't calculate it. You get the figure from the supplier, and use that to calculate the thickness needed to give the heat flow you want (usually a question of keeping heat flow low). If the results are not to your liking you can then try other suppliers/marerials.

Cheers.....Codey