The thermal resistance is an R value. U = 1/R. You need to know the R values of every element of the 'sandwich', and add in Rsi and Rse (internal and external surface resistances - about .15 between them, I think). Add all the R values, then 1/R = U. Remember, though, that in, say a stud construction, the U value you calc will be gross, and effectively needs to be netted off for thermal bridging (the fact that the studs are a poorer ins value than the ins material between).

Hi, can you check the spec to see if it is quoting a lamda value which when combined with the thickness gives a U or R value which ever they are using. Remember you can add R but not U to get the total. So add all the R then invert the answer.
If they quote the lamda it will be for 1m of thickness so the value for your thickness is the lamda x thick in mm/1000mm then invert this for the R for that thickness.
cheers
mike up north