So how are U-values calculated?

Most materials have known and published values for the rate at which heat is conducted through them under specified conditions. These values are known as k-values. They are a measure of thermal conductivity. In any element, such as a wall, the k-values for each material layer are divided into the thickness of that material to obtain the R-value for that layer or the thermal resistance. When they are added all together, the total resistance for the complete section of the wall is nearly found (the air resistances for the inner and outer face have also to be added on). The total resistance is then divided into one to obtain the reciprocal of resistance and this figure is the U-value for the wall.