A built assembly might have drywall, a polyethylene vapour barrier, wood or metal structure, insulation and plywood covered with siding. Figuring out the effective R-value requires us to use some math to get a more accurate reflection of performance.

To calculate the effective R-Value, we need to know the Lambda values (conductivity in W/mK) of the building materials and their thickness. We can find those values in charts like this one:

Note: the chart has completed one of the steps for us by dividing the thickness in metres by the conductivity in W/mK.

Figuring out this part requires us figuring the U-Value of the assembly, which is the inverse of R-Value. The equation below shows that by dividing 1 by the added R-Values in an assembly equals the U-Value.

The equation is visualized in the graphic below. Notice how there are layers inside and outside called Rsi and Rso, respectively. These represent the R-Value of the thin film of air on the inside and outside surfaces of the assembly.

To get the values of the R1, R2, etc...we'll need to use the material thickness divided by the lambda value. In the equation shown below, d1 and d2 are the thicknesses of layer one and two, respectively. The same can be said for the lambda 1 and 2. This seems complicated but once you've filled in the values for material values, it's straight addition and division.

The RT is the Total R-Value of the assembly or the effective R-Value.