When calculating a thermal transmittance it is helpful to consider the building's construction in terms of its different layers. For instance a cavity wall might be described as in the following table:
In this example the total insulance is 1.64 K⋅m/W. The thermal transmittance of the structure is the reciprocal of the total thermal insulance. The thermal transmittance of this structure is therefore 0.61 W/(m⋅K).
(Note that this example is simplified as it does not take into account any metal connectors, air gaps interrupting the insulation or mortar joints between the bricks and concrete blocks.)
It is possible to allow for mortar joints in calculating the thermal transmittance of a wall, as in the following table. Since the mortar joints allow heat to pass more easily than the light concrete blocks, the mortar is said to "bridge" the light concrete blocks.
The average thermal insulance of the "bridged" layer depends upon the fraction of the area taken up by the mortar in comparison with the fraction of the area taken up by the light concrete blocks. To calculate thermal transmittance when there are "bridging" mortar joints it is necessary to calculate two quantities, known as R and R.
R can be thought of as the total thermal insulance obtained if it is assumed that there is no lateral flow of heat and R can be thought of as the total thermal insulance obtained if it is assumed that there is no resistance to the lateral flow of heat.
The U-value of the above construction is approximately equal to 2 / (R + R)
Further information about how to deal with "bridging" is given in ISO 6946.