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 max and R min. R max can be thought of as the total thermal insulance obtained if it is assumed that there is no lateral flow of heat and R min 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 max + R min) Further information about how to deal with "bridging" is given in ISO 6946.