The Y-value is a figure used in SAP to indicate the amount of heatloss attributable to linear and point thermal bridging for a given dwelling. The Y-value is expressed in units of W/m2.K - the same units as a U-value. It is calculated as follows: first, for each junction the psi-value is multiplied by its length, the resulting figures for all junctions in the building are summed, to give the Htb. The Htb is the thermal bridging heatloss factor in W/K. This is converted to a Y-value by dividing by the surface area of the building. The default value is 0.15 W/m2.K and good practice design can typically reduce this by about half.
For a typical house, crunching the numbers indicates that the heatloss due to default levels of thermal bridging (i.e. a Y-value of 0.15) adds between 25% and 40% to the heatloss - a massive amount! with good practice design, experience indicates that it is usually possible to halve the Y-value; therefore some attention to detail (pun intended), could the FEEs by between 13% and 20%. Under the 2013 version of SAP the FEEs must achieve a mandatory level and this potential saving has a direct impact on that figure. the BER will also be reduced, but at a lower rate because there are additional factors in play such as lighting, hot water demand and renewables. Our research indicates that halving the Y-value would yield a typical saving of 10% on BER.
With such significant savings possible, no-one can afford not to consider thermal bridging very carefully.