These are a measure of the total heat loss expected from all of the thermal bridge losses in all of the junctions in the house.
New Approved Document L1A 2010 is expected to become effective in October 2010. This will require that all new dwellings have a 25% carbon emission reduction compared with AD L1A 2006. The Besblock Thermal Modelling system with its associated lower yvalues enables this 25% improvement to be achieved often without renewable energy sources.
The Besblock thermal modelling exercise was undertaken in conjunction with its Star Performer block to minimise the effect of Thermal Bridges (heat loss) that occur at junctions between building elements. The resultant psivalues (Ψ) have been calculated in accordance with BRE I/P 106 and BR 497 as required by SAP 2009.
A detached house has a total exposed element area of 357.20m^{2}, made up as follows:
Element  Area m^{2}  Uvalue  Total heat loss (W/K) 

Doors  1.95  2.45  4.78 
Windows  29.04  1.68  48.76 
Ground floor  71.09  0.21  14.93 
Walls  184.04  0.28  51.53 
Roof  71.08  0.12  8.53 
Total exposed Elements:  357.20  Total fabric heat loss, (no correction): 
128.55 
Yvalue^{}  Estimated heat loss at junctions (W/K) 
Revised heat loss for dwelling (W/K) 


SAP default  0.15  53.6  182.13 
Accredited

0.08  28.6  157.13 
Besblock TM system  0.035  12.5  141.06 
In most cases the adoption of the Besblock system of construction details will result in the Heat Loss Parameter (HLP) being < 1.10 thereby gaining 2 credits.
There are two steps to the effective use of Numerical Thermal Modelling:
Heat escapes from buildings by:
Traditionally at these junctions structural considerations and buildability have taken priority, resulting in concentrations of dense construction materials which often completely interrupts the insulation system.
It is this third item that Numerical Thermal Modelling addresses.
The most common method of evaluating thermal bridge losses in the SAP calculation is to use a prescribed coefficient called the yvalue, which basically increase the total heat loss area by a fixed percentage and then designates that area as the thermal bridge having a Uvalue of 1.0 W/m2.K SAP 2005 has two set yvalues:
y = 0.15 is SAP default for all forms of construction in the absence of any information about the junction details, and;
y = 0.08 can be used where details can be shown to be consistent with published junction details known as accredited construction details (ACDs) for masonry, timber frame and steel frame buildings.
Alternatively, and less commonly used, the SAP calculation can consider each thermal bridge individually, where a detail specific coefficient is multiplied by the length of the junction to produce the linear thermal loss. These coefficients are known as Ψvalues, and they are specific to construction details and construction method.
In 2010, SAP 2009 will no longer permit the use of y = 0.08 for accredited details. Instead a set of Ψ
values published in Appendix K of the SAP Specification will have to be applied to all junctions, and
the results summed to calculate the total thermal bridge heat loss for the building.
y  values by their universal nature are crude and generally conservative values to use to estimate thermal bridge losses. Ψvalues offer a more considered and accurate method to evaluate these losses. Using system specific Ψvalues, that have been calculated using finite element heat flow methodology are going to be more accurate still, and they will reward good design practice.
Numerical Thermal Modelling Calculations are performed in accordance with:
and with reference to the following publications: