Architectural Masonry

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Y Values and Thermal Bridging

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.

The Besblock thermal modelling system, CSH 3 and L1A 2010

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 y-values 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 psi-values (Ψ) have been calculated in accordance with BRE I/P 106 and BR 497 as required by SAP 2009.

  • Often, designers will increase fabric insulation in an attempt to attain the 25% carbon emission reduction.
  • With U-values often as low as 0.14, 0.18 and 0.27 for the various elements of the fabric, by the law of diminishing returns this is usually not the most cost-effective option.
  • A more effective method is to first examine and minimise the heat loss at thermal bridges, openings and junctions by adopting junction design details with low Ψ values
  • The Besblock Thermal Modelling system does exactly this, delivering Ψ values typically 50% better than the standard accredited values.

What is a y-value?

  • For a visual illustration of this click here
  • The y-value in the SAP is a factor to estimate the heat loss at junctions in new dwellings. Besblock y-values are always derived from the total thermal bridge losses calculated using the Ψ multiplied by the total length of each junction in the house type. The lower the y-value, the less heat is lost. Currently under SAP 2005
  • Default value is y = 0.15. For Accredited Construction detail, y = 0.08. With the Besblock thermal modelling system, y-values of 0.035 and below are often achieved.
  • In SAP 2009 where details of the thermal bridges are unknown a default y-values 0.15 should be used.
  • All other thermal bridge losses should be calculated using Ψ values. Where junctions conform to accredited construction details assessors should use the Ψ values shown in table K1 of SAP 2009.
  • Alternatively Ψ calculated in accordance with BRE IP 1/06 and BR 497 may also be used. This is the Besblock TM system and these Ψ are typically 50% better than those in table K1 of the SAP.
  • It must be emphasised that our junction details must be used on site if the Besblock TM values are used in the SAP calculation.
  • Junction details and values based on our Accredited Details can be downloaded here

What does this mean?

  • The SAP calculates the total area of all exposed elements of the building fabric and multiplies each element by its U-value to give the heat loss for that element. The sum total of the elements gives the total heat loss for the fabric.
  • Whichever y-value is adopted, the total fabric area will be multiplied by that figure and the result is added into the total heat loss calculation.

Examining the example below:

  • Default: y = 0.15: Fabric is multiplied by 0.15. The resultant figure is added into the heat loss which becomes 182.13 W/K
  • Y = 0.08 (accredited construction detail) Fabric is multiplied by 0.08. The resultant figure is added into the heat loss which becomes 157.13 W/K
  • Besblock TM system y = 0.035: Fabric area is multiplied by 0.035. The resultant figure is added into the heat loss which becomes 141.06 W/K


A detached house has a total exposed element area of 357.20m2, made up as follows:

Element Area m2 U-value 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):

Estimated heat loss at junctions for this dwelling

  Y-value Estimated heat
loss at
junctions (W/K)
Revised heat loss
for dwelling (W/K)
SAP default 0.15 53.6 182.13

Construction Detail

0.08 28.6 157.13
Besblock TM system 0.035 12.5 141.06

Building Fabric CSH ENE 2 non mandatory

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.

Effective use of Numerical Thermal Modelling

There are two steps to the effective use of Numerical Thermal Modelling:

  • Accurately model the system junctions to produce lower Ψ-values than the published generic values.
  • Use the results of the numerical modelling process to develop more thermally efficient junction details.

Numerical Thermal Modelling Calculations - A Summary

Heat escapes from buildings by:

  • migrating through the main building heat loss elements (walls, ground floors, roofs, windows and doors);
  • warm air exiting through both the designed ventilation and through un-planned holes and gaps in the building envelope, and;
  • migrating through highly conductive junctions between the main heat loss elements.

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 y-value, which basically increase the total heat loss area by a fixed percentage and then designates that area as the thermal bridge having a U-value of 1.0 W/m2.K SAP 2005 has two set y-values:

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 Ψ-values, 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. Ψ-values offer a more considered and accurate method to evaluate these losses. Using system specific Ψ-values, 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:

  • EN ISO 10211-2007 (British Standards)
  • IP 1/06 & BR497 (BRE Press)

and with reference to the following publications:

  • EN ISO 6946 (British Standards)
  • BR443 (BRE Press)

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Besblock Limited - Heslop, Halesfield 21, Telford, Shropshire, TF7 4NF
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