Psi values, Y values and Thermal Bridging

Psi Value

Psi values (W/mK) are used to calculate the Y value (W/m2K) for use in the SAP 2009 calculator for the effect of none repeating thermal bridges. Accredited Construction Details and associated Psi values for a new build design are available from the Planning Portal web site.

These Psi values are based on standard construction details but the default Psi values can be enhanced through the use of thermal modelling by a third party accredited assessor. Pinewood Structures has modelled its popular QuantumTM details which are available on request and in all cases Pinewood's own modelled Psi values are much improved upon the default values. Please contact us for more information.

Improved Psi values can help eliminate the need for costly eco-bolt ons such as PV panels and are well worth exploring as a way to reduce the cost of a new build whilst still meeting the onerous energy requirements of Code 3 and Code 4 homes.

You can be assured that significant cost savings can be achieved by reducing the amount of renewables on Code 3 and 4 schemes, by simply moving away from default values in SAP and using Pinewood's excellent thermal bridging Psi values. We can also help you calculate Y-values for a given design should this be of interest.

Additionally, by using Pinewood's Psi values, the repeating thermal bridging factor can also be reduced. This allows for an

"Enhanced U-value".

Nominally the U-value is uplifted by 10% by simply using our Psi values and detailing. For more information, please download the document at the foot of this page.

An explanation:


With the introduction of the new Approved Documents L1A as part of SAP 2009 at the end of 2010 and the 25% improvement on CO2 generation, the challenge is how to achieve the target SAP value whilst minimising build costs. The Zero Carbon Hub has focused on the need to construct a high performance building fabric before renewables are brought into the equation. We wholly support this approach and believe timber frame offers outstanding performance in this regard.

With this in mind, at Pinewood we are often asked to specify a target U-value as part of our commercial offer. And whilst the U-value is very important, in SAP the role of the Y-value is equally important although we are rarely ever asked this despite the fact it can make a significant difference to the SAP result. Optimising the two can ensure target SAP value are achieved and costs considerably reduced.

What is a Y-value?

Y-value is a proxy for the heat loss through the non-repeating thermal bridging areas of a building.  The default value is 0.15. Using Accredited Construction Details in conjunction with the Psi values in Appendix K of SAP 2009 document Part L1A Section 5.12, this can be reduced to 0.08. At Pinewood, by thermally modelling our junction details, for most house types, we can get this figure down to less than 0.04.

What does this mean in practice?

Instead of driving down the U-value, say to 0.15 W/m2K or below, using a Y-value of 0.04 can result in a less stringent U-value requirement of above 0.17 W/m2K or possibly higher. This means building with a lower cost fabric without compromising on the SAP value. Or alternatively, staying with the same U-value of 0.15 W/m2K  and significantly reducing the square meter requirement of PVs, or similarly reducing the need for expensive windows or indeed both.

Take a few examples:

Moving from a Y-value of 0.08 to 0.04:

  1. In a detached house, this can result in a reduction in the window U-value requirement by 0.42W/m2K. This is the difference between using double glazing instead of triple glazing. Alternatively, the building fabric U-value requirement could be reduced by 0.09W/m2K. Either that or eliminate 3m2 of PVs.
  2. For a mid-floor flat, because of the reduced external wall area, the benefits are less pronounced but still significant. Window U-values can be reduced by 0.28W/m2K, or the wall fabric requirement by 0.04W/m2K or between 0.4 and 0.8m2 of PVs dependent upon heating system used.

More details:

Thermal Modelling

Pinewood Structures has modelled the thermal bridging on its standard 0.17 U-value QuantumTM wall system. Each junction has been modelled in accordance with BRE I/P 106 and BR 497 as required by SAP and an associated Psi value calculated.

More on Psi Values?

Psi (Ψ) values are a measure of the linear thermal transmittance of a thermal bridge and have units of W/mK where m is the linear metres of the thermal bridge. To calculate the heat loss through a particular thermal bridge, then the length of the thermal bridge in metres must be multiplied by its respective Psi value.

More on Y-values

In order to calculate the Y-Value of a building, we must first measure the length of each thermal bridge and multiply it by the respective Psi value. We then aggregate these results. This aggregate figure is known as the transmission heat transfer coefficient or HTB. To calculate a Y-value the HTB, for which the unit is W/K, is divided by the total external heat loss area of the building, not including the party walls or in the case of a cold pitch roof, the warm ceiling area. This will then give a Y-value, e.g. 0.04 and in the same units as the U-value i.e. W/m2K.

So how is the Y-Value used?

In simple terms, if the heat loss of a building is calculated using the U-value for each external element with the resulting calculation measured in W/K, and say for a typical house this can be 100 W/K. We must then add to this figure the heat loss through the non-repeating thermal bridges using the Y-value. If the area of the building is say 300m2, then this figure is multiplied by the Y-value. So if the Y-value is 0.15, the result is 45W/K. This figure is then added to the 100W/K to get a total heat loss of 145W/K. This has then increased the heat loss of the building by 45% in this example.

If Accredited Details were used, with a Y-value of 0.08, then the increase in heat loss would be 24% and if Pinewood’s details were used with say a Y-value calculated at 0.04, then the heat loss increase could be reduced to 12% giving a total heat loss of only 112 W/K.

Further Reading

In February 2016 the Zero Carbon Hub has produced an excellent guide explaing how thermal bridging, Psi values and Y-values work in practice along with a practical guide on tips and techniques for design and site works. Please visit the following external link "Zero Carbon Hub Thermal Bridging Guide"  to download the document.

Related Documents
Psi and Y Values, an explanation (195 KB)
Psi and Y-values explained
Download Psi and Y Values, an explanation
Getting the Fabric Right (225 KB)
See how Pinewood's Quantum system can drive down Y-values
Download Getting the Fabric Right
Enhanced Psi Values (218 KB)
Enhanced Psi values - repeating thermal bridging factor
Download Enhanced Psi Values