A Psi-Value or ψ-value is a measurement used to determine the rate of heat loss through a junction (where two thermal elements meet e.g. external walls and ground floor). The heat loss at these junctions is refered to as non-repeating thermal bridges which can form up to 25% of the total fabric heat loss of a building. There are currently 42 recognised thermal junctions in SAP 2012
When inputting data into a SAP calc one of the first things we look at is Thermal Bridging, including the use of Accredited Construction Details (ACDs) or Enhanced Construction Details, as well as psi values produced by the various insulation and building material manufacturers.
Thermal bridging is a major factor in the heat loss within a building – More in-depth detail on Thermal Bridging can be found here.
Bridging occurs when two exposed thermal elements meet; this allows heat to transfer through to the outside world. The image shows a typical ‘cold bridge’ junction.
Thermal Bridges are the non repeating junctions affecting external elements. Here are some examples:
In SAP we input the lengths of each of the relevant junctions, which are totalled by the software to give an overall lambda value. In the past we could use default lambda values and still achieve a pass. Under the more recent versions of SAP the need to calculate thermal bridges has become more necessary.
A Psi-Value or ψ-value is a measurement used to determine the rate of heat loss through a junction (where two thermal elements meet e.g. external walls and ground floor). The heat loss (measued in W/mK) at these junctions is refered to as non-repeating thermal bridges. These can provide up to 25% of the total fabric heat loss of a building. There are currently 42 recognised thermal junctions in SAP 2012.
Each thermal bridge junction has a psi value associated with it which when multiplied by the length of the bridge gives each element a heat loss value in SAP.
Generally the more complex a building is, the more thermal bridges it will have. Where thermal bridging details are not specified or calculated then a default is applied by SAP, which effectively penalises the building, usually resulting in a failure in the SAP calculation in both DER/TER (CO2 emissions) and DFEE/TFEE (Fabric Energy Efficiency).
Accredited Construction Details are a set of freely available standardised junctions which architects and builders can use to help build using decent thermal bridges. The complete set of ACD's are available to download from the Planning Portal.
The Energy Saving Trust has also produced their own, improved set of junctions. Called Enhanced Construction Details (ECDs) they can be downloaded here.
ECDs can be used in conjunction with ACDs, and are interchangeable.
Several insulation manufacturers (including Xtratherm, Celotex, Knauf & Kingspan) now offer a set of psi values for many thermal bridge junctions, based on the assumption that you use their products in the building's construction. These psi values usually offer an improvement over ACD’s and ECD's and can be a good solution to helping a SAP calculation pass.
ACD's and ECD's can help a building to pass in SAP. This can even mean insulation levels can remain the same or even the omission of other, more expensive solutions (e.g. renewables).
Under current regs most standard builds will require some thermal bridging values to be input to be able to pass.