Thermal Bridging & Psi-Values Calculations

Thermal Bridging

What is Thermal Bridging?

Thermal bridging which is also commonly known as a cold bridge is an area of construction that has significantly higher heat transfer than that of the surrounding materials. This usually occurs where breaks in the insulation are present such as timber studs in a timberframe construction or timber rafters in a warm room in roof construction. The thermal conductivity of these materials is often higher than that of the insulation and therefore heat is transferred at a higher rate. Additionally, thermal bridges occur at junctions and abutments such as windows heads, cills and jambs. Below is a list of some typical of thermal bridges:-

  • Lintels over windows
  • Sills under windows
  • Jambs to window sides
  • Ground floor junctions
  • Intermediate floor junctions
  • Eaves with insulated rafter
  • Eaves with insulated ceiling
  • Gable with insulated rafter
  • Gables with insulated ceiling
  • Flat roof junctions
  • External wall corners
  • Internal wall corners
  • Party wall junctions
  • Party floor junctions 

Heat loss at a thermal bridge is expressed as a linear thermal transmittance (ψ-value) with a standard unit of measure W/mK.

Why are thermal bridges important?

The Building Research Establishment (BRE) estimate up to 30% of heat loss in buildings occur through thermal bridges, so for a building to be energy efficient these need to be reduced as much as possible. Typically a thermal bridge will result in a lower surface temperature which increase the risk of condensation, moisture and mould growth, all of which are recognised to cause harm to the building fabric and occupants within.   

Psi-Values (Ψ)

Psi-values are the calculation given to assess the linear thermal transmittance measured as W/mK (Watts per meter Kelvin) in accordance with BRE BR 497 (Conventions for calculating thermal linear transmittance and temperature factors). These values are used within the design SAP calculation to calculate the buildings performance, alternatively default data from (SAP Table K1) is used for the common junction types, however these are generally significantly worse than standard practice.

Our assessors are able to use specialist software to calculate either 2d and 3d junctions and provide you with actual Psi-Value for proposed construction detials enabling you to benefit from enhanced values compared to the default values used with the SAP software, helping to increase your buildings preformance.


  1. Can I use Accredited Construction Details (ACD's)? Yes, whilst these offer a slight improvement over the default values within SAP, these are being phased out in SAP 10 which is et to be introduced in 2022.
  2. How long does it take to obtain a Psi-Value? Typically a building will include around 10 or more junction types. On average it will take 2-3 days to calculate these.
  3. What information to you require to calculate a Psi-Value? We will need detailed construction details which confirm the specific materials being used and how they connect, abut or interface with one another. Typically a set of architects construction drawings and construction notes will provide us with sufficient information.
  4. What if I don't yet know how i'm going to construct a detail? We can assist with recommendations to either improve a proposed construction detail or work with your architect from scratch to create a new one.
  5. How much do they cost? Typically calculate one 2d Psi-Value will cost around £250-350+VAT. We a sliding scale for greater volumes.