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Thermal Bridging Calculations (Psi-Values for SAP Compliance)
Guidance on reducing heat loss at junctions and improving Y-values. Full thermal bridging modelling launching soon.
Ideal for new builds, developments and projects needing psi-values for lintels, sills, jambs, eaves and other junctions.
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What Is Thermal Bridging?
Thermal bridging occurs where the building fabric is less insulated, such as around windows, doors, wall junctions and structural connections. These junctions can significantly increase heat loss if not modelled or detailed correctly.
In SAP, thermal bridging is represented by a Y-value, which is calculated from individual psi-values for each junction. Lower psi-values reduce the Y-value, improve the DER/TER balance and make it easier for new build homes to pass Part L targets.
While our full modelling service is not yet live, we provide clear guidance on the role of thermal bridging in SAP compliance and how improved junction performance can support better SAP outcomes.
Why Thermal Bridging Matters for SAP
For many new builds, thermal bridging is the deciding factor in whether the design meets Part L. Improving psi-values can make a significant difference even when insulation and glazing are already optimised.
- Lowering the Y-value reduces overall heat loss.
- Better junction performance improves DER and Primary Energy results.
- Enhanced psi-values often prevent unnecessary increases in insulation thickness.
- Custom junction modelling can unlock more design flexibility.
- Accurate psi-values support both design-stage and as-built SAP compliance.
If you are unsure whether your project would benefit from improved psi-values, send us your drawings and we will provide guidance before the full service launches.
What Will Be Included in Our Thermal Bridging Service
When live, our thermal bridging calculation service will provide accurate psi-value modelling and Y-value optimisation for new builds and developments.
- Individual psi-value calculations for key junctions.
- Thermal modelling using approved thermal bridging software.
- Y-value optimisation for SAP 10 compliance.
- Assessment of lintels, sills, jambs, corners, eaves, ridge, party walls and foundation junctions.
- Clear recommendations for construction detailing and insulation improvements.
- Reports suitable for submission to SAP assessors and Building Control.
These services will be ideal for developers, architects and self-builders seeking improved SAP results or a robust specification before construction starts.
Typical Junctions That Can Be Modelled
Psi-value calculations may be required for a range of building junctions. Common examples include:
- External wall to ground floor
- External wall to roof junctions
- Window and door reveals (jambs, heads and sills)
- Party wall junctions
- Ridge, eaves and verge details
- Corner junctions (internal and external)
- Steel beam penetrations
Not all junctions need bespoke modelling. Some can use accredited construction details (ACDs) or manufacturer-approved values. We can advise on the most suitable route for your project.
What We Will Need for Thermal Bridging Modelling
Once the service launches, we will typically require detailed construction information for each junction being modelled, including:
- Architectural drawings for the affected junctions.
- Construction build-ups with insulation thicknesses and materials.
- Window and door details where applicable.
- Manufacturer product data sheets.
- 3D or 2D junction details if available from the designer.
If you are still at an early design stage, you can join the waiting list and receive guidance on how to prepare suitable junction details ahead of full calculations.
Thermal Bridging Frequently Asked Questions
What is a psi-value?
A psi-value measures heat loss at a junction between building elements. Lower psi-values improve the overall thermal performance of the dwelling and reduce the Y-value used in SAP calculations.
What is the difference between psi-values and Y-values?
Psi-values represent individual junction performance. The Y-value is the combined thermal bridging factor used in SAP. Improving psi-values reduces the Y-value and helps achieve SAP compliance.
Do all new builds need bespoke thermal bridging calculations?
No. Many projects use default or accredited construction details. Bespoke psi-values are beneficial for developments seeking improved SAP results or more design flexibility.
When will your thermal bridging service be available?
We expect to offer full psi-value modelling soon. Join the waiting list to receive updates and early access.
Be First in Line for Thermal Bridging Calculations
Join the waiting list and be notified the moment we launch full psi-value modelling and thermal bridging calculations.
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