Thermal Bridging
Psi-Value Calcs
Junction heat loss to BR 497 & BRE IP 1/06.
Calculated Psi-values for every significant junction — lintels, jambs, eaves, sills, wall-floor interfaces. Replaces cautious SAP defaults with real numbers, typically improving your energy assessment result and satisfying Building Control evidence requirements.
- 2D thermal modelling to BR 497
- Psi-value & fRsi from the same model
- Practical junction guidance if values are poor
W/m·K
Psi-Value Unit
2D Model
Thermal Analysis
BR 497
Calculation Standard
Part L
Building Regulations
What are thermal bridging calculations?
Heat escapes fastest at a building's junctions — around lintels, jambs, eaves, sills, and where walls meet floors. These thermal bridges create additional heat loss that is not captured by the U-values of the plane elements alone. Thermal bridging calculations quantify that extra loss as a Psi-value (ψ, W/m·K) for each junction, so your SAP Calculations or SBEM Calculations use real figures instead of cautious default assumptions.
When no junction calculations are provided, SAP and SBEM apply a conservative default thermal-bridging penalty (the y-value) that typically overstates heat loss at junctions. Replacing defaults with calculated Psi-values typically reduces the thermal-bridging contribution significantly, and can make the difference between a marginal or failing assessment and one that comfortably meets its target.
Why do junctions matter so much?
In a well-insulated building, the proportion of total fabric heat loss attributable to junctions increases relative to the plane elements. As wall, roof and floor U-values improve, the thermal bridging contribution becomes a larger fraction of what remains — which is why Part L places increasing emphasis on demonstrating that junction details have been properly designed and calculated.
Who needs this service?
Developers, architects and SAP or SBEM clients whose assessment is marginal or failing, anyone wanting calculated (accredited) details rather than default values, and projects using bespoke junction details not covered by the standard Accredited Construction Details (ACDs). The same 2D models that produce Psi-values also underpin our Condensation Risk Analysis (fRsi) — meaning heat loss and surface condensation risk can be assessed together from a single set of models.
Default values vs calculated Psi-values.
Two routes. One adds a blanket penalty. The other reflects your actual construction.
SAP / SBEM Default Values
When no junction data is provided, SAP and SBEM apply a conservative y-value thermal-bridging penalty across the whole building envelope. This default assumes the worst and does not reflect the actual thermal performance of the junctions specified.
- Overstates heat loss at junctions
- Can push a marginal assessment to a fail
- No junction-level evidence for Building Control
Calculated Psi-Values (BR 497)
2D thermal modelling to BR 497 produces a real Psi-value for each junction. These are fed into your SAP or SBEM model as linear thermal transmittance values, replacing the default penalty with figures that reflect what you have actually specified.
- Typically reduces y-value heat loss significantly
- Junction-level evidence for Building Control
- Identifies weak junctions before they are built
Key point: in a well-insulated building, junctions account for a larger share of remaining heat loss.
As plane-element U-values improve, the relative contribution of thermal bridging grows. Using calculated Psi-values rather than defaults is one of the most cost-effective ways to improve an energy assessment result — because the gain comes from better calculation, not more expensive materials or systems. The same 2D models also produce the fRsi temperature factor for Condensation Risk Analysis, so both outputs can be produced in a single commission.
Modelling, integration, and guidance.
2D thermal modelling to BR 497, Psi-values fed directly into your energy assessment, and practical advice on improving junctions that perform poorly — all in a single report accepted by Building Control.
Done in-house alongside your energy assessment
Thermal bridging calculations and SAP or SBEM assessments are carried out by the same team, ensuring calculated Psi-values feed directly into your energy model without the errors and delays that come from coordinating separate consultants.
BR 497 & BRE IP 1/06 Compliant
All Psi-value calculations follow the conventions set out in BR 497, with modelling carried out in specialist 2D thermal analysis software. Results are reproducible, defensible, and accepted by Building Control across England, Wales and Scotland.
Done Alongside Your Energy Assessment
We carry out thermal bridging calculations in-house alongside your SAP or SBEM assessment, ensuring calculated Psi-values feed directly into your energy model for a consistent and coordinated compliance package.
Practical Junction Guidance
Where a junction performs poorly, we advise on the most cost-effective insulation or detailing adjustment to improve the Psi-value — at design stage, before it costs money to change on site.
Design-Stage Delivery
We work from your architectural drawings before work starts, so any changes to junction details can be made while they are still straightforward to implement rather than during or after construction.
Psi-value & fRsi From One Model
We produce both the linear thermal transmittance (Psi-value) for your SAP or SBEM model and the fRsi temperature factor for condensation risk evidence from the same thermal model — keeping every calculation consistent.
Fixed Price, Nationwide
Quoted per project based on the number of junction types and construction complexity. One point of contact for your full thermal compliance package, delivered across England, Wales and Scotland.
How we calculate your Psi-values.
Four clear steps from your junction drawings through to calculated Psi-values in your energy assessment — and practical guidance if any junction needs improving.
Junction Modelling
We import your architectural junction drawings — lintels, jambs, eaves, sills, party wall interfaces, wall-floor junctions — into specialist 2D thermal analysis software. Each junction is built up layer by layer from your construction specification and material conductivities, following the conventions in BR 497 and BRE IP 1/06.
Psi-Value Calculation
The software calculates the linear thermal transmittance (Psi-value, W/m·K) for each junction by comparing the total heat flow through the modelled section with the heat flow that the plane elements would carry on their own. This is the exact methodology specified in BR 497.
SAP / SBEM Integration
Calculated Psi-values are fed directly into your SAP or SBEM model as linear thermal transmittance values, replacing the default y-value thermal-bridging allowance. Where we carry out both the modelling and the energy assessment, this integration is seamless and reduces the scope for error.
Detailed Report
You receive a full calculation report documenting every junction — model inputs, material conductivities, modelling outputs, and the resulting Psi-values — suitable for Building Control submission. Where a junction performs poorly, we include practical guidance on the most cost-effective detailing change to improve it.
Quoted per project.
Pricing is based on the number of junction types to be modelled and the complexity of your construction specification. Send us your drawings and we will confirm a fixed price within one business day.
Quoted per project based on the number of junctions, construction complexity and programme requirements. Contact us with your drawings for a fixed-price quote.
- 2D thermal modelling to BR 497 for each junction
- Psi-values (W/m·K) formatted for SAP or SBEM input
- Full calculation report for Building Control submission
- Practical guidance on junctions that could be improved
- Can be combined with Condensation Risk Analysis (fRsi)
Prices vary with project size and the number of junction types modelled. Contact us for an accurate quote — we will confirm pricing within one business day of receiving your drawings.
Get your Psi-value calculations quote
Share your junction drawings and we will confirm a fixed price within one business day. Working to a Building Control deadline? Tell us and we will prioritise your project.
Thermal bridging questions.
Everything you need to know about Psi-value calculations — what they measure, when you need them, and how they interact with your SAP or SBEM assessment.
Still Got a Question? Ask Us What is a Psi-value?
A Psi-value (ψ, W/m·K) is the linear thermal transmittance of a junction — the rate of additional heat loss along the length of that junction beyond what the surrounding plane elements would lose on their own. Lower is better. Calculated Psi-values are far more favourable than the default values assumed in SAP and SBEM when no calculation has been carried out.
Will calculated Psi-values improve my SAP result?
In most cases yes — calculated values are typically far more favourable than the default thermal-bridging allowance applied by SAP when no junction data is provided. The improvement to the y-value heat loss can be significant. We cannot promise a specific outcome or that the assessment will pass, but the benefit of calculated values over defaults is usually substantial.
What is the difference between Psi-value calculations and Condensation Risk Analysis?
Psi-values (thermal bridging calculations) measure the additional heat loss through a junction — they feed into your SAP or SBEM model as linear thermal transmittance values. Condensation Risk Analysis (fRsi / temperature factor) assesses whether the coldest internal surface of that same junction is warm enough to avoid surface condensation and mould. Both use 2D thermal modelling, and we produce both results from the same model — but they answer different questions. You may need one or both depending on your Building Control submission.
When do I need Psi-value calculations?
You typically need them when your SAP or SBEM assessment is marginal or failing and you want to improve the result, or when Building Control requires evidence that junction heat loss has been properly accounted for using calculated rather than default values. They are also required where the project uses bespoke junction details not covered by the standard Accredited Construction Details (ACDs).
Which standards govern the calculations?
The primary references are BR 497 (Conventions for Calculating Linear Thermal Transmittance and Temperature Factors, BRE 2016), which sets out the calculation methodology and modelling conventions, and BRE IP 1/06, which covers the assessment of thermal bridging at junctions and around openings. All our calculations follow these conventions and are accepted by Building Control across England, Wales and Scotland.
Do I need a Psi-value for every junction?
Not necessarily. We review your project and advise which junctions will deliver the greatest improvement to your SAP or SBEM result. For some standard details, Accredited Construction Details (ACDs) are available without the need for bespoke modelling. We focus the calculation effort where it makes the most practical difference.
What information do you need to get started?
We need the architectural junction drawings for each detail to be modelled — PDF or CAD format — along with the material specification (layer build-ups and conductivities). For standard construction types, manufacturer datasheets are sufficient. We will confirm exactly what we need at quote stage and flag any missing information before we begin.
Can Psi-value calculations be combined with Condensation Risk Analysis?
Yes — and this is usually the most efficient approach. We produce both the Psi-value and the fRsi temperature factor from the same 2D thermal model, so commissioning both together from us avoids duplication, keeps your compliance package consistent, and typically costs less than commissioning them separately.