Improve SAP Results & Reduce Heat Loss
Thermal Bridging Calculations
Bespoke psi values and Y-values for Part L compliance. Reduce SAP defaults, improve energy ratings, and demonstrate junction performance.
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Thermal Bridging Calculations
What is thermal bridging?
Heat loss at junctions (corners, window reveals, floor edges) where insulation is interrupted.
What are psi values?
Linear thermal transmittance - measures heat loss per metre of junction length.
Why does it matter for SAP?
SAP uses default values that penalise your score. Calculated values can significantly improve results.
When do I need thermal bridging calcs?
When SAP is marginal, for Part L compliance evidence, or to demonstrate junction performance.
What is the Y-value?
Overall thermal bridging factor for the dwelling - sum of all junction losses divided by envelope area.
How much does it cost?
From £50 per junction. Package deals for multiple junctions. Often saves more on build costs than the fee.
SAP result marginal? Thermal bridging calculations often provide the improvement needed to pass.
What Is Thermal Bridging?
Thermal bridging occurs at junctions where the insulation layer is interrupted or less effective - corners, window reveals, floor edges, and other details where different building elements meet.
At these junctions, heat can bypass the main insulation and escape more easily. This "bridge" of higher conductivity material creates localised heat loss that affects both energy performance and comfort.
In a well-insulated modern building, thermal bridges can account for 20-30% of total heat loss. That's why Part L requires them to be considered in SAP calculations, and why addressing them properly can make the difference between passing and failing.
What Are Psi Values?
Psi (Ψ) values measure linear thermal transmittance - the additional heat loss at a junction per metre of its length, measured in W/mK.
Common Junctions Requiring Psi Values
- E1 - Steel lintel with perforated base plate
- E2 - Other lintels (timber, concrete)
- E3 - Sill junction
- E4 - Jamb junction
- E5 - Ground floor / wall junction
- E6 - Intermediate floor / wall junction
- E16 - Corner (external)
- E17 - Corner (internal)
- E18 - Party wall junction
Each junction type has a SAP default value. These defaults are conservative - they assume basic detailing. If your construction details are better than the defaults assume, calculating bespoke psi values demonstrates this and improves your SAP result.
What Is the Y-Value?
The Y-value is the overall thermal bridging factor for a dwelling. It's calculated by summing all junction heat losses and dividing by the total thermal envelope area:
SAP has three options for thermal bridging input:
Default Y-value
Y = 0.15 W/m²K for all dwellings. Simple but penalises good construction.
Accredited Details
Published psi values for standard constructions. Better than defaults.
Calculated Values
Bespoke psi values for your actual details. Best results.
Why Thermal Bridging Matters for SAP
Thermal bridging can make or break a marginal SAP result:
Impact on SAP Score
- ✓ Default Y-value (0.15) assumes poor detailing
- ✓ Good details can achieve Y = 0.05-0.08
- ✓ This improvement can shift SAP by 2-5 points
- ✓ Often the difference between pass and fail
Part L Evidence
- ✓ Building Control may request evidence
- ✓ Proves your junctions meet assumptions
- ✓ Required for Accredited Construction Details
- ✓ Demonstrates design intent to site team
Real Performance
- ✓ Reduces actual heat loss at junctions
- ✓ Improves thermal comfort (no cold spots)
- ✓ Reduces condensation risk
- ✓ Energy bills match predictions
When Do You Need Thermal Bridging Calculations?
Consider thermal bridging calculations when:
- Your SAP result is marginal and you need to find a few extra points
- Building Control requests evidence of junction performance
- You're using non-standard construction details
- The design has unusual junctions (steel frame, SIPs, ICF)
- You want to use calculated rather than default Y-value in SAP
- Your insulation levels are high and junctions become proportionally more significant
- You're targeting a specific energy rating (EPC A, Passivhaus)
Thermal Bridging Pricing
Priced per junction or as packages for multiple details.
Single Junction
From £50
Psi value calculation for one junction detail with report.
Junction Package
From £200
Multiple junctions calculated together. Discount for 5+ junctions.
Full Y-Value
From £350
All junctions calculated with overall Y-value for SAP input.
How Thermal Bridging Calculations Work
We model your junction details to calculate accurate psi values.
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1
Send us your junction details
Construction drawings showing the detail at each junction - wall build-up, insulation position, any thermal breaks, and material specifications.
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2
We model the junction
Using approved thermal modelling software, we create a 2D model of the junction and calculate the psi value according to BR 497 conventions.
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3
Receive your results
You get a psi value report for each junction, ready to input to SAP or submit to Building Control as evidence.
What We Need for Thermal Bridging Calculations
- Section drawings through each junction showing all layers
- Material specifications (insulation type, conductivity, thickness)
- Lintel specifications if steel lintels are used
- Window/door frame details (frame depth, thermal break position)
- Any cavity closers or thermal break products
- Information on mortar joints and fixings if relevant
Don't have detailed drawings? We can work with sketch details and make reasonable assumptions where needed.
Thermal Bridging FAQ
Can I use Accredited Construction Details instead?
How much can thermal bridging improve my SAP?
Do I need thermal bridging calcs for extensions?
What software do you use?
Can you help optimise junction details?
Need thermal bridging calculations?
All thermal bridging calculations completed by qualified assessors using approved software.