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Part L Compliance

PSI Values &
Thermal Bridging

Bespoke Junction Calculations for SAP Assessments

Thermal Bridging Calculations

Bespoke PSI Values for
Part L Compliance

PSI (Ψ) value calculations are required for all new residential and commercial buildings under Part L of the Building Regulations. With the removal of Accredited Construction Details from Part L 2021, every project now needs bespoke PSI values calculated for each construction junction to demonstrate compliance — using default values is no longer sufficient.

Building construction junction

What is a PSI Value?

Measuring Heat Loss
at Construction Junctions

A PSI (Ψ) value measures the rate of heat loss through a linear thermal bridge — a junction between building elements where heat escapes more readily than through the main wall, floor or roof fabric.

Every building has dozens of these junctions: where walls meet floors, where roofs meet walls, around window and door reveals, at eaves, balconies and corners. Each one allows heat to bypass the insulation layer, reducing the effective thermal performance of the building envelope.

The PSI value quantifies exactly how much heat is lost at each junction per metre of length. The lower the PSI value, the better the thermal performance at that junction — and the better the overall SAP rating of the building.

Since Part L 2021: Accredited Construction Details have been removed. Every new project must now have bespoke PSI values calculated — there is no compliant default route.

Improves Your SAP Rating

Accurate PSI values feed directly into SAP calculations. Using calculated values instead of conservative defaults can significantly improve a building's SAP score, reducing the need for additional energy efficiency measures and lowering project costs.

Required for Part L Compliance

Since the removal of Accredited Construction Details from Part L 2021, bespoke PSI values are mandatory for new dwellings. Without them, default values apply — which are deliberately conservative and can prevent a building passing SAP.

Reduces Construction Costs

A poor PSI value means a worse SAP score, which often forces developers to upgrade insulation, glazing, or heating systems. Accurate thermal bridging calculations can demonstrate compliance at lower specification — saving money on every unit.

Building construction detail

What We Assess

Construction Junctions
We Calculate

We calculate PSI values for all types of construction junctions across residential and commercial buildings:

Wall to Floor

Ground floor perimeter junctions including slab edges and suspended floors

Wall to Roof

Eaves, verge, gable and parapet junctions at roof level

Window & Door Reveals

Jamb, sill and lintel junctions around all openings

Intermediate Floors

Floor zone junctions between dwelling levels and party floors

Corners & Returns

External and internal corners where wall elements meet

Balconies & Parapets

Cantilevered elements and balcony slab connections through the envelope

Architect reviewing drawings

How We Work

Our PSI Value
Calculation Process

We use 2D thermal modelling software meeting BRE 479 standards to produce accurate, certifiable PSI values for every junction on your project:

01

Review Your Drawings

We review your architectural drawings and construction specification to identify all thermal junctions requiring assessment.

02

2D Thermal Modelling

Each junction is modelled in 2D thermal simulation software, inputting accurate material U-values, geometry and boundary conditions.

03

PSI Value Extraction

The linear thermal transmittance (Ψ) is calculated from the thermal coupling coefficient L²D for each junction.

04

Report & SAP Input

We provide a full thermal bridging report with all PSI values ready for input into your SAP assessment — and can liaise directly with your SAP assessor.

New build housing development

Who Needs PSI Values?

Required for All
New Build Projects

PSI value calculations are now a mandatory part of the compliance process for any new residential or commercial building in England. You will need them if you are:

Need PSI Values for Your Project?

Get in touch with our team to discuss your thermal bridging requirements. We cover all junction types for residential and commercial new-build projects across the UK, and can work alongside your existing SAP assessor.

Get a Quotation
01332 492082
07886 277818
Send us an enquiry
11 Caithness Crescent, Derby DE22 3XS