Project details
Service: PSI value calculations
Client: UK residential developer
Project: 24 new build homes (mix of semi-detached and terraced dwellings)
Location: Hawthorn View Development, Sevenoaks, Kent TN13 (fictional)
Regulation driver: Building Regulations Part L (SAP compliance and energy performance assessment)
ATSPACE team: Building energy compliance consultants, SAP assessors, thermal bridging specialists
The headline result
ATSPACE delivered PSI value calculations for a 24-unit residential development, supporting SAP assessments that achieved compliance first time across all dwellings.
The project required accurate thermal bridging inputs to ensure SAP calculations reflected real-world construction details and met Part L requirements. Through detailed PSI value analysis and coordinated energy modelling support, ATSPACE enabled the design team to secure Building Control approval without revisions or delays.
ATSPACE delivered:
Full PSI value calculation set for all key junctions
Thermal bridging inputs for SAP energy modelling
Part L compliance support across all dwelling types
Review of junction details and construction assumptions
Optimised thermal performance inputs for SAP submission
Building Control-ready compliance documentation
Project overview
This residential development consisted of 24 new build homes, including a combination of two-storey semi-detached houses and rows of terraced dwellings. The scheme was designed to meet current energy efficiency standards under Building Regulations Part L and achieve SAP compliance prior to construction.
SAP (Standard Assessment Procedure) is the UK government methodology used to assess the energy performance of new homes. It incorporates fabric performance, heating systems, ventilation, and importantly, thermal bridging through PSI values.
The developer required PSI value calculations to ensure that thermal bridging was accurately represented in SAP assessments and that all homes would pass compliance first time.
ATSPACE was appointed during the technical design stage to provide PSI value calculations and support the SAP submission process. The key objective was to avoid any risk of SAP failure, which could delay Building Control approval and impact the construction programme.
Why the service matters
PSI value calculations are a critical component of SAP assessments under Part L of the Building Regulations. They measure heat loss at junctions where building elements meet, such as walls, floors, roofs, windows and doors.
Even when insulation levels are compliant, poor thermal bridging assumptions can negatively impact SAP results and lead to failure in energy performance targets.
PSI value calculations ensure that:
Thermal bridging is accurately represented in SAP models
Heat loss at junctions is correctly quantified
Energy efficiency targets are achieved
Building Regulations compliance is demonstrated
This service is typically required for:
New build residential developments
SAP submissions for Building Control approval
Energy performance compliance reporting
Developments using modern insulation standards
If PSI values are not correctly calculated or are based on default assumptions, SAP calculations may fail, resulting in redesign, delays, or additional cost to improve fabric performance.
The problem
When ATSPACE was appointed, SAP modelling had already begun using preliminary design assumptions. However, several issues were identified relating to thermal bridging inputs.
Key concerns included:
Use of generic PSI values rather than project-specific calculations
Unverified junction details in architectural drawings
Inconsistent assumptions between SAP assessor and design team
Potential overestimation of heat loss at key junctions
Risk of SAP failure due to conservative default values
The development was also operating under a tight delivery programme, with Building Control submission scheduled shortly.
Without accurate PSI value calculations, there was a significant risk that SAP results would not achieve compliance, potentially requiring design changes or delays to approval. The developer needed certainty that the energy model reflected accurate and achievable performance data.
What ATSPACE was asked to do
Carry out project-specific PSI value calculations
Provide accurate thermal bridging inputs for SAP modelling
Review junction details across all dwelling types
Confirm Part L compliance requirements
Identify and reduce risk of SAP failure
Support Building Control submission process
Deliver clear, audit-ready compliance documentation
Our approach
Step 1: Understanding the compliance requirement
ATSPACE reviewed the SAP modelling approach and confirmed where PSI value calculations were required to replace default thermal bridging assumptions. We defined a structured calculation methodology aligned with Part L requirements.
Step 2: Reviewing junction details and construction assumptions
We analysed architectural drawings and construction details, focusing on key junctions including wall-to-floor connections, roof-to-wall junctions, window and door reveals, and party wall interfaces. This allowed us to identify where thermal bridging performance could be improved or refined within the SAP model.
Step 3: Optimising PSI values for SAP performance
Before finalising calculations, ATSPACE provided practical recommendations to improve SAP outcomes, including improving insulation continuity at key junctions, refining window reveal detailing assumptions, reducing thermal bridging at floor slab edges, and aligning construction details across dwelling types. These improvements were incorporated into PSI calculations to support better SAP performance without design changes.
Step 4: Clear SAP-ready reporting
ATSPACE completed PSI value calculations and delivered structured outputs suitable for direct SAP input. The documentation clearly demonstrated compliance with Part L and supported first time SAP pass across all dwellings.
Real issues we faced and how we solved them
Issue A: reliance on default PSI values in SAP model
The initial SAP assessment used generic thermal bridging values.
How ATSPACE helped: We replaced default assumptions with project-specific PSI calculations, improving accuracy and compliance confidence.
Issue B: inconsistent junction detailing
Different house types had slightly varied construction details.
How ATSPACE helped: We standardised PSI calculations across dwelling types to ensure consistent SAP inputs.
Issue C: risk of SAP compliance failure
Initial modelling results were close to regulatory thresholds.
How ATSPACE helped: We refined thermal bridging inputs to improve overall SAP performance without altering the design.
Issue D: tight submission deadline
The project required fast turnaround for Building Control submission.
How ATSPACE helped: We prioritised PSI calculation delivery to align with SAP reporting deadlines and prevent delays.
Results
PSI value calculations completed for all 24 homes
SAP compliance achieved first time
No redesign required
Building Regulations approval supported successfully
Thermal bridging inputs fully validated
No delay to construction or submission programme
Consistent compliance across all dwelling types
What this project proves
This project demonstrates that PSI value calculations play a critical role in achieving successful SAP compliance for residential developments.
When completed accurately and early, they ensure that SAP assessments reflect real construction performance, thermal bridging does not negatively impact compliance, Building Regulations approval is achieved first time, and project delays and redesign risks are avoided.
PSI value calculations are not just a technical requirement, but a key factor in securing reliable SAP performance outcomes in modern housing developments.
Mini FAQ
What are PSI value calculations in SAP?
They are thermal bridging calculations used to determine heat loss at junctions in SAP energy assessments.
Why are PSI values important for SAP compliance?
Because they directly affect energy performance results and can determine whether a dwelling passes or fails Part L requirements.
Do all new homes need PSI value calculations?
Yes, most new build homes require PSI values for accurate SAP compliance.
What happens if PSI values are not used?
SAP calculations may rely on default values, which can reduce accuracy and risk non-compliance.
Can PSI values improve SAP results?
Yes, accurate PSI calculations often improve energy performance outcomes compared to generic assumptions.
When should PSI calculations be completed?
They should be completed during the design stage before SAP submission to Building Control.
Call to action
Need PSI value calculations for SAP compliance on your residential development?
ATSPACE helps developers, architects and SAP assessors achieve fast, reliable Part L compliance with accurate thermal bridging calculations and no unnecessary delays. Send your project details and we will confirm requirements and next steps.
