Residential development in Brampton Huntingdon


Vistry Partnerships
Photo credit: Vistry Partnerships

Modelling bespoke Psi values leads to a 68% improvement in thermal loss calculations

Darren Evans saves developer from over investing in fabric.

The challenge

The developer was concerned they were about to over-invest in the fabric specifications for a new residential development of 210 three-, four- and five-bedroom homes.
  • Accredited Construction Details: the application of generic nominal thermal junction heat loss values was leading to an over-specification of the ground floor, walls, roof and window U-Values.
  • Inaccurate reporting: If left unchecked, this would lead to the overuse of resources and inaccurate reporting of performance against Part L.

The solution

  • Modelling Psi values: to more accurately describe the high energy-efficiency standards of these new homes. Great care was taken in the design of the junction details to prevent heat loss, but this was not reflected in the homes’ energy calculations.
  • Collaboration: together, Darren Evans and Vistry Parnerships identified a suite of four Psi values to calculate and use across all 210 homes. The design of the thermal junctions enabled us to achieve excellent Psi value results.


  • Improvement of 68%: modelling the four Psi values achieved an average of 68% improvement over Accredited Construction Details (ACD) values.
  • High-quality design and build accurately reflected, enabling the homes to easily comply with both TFEE & TER Part L thresholds without the need to over-specify in project U-Values or increase specification from fixed building services.
  • Cost-effective compliance: The solution cost £1.19 per junction per plot across the site. All other future sites built to these same junction designs can reuse these bespoke details, improving return on investment further.
“Working with Darren Evans to identify and develop Psi values for critical junctions has delivered significant benefits to SAP and our financial model. “These have been a huge benefit and one that is to be repeated on new projects. Darren Evans’ technical expertise has assisted us to maintain a fabric first approach to compliance with ADL1a.”
Simon Jones, design and construction coordinator
Vistry Partnerships

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