Understanding PSI Values: Enhancing Building Performance and Energy Efficiency

Under Approved Document L Volume 1 (2021 edition incorporating 2023 amendments), paying attention to the thermal performance of the building envelope is a statutory requirement for Building Regulations compliance. One key factor in evaluating the energy efficiency of a building is the psi value, which determines linear heat transmission and directly affects the Dwelling Fabric Energy Efficiency (DFEE) rate and Dwelling Primary Energy Rate (DPER).

What are PSI Values?

PSI values, also known as Ψ-values or linear thermal transmittance, represent the thermal bridging effect within a building's construction, expressed in Watts per metre-Kelvin (W/mK). Thermal bridges occur when there is a discontinuity in the insulation layer, resulting in heat loss or gain through the building envelope. Incorporating a calculated psi value into SAP 10.2 energy models replaces standard government default penalties of 0.15 W/(m²K), reflecting true junction performance.

Significance of PSI Values:

1. Energy Efficiency: By understanding and improving PSI values, builders and designers can reduce thermal bridging and minimise heat transfer between different elements of the building envelope. This, in turn, helps in enhancing the energy efficiency of the building and reducing heating and cooling demands. In SAP 10.2, linear thermal bridging accounts for up to 30% of total dwelling fabric heat loss. Modelling a specific psi value for standard junctions—such as E1 lintels, E2 sills, E3 jambs, and E4 ground floors—prevents Building Control defaulting to severe backstop heat loss penalties.
2. Comfort and Indoor Climate: Thermal bridges can lead to localised temperature variations, creating uncomfortable conditions for building occupants. By addressing thermal bridging through optimised PSI values, a more uniform indoor climate can be achieved, ensuring occupant comfort by eliminating cold draughts near external wall junctions, party walls, and window reveals.
3. Condensation Control: Thermal bridges can contribute to condensation formation, potentially leading to moisture-related issues such as mould growth and material deterioration. Managing PSI values helps prevent condensation by minimising temperature differentials and maintaining appropriate surface temperatures within the building envelope. Evaluating the psi value determines the internal surface temperature factor (fRsi), which must remain at or above 0.75 in residential buildings to prevent surface mould in line with BRE IP 1/06.
4. Building Durability: Thermal bridges can cause uneven stress distribution on building materials, potentially leading to structural damage over time. By reducing heat transfer across thermal bridges, the overall durability and longevity of the building can be improved by preventing interstitial moisture accumulation that causes plasterboard damp, structural timber rot, and freeze-thaw masonry degradation.

Improving PSI Values:

1. Design Considerations: During the design phase, architects and engineers can minimise thermal bridges by incorporating continuous insulation, optimising the layout of building elements, and utilising materials with lower thermal conductivity such as insulated cavity closers at window reveals and fully filled, sealed party wall cavities to achieve a zero heat-loss rate (0.00 W/m²K) in line with Approved Document L Table 1.1 notional dwelling targets.
2. Construction Techniques: Proper construction practices, such as carefully sealing joints, using thermal breaks, and ensuring effective insulation installation, serve an essential function in reducing thermal bridging. Under Appendix B of Approved Document L, contractors must capture geo-located, timestamped photographs before concealment to verify that insulation continuity and airtightness seals match design details.
3. Advanced Materials and Technologies: The market offers various innovative solutions for reducing thermal bridging, such as high-performance insulation materials, insulated cladding systems, and thermally broken components. Utilising these technologies can significantly enhance PSI values, particularly through aerogel insulation strips, phenolic cavity boards, and structural thermal breaks in parapets and balconies.
4. Simulation and Analysis: Advanced building energy modelling software can help simulate and analyse the impact of thermal bridges on a building's performance. These tools enable designers to optimise PSI values by assessing different design alternatives and identifying areas for improvement. ATSPACE energy assessors use numerical 2D and 3D finite element analysis to model non-standard architectural junctions, with design-stage SAP calculations and thermal bridge reports completed within 5 to 10 working days.

Conclusion:

PSI values are essential for assessing and improving the thermal performance of a building. By addressing thermal bridging through optimised PSI values, energy efficiency can be enhanced, occupant comfort can be improved, and the risk of condensation and durability issues can be mitigated. Establishing a verified psi value for critical structural junctions enables designers and housebuilders to meet Approved Document L Part L1A requirements without relying on costly over-specification of mechanical building services or risking Building Control rejection of the as-built BREL report.

About the Author: Thomas Thornburrow is an Energy Assessor at ATSPACE Ltd specialising in SAP calculations, thermal bridging analysis, and Part L compliance for residential developments across England and Wales.