What is the Performance Gap?

The gap between how a new home was designed to perform and how it actually does

The Performance Gap is the difference between the energy performance a new home was designed to achieve and what it actually delivers once built and occupied. Field studies over the past decade have repeatedly found UK homes using more energy in use than their SAP calculations predicted, sometimes by 20% or more. That discrepancy is the Performance Gap, and it's the single biggest reason why Part L and the Future Homes Standard now lean so heavily on photographic evidence of what was actually built.

Why the Performance Gap exists

A SAP model is a prediction based on specifications. A finished home is the sum of what was actually installed. The gap between those two things has plenty of room to grow:

  • Substituted materials and products. The boiler specified on the design doesn't arrive in time; the contractor uses what's in the merchant's that morning. The as-built efficiency figure drops.
  • Incomplete or poorly installed insulation. Gaps behind sockets, compressed mineral wool in the loft, cavity insulation missed around window reveals. The fabric under-performs.
  • Thermal bridges designed out on paper but built back in. A thermal bridge detail that was calculated at a psi-value of 0.04 ends up behaving like 0.12 because the junction wasn't built as drawn.
  • Airtightness shortfalls. Tapes, membranes and seals done at speed rather than carefully. The pressure test result is close to the design target but not at it.
  • Commissioning issues. A heat pump installed but set up on default parameters rather than tuned to the specific dwelling. MVHR balanced incorrectly. Controls left on factory settings.
  • Occupant behaviour. Not technically a construction issue, but how people use the home changes measured performance. SAP assumes standard patterns.

None of these are individually large. Combined, they add up.

How the Performance Gap is measured

Usually retrospectively, by comparing in-use energy data (from meters or monitoring equipment) against the design and as-built SAP predictions. Post-occupancy evaluation studies from the Building Performance Network, BRE, Leeds Beckett and UCL have all reported Performance Gaps on UK new builds, with typical ranges of 10-60% excess energy use depending on the study.

For an individual dwelling, there's rarely hard in-use data available. The Performance Gap lives in the difference between predicted and actual - both of which are moving targets.

Why it matters under Part L

Part L compliance has traditionally been largely a design exercise: prove the design meets the target emission rate, build it, get signed off. The Performance Gap has forced a shift. If the as-built home doesn't perform as designed, the compliance calculation is fundamentally misleading - and the occupant ends up paying higher bills and emitting more carbon than promised.

The regulatory response is layered:

  • As-built SAP replaces design-stage SAP for the final sign-off. The OCDEA updates the model to reflect what actually got installed before the BREL report is produced.
  • Photographic evidence - Appendix B - forces site-level verification of fabric, insulation, airtightness measures and services. Without photos, there's nothing to back up the as-built calculation.
  • Mandatory airtightness testing on a proportion of plots brings measured reality into the loop.
  • The Future Homes Standard sharpens all of the above. Stricter evidence expectations, more detailed thermal-bridging calculations, tighter airtightness targets, and the beginnings of mandatory post-occupancy energy data on some projects.

Buildsnapper helps narrow the Performance Gap at its source: on site, as evidence is captured. Section-by-section Appendix B capture, GPS and timestamps automatic, straight to your OCDEA. From £6.67 per month, 14-day free trial. See how it works.

What builders can do to reduce the gap

  • Build as designed. Flag any substitutions to the OCDEA so the as-built SAP reflects reality.
  • Photograph each Appendix B section properly, before it gets covered up - not after.
  • Take airtightness seriously. Tapes, membranes, socket boxes, service penetrations. A good airtightness result is the fastest way to close the gap.
  • Commission properly. A heat pump set up correctly can shift measured performance by 15-20% compared to a default setup.
  • Train site staff on the things that matter most. Cavity closers at window reveals, continuity at wall-floor junctions, MVHR balancing.

Frequently asked questions

What is the Performance Gap in UK construction?

The Performance Gap is the difference between a dwelling's designed energy performance (as calculated by SAP) and its actual as-built performance in use. UK field studies have typically found 10-60% excess energy use compared to predictions.

Why does the Performance Gap happen?

A combination of substituted materials, imperfect installation of insulation, thermal bridges that weren't built to the designed detail, airtightness shortfalls, incorrect commissioning of heating and ventilation systems, and occupant behaviour that differs from SAP's standard assumptions.

How does Part L address the Performance Gap?

Through as-built SAP (updated to reflect what was actually installed), mandatory photographic evidence under Appendix B, airtightness testing, and - under the Future Homes Standard - stricter assessor scrutiny and tighter construction standards to leave less room for shortfalls.

Is the Performance Gap shrinking over time?

Gradually. Every update to Part L has introduced more robust evidence requirements. The Future Homes Standard represents the biggest shift so far, combining tighter construction standards with greater scrutiny of as-built evidence.

Can builders be held liable for a Performance Gap?

Not directly in most cases, because the gap is distributed across many factors and occupant behaviour is part of it. But building in breach of the design specification - or supplying false evidence - can trigger Building Control sanctions, warranty claims, and legal liability under building regulations law.

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