When a new building earns an ‘A’ on its Energy Performance Certificate (EPC), it should, in theory, be highly energy efficient. Yet many such buildings still use far more energy than predicted once occupied. This mismatch – between design expectations and real-world energy use – is known as the performance gap.
The performance gap represents a significant and costly shortfall between a building’s predicted energy use and its actual operational performance. Even fully compliant buildings can underperform by 50% or more. Understanding why this happens starts with two core ideas:
- Assessed Performance (Prediction) – what the building should do based on design and simulation.
- Measured performance (Verification) – what the building actually does once built and occupied.
Bridging the gap between these two is essential for eliminating wasted energy, avoiding future risk, and achieving genuine sustainability. That’s why developers, designers, and asset owners must now prioritise both Assessment and Measurement throughout the project lifecycle.
Stroma provides the expertise to support both Assessment and Measurement requirements, offering a single, trusted partner for delivering buildings that perform exactly as promised.
What’s the Difference? (Assessment vs. Measurement)
Assessed and Measured performance may sound similar, but they happen at different stages and serve different purposes. Assessment predicts efficiency during design, while Measurement verifies it once the building is in use.
| Feature | Assessed Performance (Prediction) | Measured Performance (Verification) |
| Stage | Design and planning | Post-construction / In-use |
| Input Data | Design drawings, material specifications, standard occupancy profiles | On-site physical test results, temperature readings, energy meter data |
| Key Tools | SAP, SBEM, Dynamic Simulation Models (DSM) | Thermal imaging, air tightness testing, heat flux measurement |
| Goal | Achieve regulatory compliance (Part L), estimate EPC rating | Verify construction quality, quantify actual heat loss |
The problem with relying only on Assessment is that design models assume perfect construction and ideal conditions. In reality, flaws like thermal bridging, poor sealing, or missing insulation can drastically affect efficiency – yet they’re not considered in design simulations. Measurement reveals these issues, providing the insight needed to ensure buildings perform as intended.
Pros & Cons of Each Approach
Both Assessment and Measurement have essential roles in achieving efficient, compliant, and comfortable buildings. Understanding their strengths and limits helps determine when and how to apply them.
Assessed Performance
Pros:
- Required for compliance and building approval
- Provides an early benchmark for design optimisation
- Inexpensive to model multiple design scenarios
Cons:
- Based on assumed conditions, rather than verified data
- Rarely matches real energy bills or comfort levels
- Gives a false impression of performance if not later validated
Measured Performance
Pros:
- Confirms construction quality and workmanship
- Provides real world data for setting operational carbon targets
- Identifies and locates defects for precise remediation
Cons:
- Usually carried out post-construction, when fixes cost more
- Requires specialist equipment and expertise
- Adds upfront costs (but often prevents greater losses later)
Methods of Measuring Building Performance
Design-stage predictions are valuable, but they only tell half the story. Measuring building performance gives tangible, in-use data that verifies efficiency and identifies where energy is being lost. Below are key methods used to measure real-world performance.
Air Tightness Testing (ATT) / Air Permeability:
Air tightness testing measures unwanted air leaks through walls, floors, and junctions. A fan is used to pressurise the building, revealing gaps that let heat escape.
Air leakage is one of the main causes of wasted heat loss. Minimising it improves comfort, reduces energy demand, and ensures ventilation systems perform as intended.
Heat Transfer Coefficient (HTC) Testing:
HTC testing measures the rate of heat loss through the entire building fabric. It compares indoor and outdoor temperatures over time to calculate how efficiently the structure retains heat.
This gives a real-world benchmark for overall thermal performance – a direct check against the design model and a key indicator of true energy efficiency.
Thermographic Surveys (Thermal Imaging):
Thermal imaging uses infrared cameras to visualise temperature differences across surfaces. Bright or dark patches show where insulation is missing or air is leaking.
It’s a fast, non-invasive way to spot defects and verify workmanship that computer models and visual inspections can’t easily detect.
Retrofit & Existing Buildings:
Older buildings rarely have accurate performance data. Measurement establishes a baseline before retrofit works and verifies the impact of upgrades afterwards.
Repeating relevant tests before and after improvements provides measurable proof that any upgrades have delivered the intended results.
Infrared U-Value Measurement of Walls
Heat3D precisely measures heat flows and U-values of building elements using a patented, quick, and non-invasive method that follows ISO9869-2. It can be used to detect heat flow rates, thermal bridging, poorly performing structures, as well as assessing existing levels of insulation.
Combining Both Approaches to Close the Gap
True energy performance depends on both Assessment and Measurement working together. Assessment sets the design intent; Measurement confirms it was achieved.
- Early Design: Use Stroma’s SAP / SBEM assessments to set realistic performance targets.
- Mid-Construction: Schedule interim site inspections and thermographic checks to catch issues before they’re concealed.
- Completion: Carry out final ATT and HTC testing to confirm that the building meets design expectations.
Combining these stages moves projects beyond compliance into verified performance – reducing risk, avoiding tenant complaints, and protecting long-term asset value.
Stroma: Your Partner for Assessment & Measurement
Assessment is necessary for compliance, but Measurement is essential for true energy efficiency.
Stroma Built Environment provides expertise in both – from early-stage modelling (SAP, SBEM, DSM) to on-site testing and verification (ATT, HTC, Thermal Imaging). With Stroma, you get a single, trusted partner to ensure your buildings perform exactly as promised.