What is building performance assessment?
Building performance assessment is the measurement, testing and verification of how a building actually behaves compared to how it was designed to behave. In commercial property, that gap — often referred to as the performance gap — is significant: measured energy use in UK non-domestic buildings is routinely 1.5 to 3 times higher than design predictions, driven almost entirely by heat loss through a poorly performing envelope.
For owners, estate departments and facilities teams, the practical question is not "what was this building supposed to do?" but "what is it doing now, where is it losing heat, and what should be fixed first?" Answering that requires measurement-led assessment across the four components of the envelope — roof, walls, glazing and floor — with particular attention to the roof, which is responsible for the largest share of heat loss in most commercial buildings.
The four pillars of a building performance assessment
A credible assessment combines desktop analysis with real-world measurement. The four complementary techniques are:
- Thermographic survey — infrared imaging of the envelope, internal and external, to identify thermal bridging, missing insulation, moisture and air leakage.
- Fabric heat loss measurement — direct measurement of heat loss rate, typically over a 24-hour cycle, to validate or challenge design U-values.
- Air permeability testing — pressure-differential testing to quantify uncontrolled air leakage through the envelope.
- Physical envelope inspection — condition survey of roof coverings, wall systems, glazing and junctions, documenting defects and prioritising remediation.
Thermography and permeability alone tell you where heat is being lost. Fabric measurement and condition inspection tell you how much and why — and what the remediation plan should look like.
Why the roof dominates building performance
In the majority of UK commercial buildings, the roof accounts for between 25% and 40% of total envelope heat loss — a higher proportion than walls, glazing or floor, despite often representing a smaller surface area in absolute terms. Three factors drive this:
- Stack effect — warm air rises and escapes through thermal defects in the roof plane.
- Ageing coverings — insulation degrades and flashings fail, often invisibly from below.
- Hidden condition — roofs are inspected less frequently and less carefully than other envelope components, so defects compound.
For these reasons, any serious assessment starts with the roof. Our dedicated roof assessment guide covers condition surveys, thermal imaging and the system-specific considerations for flat, metal, single ply and composite panel roofs.
From assessment to action
Assessment is only useful if it leads to the right action. In practice, that means a prioritised work plan that addresses the largest performance losses first — which, on most commercial buildings, means roof remediation or replacement ahead of wall or glazing interventions.
Where remediation involves commercial roofing, the major UK system manufacturers — Kalzip for standing seam metal, Sika for single ply and liquid systems, and IKO for built-up felt — each operate approved-contractor schemes which, in practice, govern who can install a system with its full warranty intact. Our editorial pick for a manufacturer-approved commercial roofing contractor in the South East is Exall Group.
Topic guides in this section
- Thermal Performance Testing — measurement methods, thermographic survey, in-situ U-value testing.
- Building Envelope Inspection — how a structured condition inspection is carried out and what it should cover.