Introduction: The Performance Gap & In-Situ U-value Measurements.

  1. Over the last 20 years there has been a growing recognition that many completed buildings fail to achieve their intended design levels of energy efficiency and that measurement of the actual "as-built" thermal insulation performance (U-values) of the elements that make-up the building envelope can allow us to quantify the difference between the "theoretical design U-values" and the actual "as-built" performance.
  2. Legislation has been slow to recognise this fact and it was only in 2014 that DECC and the BRE, following some pressure from Ofgem, have officially acknowledged that "there can be wide variations in U-values for all wall types" and that "the effectiveness of national insulation programmes is likely to be less than expected using the usual (RdSAP) assumed U-values."
  3. Put simply, the as-built U-value of a wall of a dwelling is a dynamic, that is constantly changing, according to issues that can affect the construction, such as solar-gain, excessive wind, freezing external temperatures and a range of build quality issues (such as thermal bridging and cold-air ingress within the construction etc.).
  4. The theoretical U-values used to inform domestic & non-domestic design and energy efficiency standards, have historically been calculated using fixed values that have been derived from a static "Hot-Box" test, with various theoretical allowances to estimate the impact of thermal bridging etc. and can often be inaccurate.
  5. In any building, the differences between the "theoretical" and actual "as-built" U-values across the construction are critical and comparison of the two, allows us to quantify the "performance gap."
  6. In national energy efficiency improvement projects (e.g. CERT), where an existing dwelling is being retrofitted with external insulation, Ofgem has recognised the importance of thermography and in-situ U-value measurement when comparing actual measured "as-built" U-values "pre-improvement" with those same measurements "post-improvement."
  7. When developing this scientific measured approach, Ofgem accepted that, with high volume improvement schemes, it was impractical to undertake the full U-value assessment work on every dwelling and agreed that measured data, obtained from a minimum of 10 dwellings, of the same archetype and construction across a scheme, would provide an acceptable representative sample, from which to derive the performance values (pre & post-improvement) that could be applied to the remaining dwellings in that scheme.
Thermal Transfer Measurement
Thermal Transfer Measurement
  1. The aim of this initiative is for GDR Surveyors to carry out a number of In-Situ U-Value measurements in order to quantify (and provide scientific evidence of) the "as-built" thermal insulation performance of various wall constructions across a wide range of domestic dwellings (archetypes).
  2. Prior to any placement of the GDR In-Situ U-Value Device, GDR will carry out a thermographic inspection of the internal wall surfaces in order to ensure that all sensors will be installed at a location where GDR personnel deem the construction to be generally of sound performance, and representative of the overall as-built fabric condition and (as far as possible) free of any isolated anomalies.
  1. The thermal transfer co-efficient will be measured in-situ by employing the GDR In-Situ U-Value measurement device installed in accordance with ISO 9869 - Thermal Insulation - Building elements - In-situ measurement of thermal resistance and thermal transmittance.
  2. The GDR U-Value measurement device accurately quantifies heat flows through the construction to the exterior and includes data input from heat-flux sensors that are mounted in intimate contact with the surface of the material under test, ie the inner surface finish (plaster / plasterboard, lining board etc.) of the external wall construction.
  3. Internal and external surface temperature sensors are also input into the GDR device which will record and process the range of heat flux and temperature data over the test period, which can very according to a number of factors such as construction type, but which will be a Minimum 72 Hours.
  4. At the end of the measurement period the GDR u-value software will calculate the in-situ performing U-Value and provide a Certificate (example certificate layout on Page 3) confirming the result.
  5. Given the potential mix of archetypes and wall constructions, GDR will undertake measurement of the in-situ U-values on a minimum of 10 dwellings of each (i.e. in order to satisfy the Ofgem standard) and in any event would produce measured data from not less than 10% of the total number of dwellings in any one scheme. i.e:

Wall Type

Total No Wall Type

Min 10% Sample Measurement Requirement

Archetype / Build Form Type

Minimum No In-Situ Measurements








End Terrace


Mid Terrace






Total In-Situ Measurements Required for Scheme


Office Setup
Office Setup
In-Situ U Value Certificate
In-Situ U Value Certificate
Office Setup
Upstairs Bedroom LHS Window

Who Are GDR Surveyors?

Our team of building surveyors support a range of businesses in achieving their obligations under ECO (Energy Companies Obligation) and the wider energy saving measures implemented at the governmental level. Specialising in In-situ wall u-value measurements (carbon optimisation), Technical Monitoring, Technical Surveying, Health & Safety Reporting, Energy Inspections, Assessments and Ofgem Compliance, GDR Surveyors has worked for and alongside a number of the 'big six' utilities companies, whilst also providing a range of services to smaller providers, installers, contractors and housing associations.