Heat Halo has been independently tested under BS EN ISO 9869 — the recognised standard for measuring real-world wall thermal performance in occupied buildings. The figures below describe different aspects of that test, and together explain both the independently measured result and what it means for typical solid-wall homes.
Warmer walls, reduced condensation risk, and independently verified performance — explained without jargon.
Measurable retrofit evidence for hard-to-treat stock, with a full independently authored test report available to download.
The independent assessment
Heat Halo was assessed by Build Test Solutions Ltd (BTS), an independent building performance testing organisation. The test was conducted at a property in Binfield, Berkshire, completing on 24 April 2026 following an 18-day continuous monitoring period.
BTS used the in-situ heat flux monitoring methodology set out in BS EN ISO 9869: Thermal insulation — Building elements — In-situ measurement of thermal resistance and thermal transmittance. This is the established standard for measuring the real-world thermal performance of walls in their actual condition — not in a laboratory or simulation.
In-situ testing measures what actually happens through a real wall, under real weather conditions, in a real home. That makes it highly relevant for real-world retrofit decision-making.
Heat flux sensors and thermocouples were fixed to an external solid wall in a room selected for treatment. A continuous baseline measurement of U-value (thermal transmittance) was established over a multi-day period before any Heat Halo was applied. This confirmed the pre-installation wall performance.
A second room on the same external wall was monitored throughout the entire test period without receiving any treatment. The control room remained at a steady U-value of approximately 0.85 W/m²K throughout. This confirmed that any change in the treated room was attributable to the Heat Halo system — not to changes in ambient temperature, occupancy, or weather. Note: the treated and control wall sections had slightly different starting U-values (0.725 vs ~0.85 W/m²K), which is normal in real buildings.
Heat Halo was applied to the selected wall by approved specialist contractors. Heat Halo was spray-applied in layers to a wet minimum depth of 2 mm. No plaster skim or decorative paint was applied during the test — this was a deliberate decision to isolate the thermal contribution of the Heat Halo coating itself, rather than measure the combined performance of the full finished system. Sensors remained in place throughout.
Important context. A real Heat Halo installation in a home is finished with a conventional plaster skim and emulsion on top of the coating. The 16% U-value reduction reported here reflects the Heat Halo coating in isolation. The plaster skim and emulsion applied during normal installation add a small additional thermal resistance on top of this measured improvement, but were not assessed in this test.
Heat flux and temperature measurements continued uninterrupted for a further 18 days after installation. The ISO 9869 standard requires data to be collected over a sufficiently long period to average out short-term fluctuations. An 18-day dataset provides a robust, statistically stable result.
BTS processed the time-series data to calculate the mean U-value of the treated wall before and after installation, alongside internal surface temperature differentials. The full dataset and analysis are set out in the published test report.
Context and caveats
We publish these figures because they are real, independently obtained, and relevant to the decision homeowners and housing professionals are making. We also believe it is important to set them in context.
The test was conducted on a specific pre-1919 solid-wall property with a pre-installation U-value of 0.725 W/m²K. Properties with thicker or thinner walls, different construction, or different internal conditions will produce different baseline and post-installation U-values. The percentage improvement will also vary.
A solid wall with a significantly better pre-installation U-value will show a smaller absolute improvement. A wall with a worse starting point may show a larger one. Heat Halo conducts a suitability survey on every property before proceeding.
The 16% U-value reduction is the primary finding of the BTS report — the directly measured before-and-after improvement on the treated wall (0.725 → 0.608 W/m²K). The ~28% figure is a post-installation comparison between the treated wall (0.608 W/m²K) and the untreated control wall (~0.85 W/m²K) under the same external conditions. The two wall sections had slightly different starting U-values (0.725 and ~0.85 W/m²K respectively), which is normal in real buildings, so this is a comparative result describing relative performance after installation rather than a standalone improvement percentage.
The evidence base currently comprises one independently monitored installation. Further testing across additional properties is planned. We will publish updated evidence as it becomes available. We present the current results because they are real and independently verified — not because we consider the evidence base complete.
Unlike laboratory assessments, in-situ testing captures actual conditions: occupancy patterns, varying external temperatures, moisture behaviour, and real wall construction. This means results are more practically meaningful but also subject to the specific conditions of the test. The 18-day monitoring window and simultaneous control room measurement were specifically designed to control for these variables.
Our position
We believe the honest presentation of independently tested evidence — including its limitations — is more useful to decision-makers than marketing claims without a documented basis.
The full BTS test report is available to download. It contains the raw data, methodology, and findings. We encourage specifiers, housing professionals, and technically minded homeowners to read it.
About the testing body
Build Test Solutions Ltd is an independent building performance monitoring and testing organisation. BTS provides in-situ thermal performance testing using calibrated heat flux monitoring equipment in accordance with BS EN ISO 9869. They have no commercial relationship with Heat Halo beyond the provision of this test.
The standard specifies in-situ measurement of thermal resistance and thermal transmittance of building elements using heat flux meters. It is the recognised European and international standard for this type of field measurement.
Laboratory testing measures idealised samples under controlled conditions. In-situ testing measures actual installed performance in an occupied building — which is the figure that matters for energy performance and occupant comfort.
Heat flux sensors measure the rate of heat energy passing through a surface in real time. Combined with internal and external temperature data, this allows U-value to be calculated from actual wall behaviour over an extended period.
Testing a treated and untreated room on the same wall simultaneously is considered best practice for isolating the effect of an intervention. It eliminates variables such as weather, occupancy and heating patterns that would otherwise confound the results.
See the full evidence
The complete independently authored report — raw data, methodology, findings — is available for download. No registration required.
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