A first step for the town hall
Set in the heritage quarter, the Market House was once at the centre of a thriving mining community. Designed by Cope and Eales of London, it is constructed from solid granite ashlar and is locally recognised for its distinctive vermiculated quoins and voussoirs.
Today, with Cornwall Council’s long-term vision for the Market House defined, the first priority would be to improve the thermal performance of the large first-floor assembly room, formerly the town hall.
The challenge
Improving thermal performance meant tackling significant heat loss through the building’s solid, uninsulated walls. Granite’s high thermal conductivity and long periods without heating left the space cold, damp and in decline.
With large arched windows and high ceilings, the space had striking character, but the walls and window reveals contributed to significant heat loss.
With conventional internal or external insulation not feasible, any intervention needed to be compatible with the building’s fabric, preserve architectural details, and avoid trapped moisture and condensation. A breathable insulating plaster was the ideal solution to improve thermal performance overall and reduce thermal bridging at window reveals, without compromising the fabric or appearance.
The solution
Working closely with Cornwall Council, the existing internal plaster, skirting boards, and surrounding ceilings were removed to prepare the space for SuperTherm.
Cornerstone’s most insulating lime plaster, SuperTherm has a K-value of 0.04 and would function as a thermal layer and basecoat, improving insulation while remaining highly vapour-permeable to manage moisture effectively.
Applied by spray and hand methods, the Natural Plastering Company applied 35mm of SuperTherm to the main walls, and 25-80mm within the window reveals. Once the base layer was complete and damp cured for seven days, a 6mm layer of SuperTherm Finishing Plaster was applied.
Performance testing and thermal imaging
Throughout the 15-month project period, independent performance monitoring was implemented pre- and post-application. The Elemental Cube™ was deployed to ensure accurate testing throughout the project, creating a one cubic meter of temperature-controlled space both internally and externally to the building.
Heat flux sensors and GreenTEG U-value kits were also used to obtain an average U-value for each wall section and to identify any inaccurate U-values to discard.
To ensure that each location tested had an accurate representation of the specific orientation and wall construction, a thermal camera was used. The camera analysed heat transmission through the façade to identify anomalies that may cause inaccurate data, such as thermal bridging, cold spots, or inconsistent insulation.
Thermographic inspection showed orange areas with high heat transfer, while light blue indicated lower transfer.
The results
For the main external wall, the initial application of SuperTherm improved the U-Value from an average of 2.18 W/m²K to 1.07 W/m²K, an average thermal improvement of 68.3% compared to the solid stone walls with no insulation. Once cured, the U-Value further improved to 0.74 W/m²K, an additional 30.8% improvement in thermal performance.
The U-Value of the window, prior to SuperTherm being applied, was approximately 38.5%
higher than the U-Value of the front external wall. Although a smaller section of the external facade
is made up of window reveals, this is a significant increase in heat escape when compared to the main
walls.
For the window reveals, the initial application improved the U-Value from an average of 3.22
W/m2k to an improved 1.06W/m2k, an average thermal improvement of 67.08%. Once cured, the U-Value further improved to 0.81 W/m2k, an additional 23.6% improvement in thermal performance.
SuperTherm and Supertherm Finishing Plaster will continue to carbonate over time. Further monitoring will be carried out in the Summer of 2026 to gain more comprehensive results.
“Using The Elemental Cube™, we measured a 74.8% improvement in thermal performance. That’s a dramatic reduction in heat loss through solid stone walls whilst maintaining compatibility and breathability. That’s the kind of results that make a difference to how a building feels to live or work in.”
Jason Ratcliffe, The Wellbeing Surveyor in Partnership with Cornwall Council
More information
For full details on our approach and methodology at Market House, see the St. Austell Market House Research Methodology Documentation.
The Elemental Cube was featured on Grand Designs and named a Kevin’s Green Hero in 2024. Find out how it delivers accurate thermal performance data.
