The performance gap is an increasingly well-known phenomena in buildings whereby the actual thermal performance is frequently different to, and more often worse, than predicted. At present however, regulation and management of the energy efficiency of buildings is based almost exclusively on predicted and not measured performance.
In May 2020 the Building Performance Network published their State of the Nation Review of the performance of domestic buildings. They reported an average performance gap of 18% in the thermal performance of their sample of 29 buildings. As well as an average underperformance, the study found huge variation in levels of performance, with some building’s performance 30% better than their prediction and others performing more than 100% worse.
Any prediction of building performance is limited in how accurate it can be because it is based on visual surveys, when many important factors such as areas of missing or poorly installed insulation, thermal bridging or small cracks and gaps cannot be seen. Performance measurement is required to really understand the combined effect of issues such as these.
With unexpected performance comes unintended consequences; not just in greater energy consumption and consequent emissions, but also issues with ventilation, thermal comfort and cost. It is impossible to think that buildings can be effectively managed if it is not well understood how they’re actually performing in use.
The importance of understanding actual thermal performance has been reflected in recent Government documents. The performance gap was mentioned nine times in the recent Future Home Consultation, and the Government’s EPC Action Plan states that the EPC will need to move towards measured ‘in use’ performance in future.
Clearly, greater knowledge of the performance gap in general and of the performance of single buildings in particular cannot be gained without more performance measurement. The ‘coheating test’ has been the tool that has revealed the performance gap, in this test a building is vacated for a period of around two weeks in winter and is heated electrically to a constant raised set point, typically between 20 and 25oC. The coheating test also requires specialist equipment and analysis that result in per test costs of thousands of Pounds. While the coheating test has been valuable in highlighting the performance gap, it is not practical for use on a wide scale given the disruption and cost. In fact, these factors have limited the total number of coheating tests to only a few hundred.
Recently there has been increasing interest in the research community in developing a more practical alternative, and products are starting to come to the market. Build Test Solutions (BTS), a specialist manufacturer of building performance measurement products, have developed and extensively validated the SmartHTC method.
SmartHTC uses monitored internal temperature and energy consumption and a light touch building survey to inform an algorithm that calculates the thermal performance. The temperature monitoring must be carried out over a period of at least three weeks during winter, but crucially the building can be used as normal throughout.
The smart part about SmartHTC is the way that it accounts for variations in weather and the actions of residents to isolate just the thermal performance of the building. The output is a measurement of the Heat Transfer Coefficient, which is a measure of the rate of heat transfer through a building per degree of temperature difference between inside and out. The HTC is calculated in all energy models, such as the SAP model used to generate EPCs or PHPP modelling used for Passivhaus, and directly comparing modelled and measured HTC values calculates the performance gap.
Data readings for SmartHTC can be collected using bespoke temperature sensors placed in the building temporarily, or using smart technologies such as smart thermostats. The measurements are then uploaded to the cloud-hosted SmartHTC calculator to return the result. Users can either use SmartHTC through a simple browser interface or integrate SmartHTC directly into their products through APIs.
SmartHTC has been developed over the past 10 years, growing out of a research project at Loughborough University. Over the past 2 years BTS have been carrying out a major validation of SmartHTC in partnership with the University of Salford. The validation has included laboratory testing in the unique Energy House facility, an end-terraced house deconstructed in Salford and rebuilt brick by brick within a climate-controlled chamber, and a field trial in more than 200 homes. SmartHTC has demonstrated excellent accuracy and repeatability, with SmartHTC results agreeing with those of a coheating test in every case.
SmartHTC offers the same measurement as coheating tests, but without the disruption. The simple data collection and upload means that assessments are time and cost comparable to an Energy Performance Certificate visit, or a SmartHTC calculation can be integrated as an additional feature within smart home platforms to add value to data that is already collected.
Building performance measurement can be useful to all parts of the building industry, for example to provide quality assurance for customers and products, guide retrofit decision making for stock owners or provide greater assurance for lenders and insurers.
SmartHTC has already been used in some of these roles. For example, SmartHTC was used in the ECO3 Demonstration Action programme to demonstrate the benefit of a new product, Airex active air bricks. SmartHTC measurements were carried out in a field trial in 115 properties, showing an average improvement of 12% in the thermal performance of the buildings. Local authorities have used SmartHTC to target properties most in need of retrofit, and to check that the retrofits had the intended outcome.
SmartHTC has been integrated into smart technologies such as the Switchee thermostat and the Carnego Shimmy platform, delivering performance measurement as an extension to their existing function.
By removing the barriers of cost and disruption, thermal performance measurement is available in a scalable way for the first time. Knowledge of the real thermal performance of buildings enables owners and users to make better decisions on where to target energy efficiency and fuel poverty alleviation spending, get quality assurance on work and products and avoid costly reactive maintenance. The tools are now in place to better manage buildings in advance of a shift in regulations to come. buildtestsolutions.com/thermal-performance/smarthtc/