An IEA EBC Annex is proposed to provide a network of experts and facilities to study the methods of measuring the Heat Transfer Coefficients of dwellings. 

These activities will provide datasets and findings to accelerate the development of measurement methods, a methodology by which they are deemed to be valid and consensus on the approaches for calculating uncertainty. It will generate a database of real measured buildings and calibrated energy models, and where new and improved methods of HTC measurement can be developed virtually. A specific focus will be applied to the potential applications of these methods, and the requirements on them to meet user demand. Translating HTC results to actionable insights will also be investigated through the study of investigative techniques to disaggregate the HTC, to allow the performance gap issues to be studied in further detail.

The most challenging and valuable part of the proposed annex will be the collection of a large dataset across the globe of typical dwellings, under differing climates, this will enable the development of measurement techniques for cooling dominated climates; currently the research is concentrated primarily on heating dominated climates.

Background

The energy performance gap, defined as “the difference between modelled energy performance and the measured energy performance” in dwellings is now well studied. Previous research from around the globe has found gaps between the modelled or predicted value and the actual as built energy performance in the field, a selection of recent results are found in Table 1. This figure is generally referred to in percentage terms.  These values can be positive or negative, i.e., the building can perform better than the model or the model better than the building. 

 

Table 1 Global examples of recent energy performance gap studies with sample sizes and levels of performance gap included (Fitton, 2021)

Country	Sample size (N)	Average Performance Gap	Reference
Canada	1	74%	(Rouleau et al., 2018)
Germany	3400	30%	(Galvin, 2014)
United Kingdom	25	50%	(Johnston et al., 2015)
Switzerland	50000	11%	(Cozza et al., 2020)
Italy	6	45%	(Ballarini and Corrado, 2009)

 

To address the performance gap many researchers have designed methods for measurement of the average rate of heat loss of dwellings, known as the Heat Transfer Coefficient (HTC), this is defined in ISO 13789 as the “heat flow rate divided by temperature difference between two environments” (BSI, 2017). Some test methods were designed in the 1970’s and require bulky, expensive equipment and test conditions that are disruptive (empty homes, high temperatures and long test durations) (Swan et al., 2015). It is for this reason that over the past 20 years many researchers in academia and industry have worked on new in-use methods for measuring HTC. In the last years this has been accelerated by large research schemes such as CERIENE (Bouchié et al., 2014) SMETER (BEIS, 2020) and ISABELLE (Thébault and Bouchié, 2018). However, these all lack a standardized approach to uncertainty and quality assurance that past research has shown is required by stakeholders (Deb et al., 2021a)

A further significant component is the need to carry out testing on buildings in cooling dominated climates, as much of the HTC measurement work carried out globally has been carried out on buildings in heating dominated climates. Around 2.8 billion people live in hot countries, where the average daily temperature is greater than 25 °C (IEA, 2019), leaving a significant amount of the global domestic building stock that lacks a high quality method to make HTC measurements. Energy performance under cooling is becoming increasingly important as the uptake of air conditioning is increasing rapidly in many
regions and combined with an increase in global temperatures. 

While measurement of the HTC unlocks multiple use-cases, such as whole dwelling quality assurance and pay-for-performance schemes, it can be difficult for some stakeholders to use directly, especially in situations where the HTC is found to differ from predictions. At this point, methods are needed to interrogate the HTC result or disaggregate it to identify whether improvement works must be carried out. This was highlighted recently by a large study of stakeholders (Deb et al., 2021). 

This work will continue to build upon the work delivered by Annex 58 and 71 of IEA EBC and will make significant contribution to three of the five High Priority Research Themes laid out by the IEA EBC: -

• Planning, construction and management process reducing the performance gap
  The proposed annex will deliver a process whereby one of the most important causes of the energy performance gap, the thermal performance of the building fabric, can be measured with accuracy, leading to accepted validation methods and a known uncertainty.
• Low tech, robust and affordable technology
  The proposed annex will develop methods to characterise the thermal performance of buildings at lower cost and intrusion than incumbent methods. This will support the widespread testing of dwellings, closing the performance gap and empowering homeowners without the need for large quantities of research-grade equipment. The annex will also support innovators by providing unique datasets at zero cost.
• Energy efficient cooling in hot and humid, or dry climates
  A portion of this annex proposal is concentrated around cooling dominated climates, acknowledging that performance here is vital, and that HTC research is lacking in this area.

Objectives

The proposed EBC Annex will address five clear objectives.

Objective 1: To develop new knowledge and understanding of the breadth of real-world applications for in-situ building energy performance measurement techniques and the technical requirements of those applications across different sectors. Stakeholders include the building industry, government policy and regulation, innovators from around the world, and applications in heating and cooling climates. The requirements of these applications include the requisite accuracy and how different techniques can be validated and verified for use. This objective is addressed in sub-task 1.

Objective 2: To extend the current HTC estimation methods to new building typologies and climates while improving their accuracy, repeatability and robustness. This work will include high performing homes (e.g. new-build and Passivhaus, where methods are limited in accuracy due to the low heat losses) and apartments in larger buildings (with many party elements). Methods that can be used outside of the main heating season, and in cooling dominated climates, will be investigated. This objective is addressed in subtask 2.

Objective 3: To co-create a new framework for the verification and validation of in-situ building energy performance measurement techniques. This will reconcile the current disparate approaches and provide the playbook of methods required for estimating uncertainty and evaluating accuracy and repeatability in the field. Recommendations will be made for auditing, accreditation schemes, and standards that may be required to assure delivery in practice. This objective is addressed in sub-task 3.

Objective 4: To develop a new research area on building performance diagnostics, that identify the reason for the HTC performance gap. Novel in-situ methods are needed that disaggregate HTC estimates to identify the root-causes of underperformance and negate the need for expensive forensic examinations. This objective is addressed in sub-task 4. 

Objective 5: To collect and curate data sets to support the work of this EBC annex and to create a legacy resource that is free to access and that accelerates innovation and adoption of in-situ building energy performance measurement techniques. This will include new and existing simulated data, data from field trials in occupied homes and data from test houses. Best practice methods for the collection, reporting and curation of these data will
be developed. This objective is addressed in sub-task 5.

Scope and Limitations

The proposed annex will focus on residential buildings, including apartments and multioccupied blocks. It will not include non-domestic buildings, though the techniques developed here could be transferable. There will be efforts to extend the applicability of the current methods to periods outside of the traditional heating season, and to high performing dwellings with lower HTC. The impact of this extension on the accuracy and repeatability of the methods will be studied. Methods suitable for use in cooling dominated climates will be included for the first time in any major collaborative research activity. It is likely that there will be similarity in the methods used for the different climates and different building types, but also unique challenges. New best practice approaches across all aspects will be openly shared to accelerate progress and drive innovation in the measurement and diagnosis of building thermal performance.

A key potential limitation for the proposed annex is the current pace of development and appetite from policymakers and industry to use in-situ building measurement techniques in the near term. Sub-task 1 is essential to ensure the annex’s relevance and impact; it seeks to turn this potential challenge into an advantage, leveraging current interest and enthusiasm to develop and apply such methods, whilst evolving detailed research to reflect the changing research and application landscape.