Building Performance Evaluation for Energy Efficiency

BPE is a process of gathering quantitative and qualitative information that characterises the performance of a building including a wide range of closely linked parameters.

A holistic BPE process includes assessing:

• The building's context, use, design and construction, significance, and the condition of the building fabric
• 'As built' and 'during construction' performance: fabric performance and systems/services performance
• 'In use' performance: indoor air quality; indoor environmental quality and ventilation; occupants' perception and experiences; energy and water use

It measures what performance was targeted or expected, compared to what was achieved. It can be used to assess change by comparing an 'as built' baseline with 'during construction' and 'in use' phases. Depending on the assessment methodology BPE may be a continuous process, with varying levels of investigation.

Evaluating an older building is crucial for bridging the performance gap between what was intended and designed, and the performance achieved. BPE provides evidence to measure the successes of changes to existing buildings, failures, and risks against key performance indicators.

As a quality assurance process, BPE is implemented across the construction industry and recommended in standards such as PAS 2035:2023 and PAS 2038:2021. It is notably described by the Building Performance Network, the Good Homes Alliance, the RIBA and BS 40101: 2022.

A post-occupancy evaluation is an integral part of the BPE process. It establishes how an occupied building is working (in terms of thermal performance) over the first few years after construction or alteration.

The scope and comprehensiveness of BPE need to be proportionate to the building's significance and complexity, and the project's objective, such as climate change adaptation and mitigation. It is important to establish how the building's performance will be measured throughout the works and compared with reference performance such as previous or intended performance. Choosing appropriate assessment techniques is essential to build an accurate picture of the actual performance of a building.

Any performance comparators must consider the building's unique context and its environmental conditions. Using design specifications, appropriate benchmark performance or past performance data from the same building is key, especially if the building has been upgraded or is a hybrid of traditional and modern construction.

It is essential to review the project's objectives as more information is gathered on the building's actual performance, context and significance; the condition of building fabric and services; and how occupants use energy and the overall energy consumption. This iterative assessment will ensure that the proposed energy efficiency measures will not harm the building fabric or significance.

BPE is fundamental to identifying the energy performance of a building. It can be based on energy usage over a year (from energy metering or energy bills) or in situ heat transfer coefficient monitoring, combined with other measurements. However, using only one BPE technique can lead to overall building performance being misrepresented or misinterpreted.

The level of investigation at different stages of the project, or throughout the lifespan of the building, must be carefully considered because excessive data collection can be both costly and ineffective.

The scope, assessment techniques and depth of investigation for BPE will vary, based on the:

• Acceptable level of disruption for the occupants and stakeholders
• Building fabric's significance and complexity
• Existing type of problems and defects, and opportunities for improvements
• Building construction, materials, previous repairs and alterations
• Aims of the assessment and the associated level of investigation
• Aspirations and needs of the occupants

Early investment in surveys and data gathering can save on the capital costs of energy efficiency measures, which are often more suitable, simple, and cost-effective as a result.

• Consider an enabling works contract for pre-retrofit BPE
• Include in the contract the cost of BPE during and after the works
• Anticipate the need for extensive BPE and estimate the likely timescale to ensure the project runs smoothly

Engineers, surveyors and architects need to review their professional indemnity insurance (PII) to confirm they are covered for a BPE carried out after the works, whether by them or a third party. Conflict of interest may arise if the professional who carried out the energy efficiency works also conducts the BPE. However, it is a beneficial process to improve practices and replicate successful approaches.

Standard Assessment Procedure (SAP) and Simplified Building Energy Model (SBEM) are used by the UK government to estimate energy performance in dwellings and non-domestic buildings for compliance with 'Approved Document Part L: Volume 1 Dwellings' and 'Approved Document Part L: Volume 2 Buildings other than dwellings'.

Standard Assessment Procedure (SAP) is a steady state energy modelling methodology that uses monthly recorded data, standardised occupancy and behaviour data to generate an Energy Performance Certificate (EPC). Reduced data Standard Assessment Procedure (RdSAP) is the approved methodology in England to estimate energy performance and generate an EPC for an existing home. However, this assessment methodology tends to inaccurately represent the performance of older buildings.

Experience is crucial for selecting the appropriate approach for conducting a BPE, carrying out the evaluation itself and interpreting the results to inform proposals. By measuring data on site, BPE can help identify practical and cost efficient adaptations which consider the whole life carbon of the materials used, as well as opportunities that have minimal environmental impact.

In situ testing and monitoring provide valuable insights into the many aspects of building performance. Choosing assessment methodologies addressing the unique characteristics and heritage significance of the building is essential.

Techniques range from basic walk-through surveys to identify defects, to more detailed monitoring and investigative evaluations:

Energy and water use

• Monthly meter readings or review of energy bills (over a year), smart meter or building energy management system
• Monitoring water use

Hygrothermal performance

• SMETER or co-heating test: heat transfer coefficient measurement
• Thermal envelope thermal transmission measurement: U-value
• Thermal imaging: heat loss assessment, air leaks
• Remote inspection of the fabric
• Air permeability testing: air leaks, construction quality
• Fabric moisture assessment: surface temperature, moisture content of fabric, fungal and mould growth testing

Environmental monitoring

• Internal environmental quality monitoring: indoor air quality, relative humidity, surface temperature, daylight, glare, overheating, acoustics and so on
• External environmental monitoring: ecology, shading, noise, flood risk, drainage and so on

Existing building services assessment

Occupant satisfaction survey

Ongoing monitoring of building parameters

Learn about the detailed BPE approaches that underpin these techniques in the Retrofit Revisit report. For example, operational data is valuable to facility managers and building occupants, facilitating efficient building operation and reducing energy use and cost.

The BPE process should be proportionate to the building's significance and complexity, potential risks and the extent of the changes. Allow for adjustments of the BPE strategy based on the condition survey and the initial findings of the evaluation. Be mindful of resource constraints while effectively balancing accuracy of the equipment and methodology, time, and cost of the evaluation.

BSI 40101: 2022 categorises BPE into four levels of investigation and suggests measurement techniques for each level.

Four possible approaches could be considered:

1. Essential: largely based on occupant feedback, together with basic building parameters and basic energy data. It will only allow for rough insights into the impact of the improvement works and it is unlikely to provide a full picture. This type of evaluation may only be suitable for small projects that have low levels of resources and needs as well as limited outcomes.

• Site visits and high-level building condition survey
• User survey
• Energy use audit based on 1 year of bills or smart meter

2. Standard: same as above, but includes more detail to verify performance and to identify any issues. Essential scope, plus:

• 1 month winter monitoring: energy meter readings in more detail; temperature and relative humidity in sample rooms; carbon dioxide in most occupied rooms; mould risk indicator
• Airtightness testing: pulse or blower door

3. Advanced: as above, but includes granular information across larger, if not all, parameters to allow for comprehensive insight and holistic understanding. Standard scope plus:

• 1 month summer monitoring: energy meter readings in more detail; temperature and relative humidity in sample rooms; carbon dioxide in most occupied rooms
• Thermal performance transmission measurement of the thermal envelope
• Thermal imaging
• Fabric moisture assessment.

4. Diagnostic: applicable if any of the above approaches identify a particular issue that needs further investigation.

Each investigation technique has specific data collection requirements, based on the BPE objectives. Quality of input data and consistency in how data is collected is essential to allow for comparisons and to facilitate future evaluations.

Select appropriate sensor types and carefully plan their installation to avoid damaging significant or vulnerable building fabric. Essential considerations for the equipment include:

• Data collection methods
• Equipment requirements, tolerances, ranges and error margins

Those carrying out the evaluations must adhere to the General Data Protection Regulation (GDPR) because BPE involves personal and building data that may pose commercial, security, or privacy risks. The Regulation applies to data collection, processing, and retention.

Inconsistencies in data labelling and storage present significant hurdles to the effectiveness of a BPE. They may impede efforts to conduct repeat studies on a specific building, track performance over time, evaluate intervention impacts, and compare results. To address this, BS 40101: 2022 recommends the consistent capture of raw data and its storage in a manner conducive to revisiting, reinterpretation, and use in subsequent studies.

A number of interdependent and interacting factors impact an existing building's performance. Interpreting a broad range of parameters in the round and considering individual parameters are key to understanding the impact of energy saving measures.

Interpreting the data collected requires knowledge and skills proportionate to the level of investigation. A cautionary approach is needed, particularly in one-off cases or small samples such as U-value measurements. Large variations may exist due to a lack of uniformity within the fabric of the building and the occupants' behaviour and needs. Look for trends, patterns, averages and extremes, and compare results against benchmarks from World Health Organisation (WHO), Chartered Institution of Building Service Engineers (CIBSE) and so on. The benchmarks can be used as indicators of potential harm to occupant health or building fabric.

A major concern in relation to the success of climate change mitigation and adaptation measures is the discrepancy between the design intent and the actual performance of the building after the works. Many factors can lead to energy savings falling short of predictions and unintended consequences.

There could be inaccuracies with:

• Initial energy use
• Materials specifications
• Survey accuracy, performance measurement methods and benchmarks
• Occupants and/or their behaviour.

Careful interpretation of the data collected is essential to maintain the wellbeing of occupants and the equilibrium of the building environment. It ensures that lessons are learnt and applied for future projects.

Reporting the performance of buildings needs a degree of consistency to enable comparison when they are assessed by different evaluators for different building types and uses. A BPE report should include the following:

• Purpose, objectives, and scope of the study
• Roles and involvement of key parties
• Level of engagement with the occupants to gain access to the property
• Building context in terms of significance, construction, use, age, complexity, size, orientation, typology, weather and materials
• Level of BPE to be conducted and choice of comparators and metrics
• Metering and monitoring equipment required
• Timescales and duration of the study
• Performance data to be reported

The data collected should be gathered in a report and made available to occupants and facility managers. Data concerning building use, maintenance guidance and unfamiliar technologies are particularly useful.

As a project progresses, the data recorded digitally will need to be updated. In most cases, they will be seen as outdated a few years after the project is completed.

It can be difficult to draw a line at the end of a project. Ongoing monitoring helps to maintain a healthy environment for the wellbeing of occupants. As buildings continue to evolve, monitoring their performance needs to be considered.