An outstanding issue: The Performance Gap

I read that the construction industry had experimented with adding insulation to new buildings and that energy consumption had failed to reduce. This offended me – it was counter to the basic laws of physics… So I made it my mission to find out what [they were doing wrong] and to establish what was needed to do it right.

Professor Wolfgang Feist, Founder, Passivhaus Institute, Germany.

This quote from the Passivhaus Institute founder highlights the relevance, for the passive house standard, in tackling the “Performance Gap”, which has been a well documented industry problem for many years. The Passivhaus approach of designing performing building fabric is assured in design and on site by the quality assured method of certification. This approach makes Passivhaus a unique standard, and one recognised as the best energy efficiency building standard in the world. It ensures an absolute correspondence between design and expectations. With the large majority of other buildings, designed and procured to traditional methods, significant discrepancies between design predictions of energy use, and operational performances have been registered. This disparity has become known as the Performance Gap.

What the Performance Gap is and when it occurs

The “Performance Gap” consists in the difference between how a building is designed to perform and how it subsequently does in reality once built. The term usually refers to energy consumption but can refer to other aspects of building performance too (Glossary, PH+ Magazine).

An overview of the relevance of this significant issue for the construction sector is provided by the extensive collection of academic studies, research and industry reports. According to the Carbon Trust report “Closing the Gap” on 28 case studies examined from the Department of Energy and Climate Change’s Low Carbon Buildings Programme, the operational energy use was up to five times higher than estimates during design; and also, when comparisons were made with relevant industry benchmark, 75% of case study designs did not perform as well as expected.

Figure 1. Design predictions for regulatory compliance don’t account for all energy used in a building. Source: Carbon Trust, adapted from Carbon Buzz, 2011

Figure 2. Chart showing a comparison between energy benchmarks for four case study buildings and actual energy use form the monitoring data for each site. Source: Carbon Trust, 2011

Performance Gap related issues may arise throughout all phases of the building process, from the design stage to the delivering, and all along the lifecycle of the building. The Evidence Review Report (2014) drafted by Zero Carbon Hub summarised in a chart the factors and conditions which are likely to contribute creating a Performance Gap (Fig. 3).

Figure 3. As-built performance. Priority for Action. Source: Zero Carbon Hub, 2014

What emerges from the graph is that the main reasons leading to a Performance Gap to occur relate to: inadequate knowledge, skills and understanding; poor quality design, products and building fabric. In this sense, it must be noted that the Passivhaus Certification tackles all the above issues and that is the reason why passive houses perform as prescribed.

Passive House works in practice

The Passive House Standard is a science-based approach, based on a meticulous design and on careful construction details. The Standard focuses on the building envelope and building fabric, to ensure indoor comfort and low energy consumptions. It is based on 5 basic principles:

  1. Thermal Insulation
  2. Airtightness
  3. Thermal bridge free design
  4. High performance glazing
  5. Mechanical ventilation & heat recovery system

The design is verified by the software known as PHPP (Passive House Planning Package): a design tool used to calculate buildings’ thermal comfort, heating, cooling and primary energy demand, based on the inputs provided and the desired features. PHPP produces reliable and accurate results for energy balance calculations, essential for Passive House (PH) design and certification of the project.

The passive house project and design must be validated by an external certifier in order to obtain the Passivhaus Certification. The aim of this process is to ensure that all the rigorous parameters for quality design and energy efficiency (related to the 5 principles listed above) are met. The certification prosses verifies the full compliance with these high standard requirements and acts as a warranty for the client. The Certification process can be applied not only to buildings (new and retrofit), but also to Products, Designers and Tradespeople, to ensure the highest quality service and effective building’s energy efficiency, throughout the whole lifecycle of the building.

It has been identified that a major difference in measuring space heating energy losses between DEAP software and PHPP is of importance to the Performance Gap issue. The measured efficiency of the dwelling depends on how long the daily heating period is, and the temperature to be maintained by the heating system during this heated period. The time when the heating is assumed to be turned on, and to desired temperature, for DEAP software is 8 hours per day. Passivhaus (PHPP) software assumes that the building will be maintained at a constant temperature throughout the day and night, so that the heating period (Heating Duration) is 24 hours.

The second factor related to this discrepancy between the software is that the Winter Interior temperature for PHPP is calculated as 1.5 degrees higher than DEAP.

In a PHPP designed house the calculation of heat loss, is based upon an internal temperature maintained at a constant 20 degrees. For a house designed to DEAP standard, the house merely must perform at the lower temperature of 18.5 degrees for an 8-hour period in the day. It is obvious that the heat losses calculated with DEAP will be lower than PHPP. These figures are given further analysis in the report created by Dr Shane Colclough (Report The near Zero Energy Building standard and the Passivhaus standard – a case study).

 

To conclude: a dwelling designed to be heated for a 24-hour period in winter using DEAP software, will not perform in terms of its predicted energy efficiency. In cases where a house is heated, 24 hours a day, at a higher temperature then 18.5 degrees, it is inevitable, an energy efficiency Performance Gap will occur.

Figure 4. Basis of calculation methodologies for DEAP and PHPP. Source: Colclough S., 2017

The issue facing designers and certifiers, is that traditionally, buildings designed to meet building regulations have often performed poorly, in terms of ‘as-designed’ energy use and ‘actual’ energy use. When nZEB becomes the statutory performance threshold for all buildings, the buildings designed will be calculated to use much less energy. If the Performance Gap issue is not addressed, then the traditional design and construction issues, associated with poor performance, will create a larger problem for these buildings.

One solution to this scenario is to use the Passive house design and certification model, which has been proven to work in practice over 20 years. A substantial amount of monitoring of Passive buildings show that they perform as the design calculations had predicted, and in many cased better than predictions. Passive House, due to the quality management and certification procedures is not affected by any Performance Gap. As stated by the International Passive House Association, the reliability of the Passive House concept can be judged by the results of those measurements. In fact, a number of monitoring projects have been undertaken while comparing 41 low energy houses and 106 passive houses in Germany. The measurements reveal that passive houses perform as prescribed, with a good correlation between PHPP calculations and actual consumption values. The monitored houses also show much lower consumption values compared to the low energy buildings, in some cases up to 80%.

Figure 5. This diagram summarises the comparison of the consumption measurements of the reference settlement (left, 65 kWh/(m²a)) and the three Passive House developments considered (about 13 kWh/(m²a) in each case). Source: Passipedia

Passive houses are designed to last, to be comfortable and energy efficient over the whole lifecycle of the building. The PH Standard is achievable, verifiable and quality assured.

 

Conclusion

The side effects of the Performance Gap are, money, efficiency and comfort losses. As stated in the Evidence Review Report published by Zero Carbon Hub (2014) […] for the government, the Performance Gap would mean that new housing cannot be relied upon to play its expected, vital role in achieving national carbon budget targets. For owners and occupants, energy bills may be higher than expected, undermining buyer confidence in new (low carbon) homes. For planners, designers, manufacturers and housebuilders, under-performing new homes could impact on their reputation and business.

Building designed to zero energy and near zero carbon standards, represents a worthy goal to aim for, and will soon be a mandatory requirement. However, limitations such as the Performance Gap must be acknowledged and addressed.

The concern for the whole industry and public bodies, is that with nZEB, under the current statutory framework will be poorly executed in design and on site. This will substantially undermine the ‘As-built’ performance of the new buildings.

In this sense, Passive House is part of the solution to “fill the Gap”. The adoption of the method of Passivhaus certification would guarantee, for the construction industry and government, that the Performance Gap issues would be addressed. The performance issues that discredited poorly designed and executed, Low Carbon buildings in the past, can now be avoided by using the, tried and tested, Passivhaus method. The adoption of the Passivhaus method across the world is testament to its inherent success and endurance.

 

Related Reports:

Carbon Trust (2011) Closing the Gap: Lessons learned on realising the potential of low carbon building design

UCL Energy Institute & CarbonBuzz (2013) Summary of audits performed on CarbonBuzz by the UCL Energy Institute

Zero Carbon Hub (2014) Closing the Gap between design and as-built performance: Evidence Review Report

Zero Carbon Hub (2015) Builders’ Book. An illustrated guide to building energy efficient homes

SHARE