Last edited 26 Mar 2018

Performance gap in low energy housing

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[edit] Introduction

There tends to be a significant gap between predicted performance and performance in use. The Good Homes Alliance conference 'Closing the Performance gap' suggested that monitoring and post occupancy evaluation (POE) are essential tools to ensure that low energy homes are operating as designed, and are comfortable and healthy (Good Homes Alliance, 2011). Currently the building regulations only require minimal post occupancy evaluation (see energy performance certificates), and so evaluations tend to be undertaken mainly for research purposes. (NHBC Foundation, 2012).

[edit] Post occupational evaluation

One of the key objectives of the Usable Building Trust is ' make building performance evaluation a routine activity for design and building teams' (Bordass, 2011) however it notes that design and build teams are often reluctant to participate in building performance evaluation as they are concerned that their reputations might suffer.

Schemes such as BREEAM also recognise the requirement for post occupancy evaluation, under BREEAM New Construction 2011 it is a requirement that a BREEAM In Use assessment is undertaken for any building that achieves an Excellent or Outstanding rating (BRE Global Ltd, 2011).

'BREEAM In-Use is a scheme to help building managers reduce the running costs and improve the environmental performance of existing buildings. It consists of a standard, easy-to-use assessment methodology and an independent certification process that provides a clear and credible route map to improving sustainability' (BRE Global Ltd).

Although it is designed as a tool to help improve the environmental performance of the building, it can also help to bridge the gap between designer and occupant, where the occupant actually has to learn about the building, the way it works and is managed in order to pass the In Use assessment.

It is difficult to determine the reasons that buildings do not always performing as designed. It may be a consequence of the design itself, construction, the performance of materials or components (or the information provided about them), assessment techniques, or even the building users not using the building as designed.

In the case of the BRE Environmental Office, which was designed to be an exemplar, the actual energy used was around 90% higher than the designed energy demand (Ni Riain, Fisher, Mackenzie, & Littler, 2000). According to (Tuohy, 2012) there were ten key findings from the post occupancy evaluation; eight were connected with the controls of the systems, one was that the building was occupied differently to the way it had been designed and final reason was that there was no energy performance data on display.

It is very revealing that so much of the problem was attributed to controls not being implemented as designed. Juricic, Van Den Ham, & Kurvers, (2012) suggest that '...user's opportunity to influence their indoor climate is one of the most important keys to robustness'. However, it can be difficult to determine whether controls were incorrectly designed or whether they are being used incorrectly.

[edit] Improving standards

Tuohy suggests that 'there are policy initiatives aimed at improving industry processes such as: Soft Landings, BREEAM, LEED, Green Star, AGBR and BIM' (Tuohy, 2012). The Department for Communities and Local Government (CLG) Code for Sustainable Homes (CfSH) has also established itself as a 'proving ground' for new guidance and regulations, it is currently a voluntary standard except for social housing schemes in England, although some planning authorities use the CfSH as a mandatory standard.

This 'proving ground' allows new ideas or strategies to be tested before being rolled out as national policy. This has been proven in the past through the use of targets for carbon emission compliance over the regulations which in the May 09 version of the Code was set at 25% above the building Regulations Part L1a 2006, and Code 4 was 44% above. The revision of the building Regulation Part L1a 2010 revised the compliance level in SAP to be in line with this new figure, with the equivalent carbon reduction of 25% over Part L1a 2006. This is likely to carry on into the revision of Part L1a 2013 with a further 25% reduction (44% over Part L1a 2006) and the introduction of Fabric Energy Efficiency (FEE) standards (CLG, 2012) which was introduced into the CfSH in Nov 2010.

The CfSH has dedicated credits for the production of a Home User Guide in order ' promote the provision of guidance enabling occupants to understand and operate their home efficiently and make the best use of local facilities' (CLG, 2010). BREEAM has similar credits for the production of a Building User Guide ' ensure delivery of a functional and sustainable asset designed and built in accordance with performance expectations' (BRE Global Ltd, 2011), however BREEAM goes one step further and introduces credits for thermal comfort, the criteria being ' ensure that appropriate thermal comfort levels are achieved through design, and controls are selected to maintain a thermally comfortable environment for occupants within the building'. This represents a step change for thermal comfort and its contribution to the environmental credentials of a building.

[edit] Case studies

Because low energy homes are rare in the UK, and even fewer are subject of Post Occupational Evaluation, actual performance is not well understood.

Leaman states that real world research means that effects '...are more difficult to predict, e.g. unanticipated operating modes for innovative technologies, or alterations in user behaviour' (Bordass, Stevenson, & Leaman, Building Evaluation: Practice and Principles, 2010). This idea is supported by Vale, who states that 'Post Occupation Evaluation (POE) could have a significant role in the lowering of environmental impacts, but the framing of domestic POE must embrace a rating of the occupants' behaviour' (Vale & Vale, 2010).

However, research by Juricic, Van Den Ham, & Kurvers, (2012) found that there was no difference between the energy use in user-orientated buildings and those designed ignoring user profiles, and concluded that '...the idea of giving the opportunity to the user to interact with the environment is not necessarily synonym with higher energy use'. This does not mean that because the building uses the same amount of energy whether users are in control or not that the users achieve the same level of thermal comfort. Nicol, Humphreys, & Roaf, state in 'Adaptive Thermal Comfort: Principles and Practice' (2012) that '...behaviour is an important role in our thermal interaction with the environmental' including changing clothes, changes of posture and metabolic rates, moving to a different thermal environment and changing the current environment using thermal controls. It has also been found that the range of temperatures that are acceptable is around 70% wider for natural ventilated building than for HVAC buildings showing that '...higher levels of personal control makes users more tolerant of wider temperature swings' (De Dear & Brager, 1998).

[edit] Technology

Tuohy has assessed new technologies that are often used in low-energy buildings and suggests that ' technology systems do not work as well as predicted' and that '...controls are poorly designed' (Tuohy, 2012). The NHBC Foundation (2012) also cites mechanical and electrical installations as one of 7 factors that contribute to the performance gap. The is particularly problematic as efficiency is effected by the whole system, ie the boiler, pipework and inline components. It has been found that an 86% efficient gas boiler performed as poorly as 55% when the whole system was taken into account (Zero Carbon Hub, 2010).

This is particularly true in relatively new technologies such as heat pumps. Actual system efficiency has been shown to be well below the expected (designed) efficiency and actually in the summer shows a Coefficient of Performance (COP) of just above 1 which is the efficiency of a standard electrical heating system (ref COP curves for a communal Ground Source Heat Pump system (Zero Carbon Hub, 2010)).

Electrical systems generally perform poorly in SAPs as the fuel factor used is 0.52kgCO2/kWh as opposed to gas which is 0.19kgCO2/kWh (DECC, 2010). The reason heat pumps perform well in SAPs as opposed to gas systems is that the COP of around 2.5 will bring the carbon emissions in line with that of gas. With the government pushing the use of these kind of technologies through SAP, where heat pumps fairly easily gain high levels of carbon reduction and through the use of initiatives such as Feed in Tariffs (FITs) and Renewable Heat Incentives (RHIs) which pay for heat energy produced using low carbon technologies such as heat pumps.

[edit] Conclusion

The mechanisms necessary to make low energy buildings exist, are in use, and are legislated for through the evolution of the building regulations. However there remain problems with the energy use in the buildings when compared with the designed energy use. This performance gap is well-known by the industry but potential solutions such as post occupancy evaluations are not being widely adopted.

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