Energy performance: initiative for new homes

Closing the gap

10 March 2016

Rob Pannell looks at initiatives under way to solve the energy performance gap in new homes

Raising awareness of the energy performance gap in new homes has been top of the Zero Carbon Hub’s agenda since a collaborative project with those involved at all stages of the housebuilding process started to uncover the extent and impact of the problem. An evidence review report published in March 2014 was followed by an end of term report in July 2014, and current work is focused on creating guidance to address some of the issues found.

The performance gap relates to the difference in modelled energy use (and carbon dioxide emissions) at the design stage, compared to the as-built stage (i.e. just before handover), based on a standard occupancy profile. So the work was not looking at the impact of occupant lifestyle, but on the potential of the home itself to perform as designed.

Scope of the problem

The information reviewed and gathered by the project revealed widespread evidence that all stages of the process of providing new homes have the potential to contribute to the gap – from concept and detailed design to procurement, construction, commissioning and verification.

This could happen inadvertently as a consequence of conflicting drivers in the industry, or as a result of poor practice.

Although the work was carried out in the context of Zero Carbon Homes remit to deliver as-built performance, the government’s decision not to improve energy-related standards for new homes in 2016 does not change the need to address the gap. The fact is that new homes being delivered every day are at risk of not meeting their design performance – whether built in line with the 2006, 2010 or 2013 regulations.

For industry, there are potential reputational dangers, and consumer confidence in new homes could be undermined if energy bills are higher than expected. There are also risks from the government’s perspective in that a gap between a building’s energy and carbon performance undermines its vital role in delivering the national carbon reduction plan.

Three cross-cutting themes were identified as primary contributors to the problem:

  • lack of understanding, knowledge and skills
  • unclear allocation of responsibility
  • inadequate communication of information.

Concept design

At the concept design stage, team members may lack knowledge or experience of the impact their design will have on the energy performance of the completed dwelling. This might include the built form, orientation, layout, materials and finishes or variations to standard house types.

Best practice lessons from the construction phase are very rarely communicated to the concept and planning team, whose involvement in a project commonly ceases at the planning stage. Consequently, lessons are not learnt about the practical implications of their design decisions on as-built performance.

The same lack of knowledge and experience applies to detailed design team members on the impact of the buildability of the design, site conditions and tolerance levels, optimising thermal detailing, and the compatibility of construction systems, materials and building services.

There is often a lack of integrated design between fabric, services and renewables. If all elements of the design are not properly integrated, building fabric and services may not perform as expected and unintended consequences such as additional thermal bridging may result.

Concern over the competency of Standard Assessment Procedure (SAP) assessors was also flagged as an issue at this stage. For an accurate assessment to be undertaken, it is clearly important that assessors have sufficient levels of competency – accurately inputting data, following conventions, validating assumptions and evidencing their assessments. Unfortunately, the evidence suggests that this is not always the case, with errors seemingly common.

Inadequate consideration of skills and competency requirements at labour procurement can lead to issues with poor installations and lack of general energy-related knowledge and skills or care among site workers and managers. The potential impact on the performance gap is high due to the knock-on impacts for fabric and services installation quality.

Construction and commissioning

Decisions may be made on site to substitute products for alternatives that, inadvertently or not, have a different energy performance from the originally specified product. This may be caused by delivery delays, to save time or money, by mistake or due to a lack of knowledge on site.


New homes are being delivered every day that are at risk of not meeting their design performance

Product substitution may be an inevitable and necessary part of the housebuilding process and should not automatically be a concern for the performance gap. The substitution must, however, be for components of equivalent performance and any variation should be reported to the design team, particularly the SAP assessor. The ultimate impact on the performance gap will depend on the product being substituted.

Evidence indicates that fabric components are often improperly installed, compromising crucial elements of the thermal design. Common issues identified include:

  • gaps between insulation boards in walls and roofs, and between boards and inner leaves
  • lack of insulation and air barrier continuity at junctions
  • poor laying of insulation quilt in lofts
  • insulation missing around cavity trays, lintels, below the damp proof course, at floor perimeter, dormers and roof lights
  • poor sealing (e.g. at service penetrations) and lack of attention to creating a robust air barrier
  • window and door frames not positioned correctly in relation to cavity insulation.

There are also issues with the installation and commissioning of services, with incorrectly fitted or commissioned systems not performing as designed. This can occur where there is insufficient installation guidance or drawings, a lack of manufacturer installation or commissioning guidance or other detailed design guidance, or a lack of knowledge or care. This may result in the site team making uninformed decisions without proper understanding of the energy strategy.

Where the site team lacks knowledge and experience relating to energy performance, decisions may be made that conflict with the design and strategy for the dwelling. Site teams face many demands, one of which is to understand and deliver often complex fabric and services designs for optimal energy efficiency. The evidence indicated that this is usually a low priority on site: many elements are being built and fitted incorrectly, with a significant impact on the performance gap.

In addition, there may not be adequate processes or responsibility may not be taken for carrying out energy-related quality assurance on site. Evidence clearly indicates that site management does not focus sufficiently on energy performance. Aspects of construction that relate to the performance of the completed building are not prioritised.

Verification and testing

To provide a robust as-built SAP calculation, it is important that the inputs reflect the final build specification on site, or there will be an inevitable performance gap. Without being provided with, or not using, updates to specifications and design changes, SAP assessors will be unable to provide an accurate as-built SAP.

There are multiple causes for this problem, which involve all stages of the housebuilding process. These include practical issues of communication as well as process problems.

The evidence also points to a lack of robust verification focusing on energy performance, for example by building control bodies or warranty providers. This may be due to reliance on third-party information, or lack of knowledge, time or incentives to do so.

In many ways, the energy performance of a home is one of its lasting legacies. In the current drive to build large numbers of new homes it is vital that they meet their intended performance.

The evidence of a performance gap has been clearly demonstrated and its existence is now generally accepted.

Now is the time to implement strategies and solutions across all parts of the housebuilding process to close the gap.

The Hub’s Builders Book documents some of the more common areas on site that could contribute to a performance gap, alongside good practice guidance on how to minimise this.

A thermal bridging guide was published earlier this year, with further publications to follow, including a design guide and SAP guide.

Rob Pannell is Managing Director at the Zero Carbon Hub

Further information

Download a complete “Energy performance: initiative for new homes” PDF