New homes: overheating

Handling the heat

2 March 2018

Overheating in new homes continues to be a problem little recognised in regulation. Tom de Saulles considers measures proposed to address this

The potential for overheating in new homes is something we are probably all more mindful of now, partly thanks to the work of bodies such as the recently closed Zero Carbon Hub (ZCH).

But although the need to do more to tackle overheating is broadly accepted, current regulatory requirements still centre on the very general Part L1A provision to limit the effects of heat gains in summer. This is assessed in Appendix P of the Standard Assessment Procedure (SAP), the government’s methodology for demonstrating Part L compliance for the energy performance of a dwelling. Appendix P provides a basic assessment of internal summertime temperatures, and is currently being reviewed along with the rest of SAP.

Surprisingly, the National Planning Policy Framework and Part F on ventilation say little about overheating. In contrast, the 2016 London Plan includes specific design requirements to tackle the problem, while the European Energy Performance of Buildings Directive talks broadly about the need to reduce air-conditioning demand, limit overheating and use passive cooling techniques.

Ventilation rates

The SAP overheating assessment has previously seen some modest revisions, but remains largely unchanged; however, the 2016 draft does include a significant amendment to the way ventilation rates are assessed during hot weather.

If this is adopted, housing designers and developers will need to take greater account of the local environment in respect of security and noise when dealing with summertime ventilation. Where either is likely to be a problem, it will no longer be acceptable to expect windows to be left open for ventilation.

Instead, the use of trickle vents must be assumed unless the ventilation strategy has been designed to address security or noise issues, perhaps by using products such as ventilation panels. In most cases, the minimal air flow rates offered by trickle vents are likely to beinsufficient to meet the SAP criteria for avoiding excessive overheating, however.

Shortly before the ZCH’s demise in 2016, it published a discussion paper – Next steps in defining overheating – that brought together its work in this area with the views of construction professionals. The recommendations it makes are significant, particularly as they may well influence the government’s future direction. Central to these is the assessment methodology, which requires detailed thermal modelling of dwellings when an initial appraisal suggests that overheating is likely to occur.

...housing designers and developers will need to take greater account of the local environment in respect of security and noise when dealing with summertime ventilation. Where either is likely to be a problem, it will no longer be acceptable to expect windows to be left open for ventilation

If on the other hand overheating is thought to be unlikely, use of a simpler tool such as the SAP overheating assessment would be adequate. In either case, the initial judgement must be confirmed to be correct and a limit of 26°C should not be exceeded in bedrooms for more than a specified period. A different threshold would apply to other areas in the home.

TM59: Design methodology for the assessment of overheating risk in homes, a Chartered Institute of Building Services Engineers report published last year, complements the ZCH methodology by setting out a standardised approach for undertaking a detailed thermal assessment, including the specific assumptions and rules to be applied to ensure consistent, accurate results across the housebuilding industry.

Current research

In March, the Department for Communities and Local Government (DCLG) commissioned AECOM to research overheating in new homes, and identify which types are at risk, based on the TM59 assessment methodology. As part of the research, AECOM has been working with experts to agree a definition of overheating and using this as the basis for modelling a range of dwelling types with different weather files and locations.

A cost–benefit analysis of mitigation measures to reduce overheating should follow, subject to ministerial clearance, and should also help inform the DCLG as to what further guidance, regulation or other measures may be needed.

Climate challenge

UK temperature records continue to be broken regularly – June 2017 saw the longest period above 30°C since 1976. Taken alongside the overheating already being experienced in many new homes, it seems likely that we will see some regulatory changes in the near future that pick up findings from the ZCH, AECOM and others.

This would be a departure from the Conservative government position in 2015, when it rejected recommendations for new building standards from the Committee on Climate Change, citing a commitment to reduce the level of regulation on homebuilders.

We hope that a middle ground may now be found that would see relatively light-touch revisions seeking to maximise a dwelling’s performance through better building form, shading, passive ventilation and thermal mass. We should know more in the coming months.

Tom De Saulles is a senior manager in building sustainability at the Concrete Centre

Further information