Overheating in homes has been a hot topic – pun intended – for some years now in the UK. The effects of climate change are increasingly apparent, with warmer average temperatures and more frequent heatwaves. We need our homes to remain habitable, both now and as our summers get warmer.
There is strong evidence that some types of architecture fare better than others in hot weather – older homes with higher ceilings or windows on at least two sides generally cope more effectively than compact modern apartments – and our fondness for floor-to-ceiling windows speaks to our lack of understanding about the original greenhouse effect. Most worrying are examples of homes that overheat for significant periods of the year even when external temperatures are not especially warm. So, what can designers do to understand the risk of overheating?
In 2017, the Chartered Institution of Building Services Engineers published TM59, Design methodology for the assessment of overheating risk in homes, which establishes a protocol for assessing such risks using dynamic thermal modelling software.
I was one of the authors of TM59, and it's been gladdening to see how well received the methodology has been. It has been adopted by the Greater London Authority and been introduced as a requirement in the draft London Plan, becoming the standard approach in London assessments. As not all homes face an equal risk of overheating, we should be aware of the most significant factors, as follows.
Homes in London, the south of England and in city centres are more likely to overheat due to warmer weather and the urban heat island effect.
The more sun-exposed glazing, the higher a homes solar heat gain.
If windows either do not open very wide or it is noisy or unpleasant when they are open, then the cooling effect of natural ventilation is lost.
If windows only open facing one direction then there is less opportunity for cross-ventilation and for a pleasant breeze passing through the home.
Dynamic thermal modelling is used to simulate the internal temperatures under test conditions and thus assess whether threshold conditions of discomfort will be reached. Once we recognise the risks, it is usually apparent what design changes might help reduce them.
The most effective measures for reducing overheating risk in new homes include using dual-aspect design, modest glazing areas – 20-35 per cent of facade area is usually ideal – external shading where glazing areas are greater, and additional opening windows or those that open wider.
Ceiling fans can be helpful in creating a pleasant breeze, especially where window opening is more limited. Use of increased thermal mass in the building fabric can be beneficial, provided that the ventilation is carefully designed to purge the mass effectively, as it can otherwise increase night-time temperatures.
In 2019, the Good Homes Alliance (GHA) launched a tool to help gauge the overheating risk for new housing schemes, which I developed with Julie Godefroy of Julie Godefroy Sustainability and Nicola O'Connor of Mandarin Research. The tool is intended for use by designers or local authorities at the pre-planning design stage, and comprises a one-page scoresheet with 14 simple questions, split between factors that increase or decrease the risk of overheating.
There are also supporting guidance notes detailing how each question relates to overheating, as well as help with scoring, advice on appropriate mitigation options and suggestions for further reading. The scores are totted up and advice given based on whether the result represents a low, medium or high risk.
The greatest challenge tends to be with schemes in very noisy locations, and evaluating when a location is too noisy to open windows is a challenge.
The Association of Noise Consultants is working on this with the Acoustics Ventilation and Overheating Residential Design Guide, due to be published soon.
This will help acousticians, architects and engineers to collaborate more effectively in resolving the tension between windows opened for cooling and closed against noise.
There is no reason that new homes in the UK should be vulnerable to overheating chronically – we still have a relatively temperate climate, and the design skills to ensure this is not a major problem.