By Alison Ingleby, former Senior Policy and Programme Officer, Greater London Authority Environment Team (Waste and Energy)
Overheating is increasingly being recognised by policy makers and the building industry as a significant and growing challenge. For many years, we have sought to address the cold, wet British winter; designing new buildings to be warm, energy efficient and air tight. But the draughty cracks and empty cavities in older buildings actually have an advantage in helping keep the building cool in summer. With temperatures in London set to regularly exceed current ‘heatwave’ levels by 2050; we need to make sure that the homes we’re building today are designed not just for the British winter, but also for the British summer.
One of the core reasons behind improving the energy efficiency of homes is reducing the demand for energy, and residents’ energy bills. Paradoxically, in a warming climate, we could end up in a situation where the focus of fuel poverty is no longer based on the affordability of keeping a home warm, but on the affordability of keeping it cool. Use of portable fans and air conditioning systems will also place a greater strain on London’s electricity network, which is already at capacity in some areas.
The Mayor of London has set high energy standards in his London Plan and works with developers to ensure that homes built in London today are some of the highest quality, most energy efficient homes in the country. In order to be able to take a similar approach to overheating, he commissioned AECOM to develop a set of cooling benchmarks to help assess the extent to which the demand for cooling in new homes is minimised through passive design measures.
The report, ‘Creating benchmarks for cooling demand in new residential developments’, uses thermal dynamic modelling of a range of typical dwelling types to determine the average summer cooling demand for a range of design strategies. It also analyses the impact of individual design measures on cooling demand and includes sensitivity testing for internal gains assumptions, future climate change and the urban heat island effect.
The findings of the report highlight the importance of addressing overheating as a fundamental element of residential building design and highlight key aspects which have a particularly significant impact on cooling demand:
– Naturally ventilated dwellings have around 20-35% of the cooling demand of units with restricted ventilation options.
– Reducing the glazing to floor area ratio has a considerable impact on reducing cooling demand. A reduction in the glazing ratio of a curtain wall flat from 48% to 25% resulted in a 38% reduction on cooling load.
– Limiting solar gains, for example through external shading mechanisms or reducing the g-value (solar transmittance) of the glazing, is one of the most effective mechanisms to reduce cooling demand, though it should be noted that reducing the g-value has an adverse impact on daylight and heating demand in winter.
– Changes in internal gains assumptions can have a significant impact on the outcomes, highlighting the importance of including a realistic assessment of occupant behaviour when carrying out dynamic thermal modelling.
– A warming climate could have a significant impact on the long-term effectiveness of a passive design cooling strategy based around natural ventilation.
To fully mitigate the potential for overheating in new development, designs must be bespoke not just to the type and location of the building, but consider who is likely to occupy the building, and how long it is expected to be standing for. Only by doing this can the external climatic conditions likely over the building’s lifetime be fully assessed, and the demand for additional cooling mechanisms now or in the future, reduced.
The work of the London Climate Change Partnership focuses on adaptation and resilience to climate change. In the case of overheating, it is clear that adaptation does not just begin at home, but must be considered before the home is even built.