A proactive approach in a warming climate

As global temperatures rise, CIBSE is looking to update its tools for calculating overheating risk in new homes. Dr Anastasia Mylona explains how work on TM59 and weather files aims to increase buildings’ resilience

The latest Intergovernmental Panel on Climate Change report (2023) finds that there is a more than 50% chance that global temperature rise will reach or exceed 1.5oC between 2021 and 2040 across emissions scenarios. Under a high-emissions pathway – the one on which the global greenhouse gas (GHG) emissions are currently – the world may hit this threshold even sooner, between 2018 and 2037. 

With the current 1.1oC of global temperature rise, changes to the climate system are now occurring in every region of the world, with rising sea levels, extreme weather events, and rapidly disappearing sea ice. Even if we managed to reduce our global emissions to zero tomorrow, we are already destined for a certain level of climate change –because of the GHG emissions already released into the atmosphere – by the middle of this century. 

What the above is clearly telling us is that, while we are committed to reducing our carbon emissions towards a net zero future, it’s imperative that we also build resilience in all sectors of human activity, including the built environment, where most of us spend most of our lives. 

The main areas in which climate change will impact the built environment are thermal comfort and energy use, structural integrity, and water management. Warmer winters may reduce the need for heating, but keeping cool in summer without increasing energy use will present a key challenge. The industry will also have to address the impact of extreme storms and having too much water (floods) and too little (droughts). 

CIBSE tools and resources are widely used by policy and industry to inform adaptation strategies. For example, in 2022, CIBSE’s TM59 Design methodology for the assessment of overheating risk in homes was introduced as one of the methods to show compliance with the Building Regulations on overheating in England: Approved Document O. 

It’s imperative that we build resilience in all sectors of human activity, including our built environment, where most of us spend most of our lives

For the first time, the requirement to increase the resilience of buildings (new homes) to the impacts of a changing climate (increasing temperatures) was introduced in national policy (in England). 

TM59 provides a consistent methodology to assess overheating. It standardises occupancy profiles, internal gains, the treatment of blinds and shading, and the use of weather profiles. 

CIBSE is in the process of revising TM59 to coincide with the release of the Future Homes Standard, currently under public consultation. As part of the process, the authoring team debated the use of future weather (2020s: 2011-40) against historic weather profiles (1983-2013) in the assessment of overheating, to allow for mitigation solutions that would work both now and in the future. 

As the life expectancy of an average home is at least 100 years, it has been decided that a future weather profile should be required to investigate the thermal performance of domestic properties. 

Design Summer Years

CIBSE’s Design Summer Years (DSY) are annual weather profiles selected to represent three types of hot event: DSY1 – moderately warm summer; DSY2 – short, intense warm spell; DSY3 – long, less-intense warm spell.

DSYs are available for 14 locations in the UK, and there are two extra DSYs for London to capture the intensity of the urban heat island. The DSYs are also available for three future timelines: 2020s (2011-40), 2050s (2041-70) and 2080s (2071-2100), based on the UK Climate Projections 2009. CIBSE is now revising DSYs based on the latest UK Climate Projections released in 2018.  

The revision of TM59 is focusing on two key elements: the revision of the night-time criterion and the implementation of the 2050s DSY1 as the minimum required weather profile to assess overheating in homes. 

Existing studies have shown that the current criterion overestimates the risk of overheating (Lomas et al, BSER&T, 2023). The current UK bedroom threshold of 26°C is based on one small study, which is now more than 45 years old. 

CIBSE is working with 20 organisations from industry and academia to look at a higher threshold. Early results have shown that the relaxation of the night-time criterion will better represent the risk of overheating and the effectiveness of mitigation options in the future.

While increasing the challenge presented by the warmer 2050s weather profile, overall it shows similar levels of overheating as the current TM59 methodology. 

There is still a lot to be done to adapt our buildings to the rising temperatures and extreme hot events. We need to better understand the comfort and health thresholds of vulnerable people. We also need to promote more sustainable cooling systems (reversible heat pumps might be a solution), especially for homes, to avoid the mass uptake of air conditioning, which will undermine our carbon-reduction efforts. 

Lastly, but most importantly, we need to focus on making our cities greener; if we tackle increasing temperatures and heatwaves at city level, then our homes and buildings will be less vulnerable to such weather events.