Overheating and How to Cool Buildings through Architectural Design

Did you spend the summer of 2022 feeling hot, restless, and uncomfortable in your home or office? Given that it was the UK’s joint hottest summer on record, it was likely that you did experience all or some of these feelings.

Europe and North Africa also suffered from periods of unusually hot weather leading to forest fires, drought, and like the UK national emergencies in places.

When it comes to our buildings particularly new builds, offices, and south facing single aspect high rise blocks, we are not prepared in the UK for such extreme weathers or the subsequent overheating.

Uncomfortable and stifling working and living circumstances coupled with an over-reliance on air-conditioning which is detrimental to energy efficiency targets, and the predicted rise in global temperatures mean something needs to be done.

With new UK building regulations in force to address overheating, we examine the issue from an architectural perspective and answer how do we create a naturally cool building?

What is overheating in a building?

There is no agreed definition but in general terms when related to a commercial or domestic building it means the build-up of heat within that building, coupled with lack of airflow, and the resultant negative health consequences.

In 2006, The Chartered Institute of Building Services Engineers (CIBSE) defined overheating as ‘conditions when the comfortable internal temperature threshold of 28°C is surpassed for more than 1% of occupied hours or where 25°C is surpassed for 5% of occupied (working) hours.’

Overheating tends to occur more in apartment buildings with data from the English Housing Survey 2019 showing that 30% of living rooms overheated compared to 12% of houses. The prevalence of overheating in these rooms as well as bedrooms was higher in London than elsewhere.

In non-residential buildings such as hospitals and schools, there is evidence of overheating with 90% of hospital wards in England at risk according to an NHS data report 2019/20, and 90% of teachers surveyed by the Environmental Audit Committee in 2020, saying they take additional measures to reduce classroom heat.

Dynamic Stimulation Modelling software measures overheating, analysing the performance of the space against CIBSE criteria.

The analysis looks at the number of hours in a summers day where the expected temperature exceeds a comfortable level, the number of hours a building experiences elevated temperatures, and that no occupied room reaches a maximum temperature. If two of these three are reached, then the building has failed.

What causes a building to overheat?

There are a variety of reasons why buildings are susceptible to overheating:

• Urban Heat Island (UHI) effect – caused by poor city planning resulting in hard smooth dark surfaces such as roads, paved areas, roofs which absorb heat and release it at night, coupled with a lack of green space and waste heat produced by transport.
• Solar gain – when light from the sun enters the building (solar radiation) and objects absorb this and emit heat within the living space increasing the temperature.
• Prevalence of glazing – excessive use of double-glazed windows and floor to ceiling windows, traps warm air in buildings.
• Climate change – increasingly hot summers and extreme heatwaves especially in the southern areas of the UK such as London means properties in those areas are at risk of overheating.
• Internal heat gains – overuse of technology, lighting, boilers, beams, pipes, and human activity all generate heat.
• Modern building fabric standards – increased levels of air tightness to retain winter heat and windows which do not open, or which remain closed due to urban pollution and noise means a lack of air flow.
• Old housing stock – the UK has the oldest housing stock in Europe, these tend to be poorly insulated and not built to cope with hot temperatures.
• Modern and rapid construction methods – thermally lightweight construction materials such as thin metal, plasterboard, and wood, used in modular homes are quicker to respond to temperature changes making them more susceptible to overheating.
• Size of property – flats are more prone to overheating due to smaller size, denser occupation, and proximity to other homes.
• Property additions – extensions, and conservatories can reduce ventilation and increase overheating.

The human consequences of overheating in the built environment

On average we spend 142 hours a week indoors. Any extreme change in the ambient temperature within those surroundings therefore presents an immediate threat to wellbeing, health, and productivity.

Overheating can reduce work rates, lower concentration levels, increase feelings of discomfort, and at worse cause fatalities. The CIBSE predicts 4500 UK premature deaths by 2050 if this issue is ignored.

Those at significant risk are the elderly, immobile, children, and pregnant women.

When it comes to productivity, heat stress and discomfort can lead to increased breaks, reduced activity, and a lack of alertness.

It can also lead to a reduction in the quantity and quality of sleep which can affect cognitive performance, mental health, and overall feelings of wellbeing.

Architectural design to prevent overheating

In the UK design has tended to focus on keeping houses warm in the winter, building properties to retain heat as opposed to worrying about releasing that heat in the summer. The prevention of illness and fatalities has focused on those that arise in the winter as opposed to when the weather heats up.

However, as the climate continues to warm in both frequency and intensity, and bodies such as the International Energy Agency warn that ‘if left unchecked energy demand from air conditioners will more than triple by 2050’ architectural design techniques which design out overheating, cool a building and leave behind little carbon footprint are in demand.

These focus on what are known as passive design and unlike mechanical active methods such as air conditioning they do not increase home energy costs, contribute to the urban island heat effect, or add waste heat to the environment.

The most well-known approach in this vein for new buildings is the fabric first Passivhaus (passive house) movement which originated in 1990s Germany. It works to strict criteria creating buildings which are comfortable to live in whilst using little energy for heating and cooling.

At RISE Design Studio we are in the process of offering a comprehensive Passivhaus design process for our clients. Currently we use a variety of the techniques to create comfortable, healthy, efficient, and beautiful buildings.

So how do you design a building to keep it cool in hot weather?

There are several effective passive and natural techniques to balance the temperature of a building, minimise heat gain and maximise heat loss:

• External/Internal shading – reduces heat gains through windows by externally looking at awnings, overhangs, balconies, deep porches, and shutters. Internally blinds and curtains can help but are not the main source of effective shading
• Exposed thermal mass – thermal mass is the ability of heavy and mid weight construction materials to store, absorb and release heat.

Buildings with a high exposed thermal mass such as concrete, rammed earth, stone, or brick, when used correctly help to maintain the internal temperature.

They absorb heat during the day and release it in the evening allowing for night time warmth as well as cooler buildings the next day.

• Natural ventilation techniques – large and openable windows on both sides of a room encourage cross ventilation and blow heat out.
• Enhanced insulation – insulation in roof and walls is the key to keeping heat in during the winter but reducing heat gains in the summer as it slows the movement of heat.

Recent innovations have led to the creation of highly insulating material such as hempcrete (marketed under names like Hempcrete, Canobiote, Canosmose, Isochanvre and IsoHemp) which combines hemp shiv and lime binder to provide natural vapour-permeable thermal mass without the carbon emissions.

• High performance windows – triple glazing can be up to two times more efficient than double glazing for keeping the heat outside when it is excessively warm. The two air or gas-filled gaps between the panes reduces the amount of heat transfer between the exterior and interior.
• Solar control glass – this prevents heat build-up thanks to a microscopic thin coating on one side of the pane to reflect the infrared rays of the sun. It is of particular use in large, glazed areas such as conservatories or on rooflights. It also helps to reduce excessive glare.
• Constructing low level buildings – low lying buildings stay cooler as the ground maintains an even temperature.
• Orientation of the building – newly designed buildings should face south as in the summer the greatest risk from the sun is low lying sun in the east and west.
• Designing green environments – if the building sits within a cool environment with less tarmac, paving etc and more in the way of green roofs and walls, then the urban heat island effect is negated.
• Reflective surfaces – incorporating paler walls, roofs and paving into the design increases reflectivity and prevents light absorption.

Our recent Light House project in London saw us incorporate a number of these techniques. These included the use of additional insulation to the external skin of the building; incorporating solar control on the glazing and encouraging passive cooling with kitchen doors and openable rooflights to ensure cross ventilation.

Building overheating in London and the UK

In line with the rest of the UK, London faces considerable threat from climate change, with overheating sitting alongside flooding as the greatest challenge for the capital. This summer the Met Office put in place a red weather warning with temperatures across the city soaring to 40degrees.

Due to the urban heat island effect experienced in the city and the ageing inner city housing stock, which is harder to retrofit with better insulation, it can be 10degrees hotter in the capital than other areas.

Climate experts predict that the likelihood of temperatures in the UK reaching this level is now ten times more likely than in the pre-industrial era with consequences for London including 59-76% of flats and up to 29% of detached properties overheating by 2030.

The Mayor’s office has made a number of long-term commitments and policy changes to tackle these including the latest update to the London Plan which requires planning applications to demonstrate the thermal performance and energy efficiency of new developments.

It looks at limiting solar and internal heat gain, as well as providing adequate passive ventilation. Mechanical cooling such as air conditioning is seen as the last step.

In June, the Government introduced new legislation stating that on new homes, ‘reasonable provision’ must be made to limit heating in summer and ‘provide adequate provision’ to cool them.

The built environment is responsible for 44% of global carbon emissions, with air conditioning systems alone representing up to 40% of their energy output. Architects stand at the front-line in addressing this and the environmental impact of overheating as temperatures rise.

Through planning and design of both new buildings, and in retrofit projects, a careful well-considered approach can help to alleviate the risk of overheating in a sustainable sense, and create buildings that are healthy, beautiful and comfortable.

As design-led architects here at RISE Design Studio we are very much committed to this ethos. For more information and to discuss your project please contact mail@risedesignstudio.co.uk or give us a call on 020 3973 9073

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