Explaining the Urban Heat Island Effect: How to Design Heat out of Homes in Urban Areas

What is the UHI effect, what causes it and why does it matter? And what solutions are available to mitigate the issue?

The world is heating up and temperatures are rising globally. With this comes a resulting increase in building temperatures, particularly in urban areas. It seems obvious that in heavily populated towns and cities, buildings will be closer together to cope with the sheer number of residents, but it does actually have an official label: The Urban Heat Island (UHI) effect.

So what is the UHI effect, what causes it and why does it matter? And what solutions are available to mitigate the issue and keep our homes cool and comfortable?

Explaining the Urban Heat Island effect

UHIs are created when urban and metropolitan areas are warmer than the rural areas that border them, in some cases up to 8° Celcius warmer. So what causes this?

– Heat is caused by energy, from people, cars, buses, and trains, as well as surfaces which absorb heat throughout the day and then release it later.

– Cities and urban areas have lots of all of these and lots of activity, meaning more heat is generated and absorbed.

– At night, the multiple surfaces found in urban areas release the heat that they’ve absorbed during the day, creating an Urban Heat Island (UHI) effect.

– In addition, the people, cars and objects found in these areas release waste heat, adding to the overall increase in temperature.

However, the buildings themselves and the methods used to create them also play a part in creating the UHI effect. Building materials are usually very good at insulating, or holding in heat. The more insulated the building, the more heat there is for the surfaces to absorb and release, contributing to the rise in temperature.

The focus on energy

In recent decades, the building services industry has responded to societal and governmental pressure to put energy at the top of the agenda when it comes to building new homes.

As a result, vast amounts of funding and initiatives were dedicated to insulating our homes, to make them more airtight and increase energy efficiency. The less air and heat escaping from a building, the more energy efficient that building is. With the introduction of Energy Performance Certificates (EPCs) and official Building Regulations such as Part F, the race has firmly been on, to ensure that all buildings meet these standards.

However, as our homes and other buildings have become more airtight, the heat has risen, leading to other issues arguably as damaging as poor energy efficiency. Let’s take a look at the cost of increased temperatures in urban areas.

Urban Heat Islands and poor indoor air quality: What’s the cost?

The effect of increased temperature on human wellbeing and mental health ranges from a lack of concentration to the inability to sleep, resulting in increased daytime irritability and physical discomfort. Increased heat combined with poor indoor air quality (IAQ) can also lead to a host of other health and wellbeing issues, such as:

– Asthma and other respiratory disorders

– Mental health issues and dementia

– Cardiovascular diseases

What is being done to counteract the increased temperatures in properties?

Despite the continued emphasis on energy efficiency, overheating is now being recognised in the building services industry as a credible threat and an issue which needs to be addressed quickly.

The Chartered Institute of Building Services Engineers (CIBSE)’s introduction of TM59, the design methodology for the assessment of overheating risk in homes, initially released in 2017, is a positive indication that the building services industry is continuing to take the issue of overheating seriously.

Whilst not official legislation, TM59 does at least raise the issue of overheating as a genuine concern and advises on how heat can be designed out of buildings in the first place.

Let’s take a look at TM59’s guidance in more detail.

The TM59 methodology is a collaboration of TM52 and CIBSE Guide A and is driven by the following criteria:

– The percentage of hours that cannot exceed the target temperature, based on the running mean – this is applied to all occupied spaces

– The number of hours exceeding 26°C in bedrooms at night –this is the temperature above which research shows sleep patterns are disturbed

TM59 also includes other parameters for assessing overheating in homes, including:

– Prescribed occupancy profiles

– Internal gains

– Window-opening profiles

– Communal heating gains

– Future weather data

The future: Designing heat out of our homes

Excessive exposure to heat in homes is not only a result of environmental factors, but also design, which is where thermal modelling is key. In addition to core design features, certain solutions inside the dwelling can create a win-win solution e.g. Low energy lighting reduces energy usage as well as reducing internal heat gains.

Here’s a run-down of the specific design considerations which can mitigate overheating:

– Minimise internal heat generation through energy efficient design

– Reduce the amount of heat entering a building in summer through orientation, shading, fenestration, insulation and green roofs and walls

– Manage heat within the building through exposed thermal mass, limiting the amount of surfaces which can absorb heat during the day or mitigating this through opening windows or providing shading through blinds

– Use natural ventilation including openable windows and cross ventilation where applicable

– Integrate balanced whole house mechanical ventilation systems such as heat recovery with true summer by-pass

– Use active, low carbon cooling systems as opposed to air conditioning, which defeats the object of energy efficient design

– Use competent, certified and trained installers. Installation quality and design is crucial in order to combat overheating

So that’s the future, when building new homes. But what can we do now, to combat overheating and its effects in current areas where UHIs are causing a problem?

Overheating in homes: Using ventilation to mitigate overheating

Combining ventilation with active cooling can offer a low carbon solution to overheating issues in residential properties and can be designed in conjunction with dynamic thermal modelling.

The main strategy for combatting this problem would be to open a window or use solar shading such as blinds. However, in urban areas where densely populated areas and increased noise and outdoor pollution can make opening a window problematic, other solutions are needed. In most instances, a mechanical ventilation system such as MEV (Central Extract System) or more

likely MVHR (Heat Recovery) is a preferable method to reduce the effects of overheating. This can then be adapted accordingly to ensure compliance with best practice guidance such as CIBSE TM59.

Here are our seven steps to better ventilation and cooling, which can be implemented to combat overheating in homes.

Purge ventilation

Acoustically treated inline extractor fans located in habitable rooms provide additional ventilation up to 4 ACH in lieu of openable windows. Can be used in conjunction with MEV and MVHR mechanical systems.

Oversized MVHR and radial ducting

Increased extract and supply rates (typically to around 2.5ACH) to provide purge requirements without the need for additional solutions. Radial ducting ensures a balanced system and reduces noise and leakage to maintain lower running speeds.

True summer by-pass and passive cooling

Modulating bypass within the MVHR system constantly aims to improve the internal comfort temperature/humidity by allowing full/partial/no heat recovery depending on conditions. Passive cooling can be activated based on external conditions that can help maximise comfort in warmer months by boosting supply air rates.

Ground to air subsoil exchanger

Working in conjunction with the MVHR system, sealed ground loops and a water to air heat exchanger provide pre-heating in the winter and pre-cooling in the summer on the supply/intake air.

Compressor chiller and enthalpy

Working in conjunction with the MVHR system, this provides 1.5KW active air temperation and dehumidification via refrigerant cycle. Enthalpy MVHR cell passively reduces the humidity of incoming fresh air under favourable internal/external conditions.

Chilled water coil

Works in conjunction with a centralised chiller system to provide 2-6KW of cooling capacity via the supply air of a constant volume MVHR system.

Radiant heating/conditioning & active dehumidification

Complete heating, cooling and ventilation strategy providing active humidity control and scalable plasterboard embedded ceiling cooling loops using a boiler, chilled water or heat pump.

Now that overheating is being recognised as a direct contributor to a reduction in wellbeing and comfort for occupants in homes, it’s time to take action and design the heat out of our homes whilst still respecting energy efficiency and other considerations.

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