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8 Minute Read • The London Plan

Why is Summer Overheating in London Such a Hot Topic?

The London Plan
The London Plan

Why is Summer Overheating in London Such a Hot Topic?


Nobody wants to live in a draughty home. We have an expectation that new buildings will be well insulated, warm and have low energy bills.

It’s all well and good in the winter, when we crave a cosy warm home, but it can be a difficult balance to ensure the same home is able to keep us cool in the summer.

Overheating is becoming a serious problem in London. If a dwelling hasn’t been designed with summer temperatures in mind it can become an uncomfortable space to live in, which can easily lead on to health concerns and negative customer feedback.

Although more commonly affecting blocks of flats, especially those which have a single façade and/or those that are heavily glazed, overheating can also be an issue on large urban developments across the south of England.

Reducing the Risk of Summer Overheating

Reducing the risk of summer overheating is covered in Approved Document Part L, but it’s a basic assessments and isn’t well suited to the modelling complexities of overheating risk in the capital. As such, The London Plan includes its own requirements that reduce the risk of overheating in new homes. This is far more detailed, but is more appropriate for people living in the city. Thermal Dynamic Modelling should be completed on all new build major residential developments to demonstrate that the building design and its services will ensure a comfortable environment for residents.

The London Plan sets out six steps to help developers plan against overheating risks, with air conditioning at the bottom of the list, only to be used if all other steps don’t hit the mark.

1. Limited internal gain

The first thing to consider is how to minimise heat generation from inside the building. Using efficient electric devices and low-energy lighting means less heat is created. This is especially important in offices where high levels of IT equipment can quickly increase the temperature of a building. If you’re using a District Heating system then well-insulated distribution pipework is a must to reduce internal gain.

2. Limited solar gain

Next, designers need to consider how to reduce the amount of heat that can enter a building. Consideration should be given to the orientation, amount and solar transmittance of the glazing and consider additional shading measures.

3. Manage internal temperatures

These first two steps help to reduce the amount of heat in the building but doesn’t get rid of it completely. Step 3 requires designers to consider how this heat is managed through the thermal mass of the buildings construction, and introducing higher ceilings to allow the hotter air to rise above the level of the occupants.

4. Passive ventilation

Following these steps, the building services then need to be considered. Passive ventilation systems can help to regulate the internal temperature while providing fresh air to the building users. This would include opening windows to purge heat build-up at night.

5. Mechanical ventilation

In cases where sites are close to a railway or busy road, opening windows isn’t an option, so mechanical ventilation with heat recovery is often needed to help combat noise or air quality issues. These systems can also be designed to reduce the internal temperature by purging warm air out of the building.

6. Mechanical cooling

Lastly, if all these measures have been considered and the build is still at risk of overheating, comfort cooling or air conditioning can be provided. Just be aware of the additional cost and design implications, along with the negative impact air conditioning has on your energy assessment.


It’s important not to dismiss the impact an overheating building can have on the people using it. Addressing these problems at the early design stage will mean less hassle at later stage of the development.

Thermal Modelling

The best approach is to have a dynamic thermal model completed on the building, pre-planning submission. This will assess the risk of overheating in line with relevant CIBSE guidance and will flag up any potential problems early on.

To comply with an overheating assessment, there are three targets to meet. The first looks at the number of hours in a summer’s day where the expected temperature exceeds a comfortable level. The second measures the number of hours a building experiences high temperatures, and the third checks that no occupied room ever reaches a maximum temperature. If the development fails in two of these targets, then it is considered to be at high risk of overheating.

The thermal model checks these three targets using three scenarios: a moderately warm summer, a year with a prolonged period of warmth, and a year with a very intense single warm spell. Or, for the proper techies out there: 1989, 1976 and 2003. Major developments referred to the Greater London Authority are expected to examine all three scenarios.

If the models show potential issues for overheating, common fixes include changing the solar transmittance of glazing (g value), introducing shading measures, reducing internal gains and increasing ventilation rates.

Overheating homes is a challenge that is becoming more common and with summer temperatures rising, it is a problem that isn’t going to disappear. It’s essential to get the design of future buildings right first time and to avoid the situation where developers are sent back into completed buildings to install cooling systems retrospectively.

A building that is easy to keep warm in the winter and easy to keep cool in the summer is achievable. It makes a site more marketable, reduces the risk of health concerns for occupants and will hopefully result in fewer complaints from residents.

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