Beat the heat

Jason Bennett of Zehnder Group UK discusses ways to address the risk of overheating in homes, as summer temperatures begin to rise

The way we build our homes has changed. Allowing more natural light to enter the home – along with better insulation for energy efficiency – are both high on self-builders’ wish lists. Yet this combination, coupled with solar gains from south-facing aspects, makes a property high risk for overheating – a modern-day issue that is often overlooked. 

Climate change has led to increasing summer temperatures, and the Met Office’s future climate projections suggest the temperature of hot summer days could increase again by between 3.8ºC and 6.8ºC. 

Recent summers have already been characterised by extremes in the UK’s climate. Temperature records soared beyond 40ºC for the first time, and large parts of the country experienced unprecedented discomfort through overheating, causing many to leave their homes in search of cooler and safer dwellings. 

High temperatures in homes negatively impact people’s lives. It can cause serious problems with sleep as well as heat stress, and present other major health risks such as heat stroke, impaired sleep and in some cases, even premature death. 

Building design often contributes to overheating in new buildings and yet the majority of self-builders are unaware of the dangers and don’t factor these into their plans. 

Don’t be a victim of overheating

The UK Building Regulations provides guidance on mitigating overheating in residential buildings, and reducing these effects. Its prescriptive approach to optimising glazing, solar shading and natural ventilation clearly outlines preferred solutions to keeping a property cool. 

However, problems occur when external factors dictate that these passive means can’t be used to control internal temperatures and ventilate effectively. For example, if planning dictates that windows cannot be opened because of risk of noise, security, or pollution, then the regulations list alternative options. These could be acoustic facade ventilators and mechanical ventilation to mechanical cooling.


The first step is to seek an energy assessment or Standard Assessment Procedure (SAP) for the build along with an Overheating Assessment. This must be done by a qualified assessor to gauge the risk of overheating in the property and is a process that all new builds must adhere to for compliance with Building Regulations Approved Document O.

This thermal modelling will determine which spaces in the house could be prone to overheating and recommendations can be made for effective solutions – either in specific rooms or the whole living space. 

When it comes to solutions, an effective cooling hierarchy consists of stages or options that mitigate overheating issues – some of which are complete standalone solutions and others that are combined, to deliver the required level of cooling capacity.

The predicted effectiveness of these solutions must be modelled in accordance with the guidelines (TM59) in order to demonstrate compliance with the building regulations (Part O), so it is best to consult an expert before finalising plans. 

Traditional methods of combating overheating often rely on air conditioning units, but this approach can have negative effects on indoor air quality. By merely recirculating air, these systems can reintroduce potentially harmful particulates such as dust and allergens, compromising occupants’ health and comfort.

These systems use a lot of energy and therefore cost more to run. They also sit dormant through the winter months, resulting in large equipment taking up space that could be better used.

It’s time for a different approach.

Using ventilation as a strategy and solution to mitigate overheating 

Combining ventilation with a tempered air system can offer a better solution to overheating issues in residential properties and can be designed in conjunction with dynamic thermal modelling.

Adopting a ventilation-first approach to overheating, not only provides a solution to overheating issues but also protects the indoor air quality within the home. Optimising the overall climate within a home to create a comfortable and healthy living environment all year round is essential to the wellbeing of those living in it. 

People wrongly assume that inside the air is clean and safe but indoor air is typically two to five times more polluted than outdoor air due to airborne chemicals and particulate matter. Without effective ventilation, harmful gases and particles can become trapped polluting the air. Ventilation extracts this dirty air providing a constant supply of clean refreshed air into the living spaces. 

In most instances, a mechanical ventilation system such as mechanical ventilation with heat recovery (MVHR) forms the base of the strategy for overheating. This can then be adapted accordingly to ensure compliance with best practice overheating guidance – providing two benefits in one. 

To determine the best approach to take for your self-build, you should consult with an MVHR designer. They can specify what type of ventilation and air temperation system is needed, as well as offering sized plans to allow for sufficient ducting space. 

Jason Bennett is indoor air quality expert and national business development manager at Zehnder Group UK