New-build homes are more energy efficient, as a result are prone to overheat. Steve Pearce of Vent-Axia explores how architects can use smart ventilation strategies to combat this, and support the 2050 net zero goal.

Traditionally, UK homes did not struggle with overheating. Older buildings, often constructed from heavyweight materials, benefited from high thermal mass, as well as featuring relatively high air leakage rates, and low levels of thermal insulation, while large openable windows provided natural ventilation, which all helped to lower temperatures.

However, today’s new build homes are far more airtight with high levels of insulation, larger areas of glazing, low-emissivity glazing, and minimal solar shading, which all contribute to overheating risks. Additionally, there are often only a limited number of openable windows, whether due to noise, air pollution, or security considerations. Meanwhile, opportunities for overnight cooling, through either natural or mechanical night-time purge ventilation, are frequently insufficient.

The risk of overheating is further intensified by climate change, which has led to warmer summers. Meanwhile, urbanisation has also raised temperatures, especially in city centres like London, where the urban heat island effect can elevate nighttime temperatures by 10°C compared to its surrounding rural areas. As a result, occupants in some new homes are now experiencing indoor temperatures that exceed comfort, and in some cases pose health risks.

Architectural measures

Ventilation can be an important part of the solution for overheating. However, there are a number of architectural measures that can be taken first to mitigate internal temperatures. A few proactive approaches architects can take include altering a building’s orientation to reduce solar gain; reducing glazed areas; and specifying low g-value windows that manage heat ingress, while maintaining SAP compliance. Meanwhile, maximising external shading, whether from balconies, overhangs or recessed windows have the ability to significantly cut solar gain.

Increasing wall and roof insulation to minimise conduction gains; managing the heat with exposed internal thermal mass and high ceilings; and using green roof systems, can all help regulate internal temperatures. In addition, to minimise the heat generated in a home, communal heating pipework can be rerouted or insulated, especially in corridors.

The next step for architects to consider is how excess heat can be removed by maximising natural ventilation through large windows; maximising cross-flow ventilation and adding ventilation louvres in walls, and employing purge ventilation at an elevated rate. However, architects can face some obstacles. For example, openable windows are not always viable, Park K (protection from falls), Part M (security), high levels of air pollution, and environmental noise can all restrict the use of windows for ventilation.

Guidance

Recognising the seriousness of the overheating problem, the Government introduced Approved Document O of the Building Regulations in 2021. Part O specifically addresses the challenge of overheating in new dwellings, care homes, and student accommodation. Its core objective is to protect occupant health and wellbeing, not to guarantee thermal comfort, with two overarching requirements: limiting unwanted solar gains and ensuring excess heat can be removed from the indoor environment.

Crucially, Part O prioritises passive measures first, reserving mechanical cooling only when necessary. There are two calculation methods: the Simplified Method focuses on limiting solar gains and ensuring adequate natural ventilation by providing prescriptive criteria for glazing areas, window openings, and building orientation. It is suitable for sites that are not constrained by noise or air pollution. The second is Dynamic Thermal Modelling (CIBSE TM59): this is a performance-based approach used when simplified compliance is not possible. This method requires professional input and modelling, especially when building in ‘high-risk’ zones like London. A noise and air quality assessment will help confirm if a Simplified or Dynamic model is needed to comply with Part O.

When it comes to removing excess heat, Part O lists acceptable methods in order of preference, starting with opening windows, followed by ventilation louvres, mechanical ventilation, and finally mechanical cooling. When natural ventilation options are restricted, mechanical ventilation provides a valuable solution with a number of different types of technology available.

Extract ventilation

Where windows cannot be relied upon for ventilation, modern extract systems, such as specially developed unitary fans with built-in temperature sensors and complete inline solutions, which can help remove excess heat. These systems are compact, low-noise, and suitable for installation in all habitable rooms. They offer on-demand operation or automatic switching, ensuring efficient performance aligned with occupant needs.

MVHR & MVHR DX

Now a core part of low-carbon building design, the latest Mechanical Ventilation with Heat Recovery (MVHR) units deliver coolth or heat recovery efficiencies of up to 93%, making them ideal for year-round performance. Crucially for overheating mitigation, modern MVHR systems can come with intelligent automatic 100% summer bypass capabilities, offering free cooling, allowing cool night-time air to flow through the home when conditions permit.

These systems also provide excellent indoor air quality through high-grade filtration – an increasingly important consideration in urban developments with high external pollution.

However, when free cooling via an MVHR summer bypass alone is insufficient to mitigate overheating throughout the year, architects can consider all the benefits of MVHR systems with integrated ‘DX’ cooling modules. Featuring intelligent controls that automatically switch between heat recovery, summer bypass, and active cooling via the DX cooling module, the units continuously measure internal and external temperatures to maintain comfort thresholds efficiently. This helps ensure homes remain cool, comfortable, and energy-efficient. The intelligent DX cooling module can activate on-demand or automatically to prevent overheating.

Another new mechanical cooling option available to architects is a combined MVHR unit plus air source heat pump. This offers renewable hot water with a ‘free’ cooling solution, monitoring comfort temperatures defined by the user and when cooler outdoor air is not sufficient to cool the space via the 100% summer bypass, the system automatically sends a signal to the heat pump to request cooling. This will then combine the air flow from the MVHR and the cool exhaust air of the heat pump via built-in dampers. This type of system offers the highest level of comfort and functionality all year round, while complying with Part O and TM59 while being extremely energy efficient.

Steve Pearce is Vent-Axia product manager