By Andrew Moore

New homes overheating is an increasing problem, with a number of instances being reported in the national press as well as industry publications. So, why is this happening and how can we resolve the issue?

In part, the problem has arisen because modern housing is constructed to much higher standards of insulation and airtightness than was possible, or economic, historically. This is exacerbated when Passivhaus standards are adopted. The overall impact is that far less heating is needed in the winter, reducing energy bills, but conversely it is essential that heat can be let out when the weather is warmer or the sun is shining – otherwise the building will overheat.

How do we fix this and why do we need a natural ventilation system alongside mechanical ventilation?

All too often, I hear people ask why we need cooling or openable windows when we have a ventilation system with grilles visible in the ceiling? This highlights a common assumption that the mechanical ventilation system (MVHR) provides all the cooling for the home, so the windows do not need to be opened – all reinforced by the message that you must keep the windows closed in air conditioned buildings. This misconception typically arises because the building concept hasn’t been explained properly. Most modern homes will need opening windows.

The MVHR provides background ventilation only, to meet the requirements of Part F, and includes heat recovery to comply with the energy efficiency requirements of Part L. This arrangement helps to reduce the heating requirement for modern buildings, but cooling is not normally provided and the ventilation rate is typically much lower than is necessary to give comfortable conditions in summer. Removing heat and controlling temperature therefore needs to be addressed separately, and can be dealt with in a similar way to the rapid ventilation needed to clear smells (such as smoke from burnt toast).

Rapid, or purge ventilation must be provided under Part F of the Building Regulations, and is normally achieved by opening windows. While this can also help to remove excess heat from rooms, the requirements for an effective natural ventilation system are more complex and need proper design to work well. The simple provision of a minimum open area may not be effective.

Good natural ventilation solutions take into account both the façade arrangement and the likely external conditions. Broadly, the more glass and the hotter it is outside, the more opening area you need to stay comfortable. With climate change likely to produce hotter summers it is increasingly important that robust ventilation approaches are adopted – if we are to avoid a generation of housing that is unsuitable in years to come. We also need to educate building users that ventilation grilles do not necessarily mean air conditioning, and that in modern buildings it is necessary to open windows, even when it is cold outside, contrary to what is needed in older buildings and the instructions to keep windows closed in air-conditioned buildings.

How can we get to the right design?

Delivering an appropriate design will mean considering all the major issues from the start, which can include the external acoustic environment, and its impact on the ability to simply open windows when needed. This is an important consideration in busy urban locations and can lead to solutions incorporating acoustically treated ventilators, rather than simple windows.

Of course, you could argue to that installing comfort cooling will solve the problems, and it might – at least locally. While this may be the best solution in some instances, it conflicts with the goal of reducing our carbon footprint, and local policies such as the London plan, which aim to avoid the need for cooling.

Ultimately, there is no single one-size-fits-all solution, but being aware of all of the issues and allowing for the associated costs from the start of the design will go a long way to delivering buildings which work well, and communicating key principles to the occupier will avoid some common misunderstandings which can otherwise result in uncomfortable or inefficient buildings.

Contact Andrew Moore,, with any questions on overheating homes or to find out how Cundall can help.

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Andrew Moore, Residential, Sustainable Design


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