Let’s be specifc: Overheating Q+A with Stuart Smith, Zehnder Group UK

Overheating is becoming an increasingly important consideration in building design, with rising summer temperatures adding to the challenge of creating comfortable, energy-efficient buildings. But addressing the risk is not simply a question of adding cooling. Decisions around orientation, glazing, solar shading, ventilation and the building fabric can all influence how much heat enters a building and how effectively it can be managed.

Building Design speaks to Stuart Smith, commercial director at Zehnder Group UK, about designing for overheating from the outset, the balance between passive and active measures and why creating buildings that can respond to a changing climate requires a whole-building approach.

How should overheating risk be considered at the early design stage, and why is it important not to leave it until later in the project?

Overheating is fast becoming one of the most significant challenges facing the construction industry. Climate change has led to increasing summer temperatures and Met Office future climate projections, under a high emissions scenario, suggest the temperature of hot summer days could increase again in future decades.

This needs to be tackled at the design stage rather than treated as something to solve once the building is already taking shape. For residential projects, that means modelling against the requirements of Part O and TM59, while commercial buildings should be assessed against the principles set out in TM52.

For buildings at risk of overheating, this could mean fundamental changes to the design. Optimising glazing, solar shading and natural ventilation should be considered as preferred solutions to keeping the building cool. Where passive cooling cannot adequately control internal temperatures, for example because windows cannot be opened due to noise, security or pollution, active cooling systems may need to be considered.

The earlier this is done, the more options the design team has and the lower the risk of costly compromises later in the project.

What role should passive measures such as orientation, external shading, solar control, glazing and thermal mass play in keeping buildings cool?

It is useful to think about overheating through the idea of a ‘cooling ladder’. The most effective way to keep a building cool is to start outside, looking at the microclimate and external measures before moving towards passive measures within the building, and only then considering active cooling.

Cooling can start with quite simple interventions, such as trees, green spaces and water, which can influence solar gain. Similarly, lighter reflective materials can help reduce the amount of heat absorbed compared with dark stone or asphalt.

Heat also does not simply disappear when the sun goes down. Floors, walls and ceilings store heat and release it slowly, so good ventilation and overnight purge strategies have an important role to play in removing stored heat.

Where does ventilation fit within the cooling hierarchy, and how should it work alongside other passive measures?

When passive strategies have been properly considered and exhausted, it is time to consider active cooling solutions. Air conditioning is often the obvious choice, but it can be energy intensive and requires additional building services.

Modern ventilation systems can help address both temperature and indoor air quality. Mechanical ventilation with heat recovery (MVHR), for example, provides a continuous supply of filtered fresh air while also providing pre-heating in winter and pre-cooling of the supply air in summer.

Where additional cooling is required, this can build on the ventilation strategy. The important point is that cooling should be considered as part of the wider indoor climate strategy from the outset rather than as an afterthought.

How can architects and specifiers balance the need for airtight, energy-efficient buildings with the need to provide effective ventilation and prevent overheating?

It doesn’t have to be a choice between energy efficiency and effective ventilation. As buildings become more airtight and better insulated, greater emphasis needs to be placed on having a properly designed ventilation strategy alongside the building fabric. Without this, airflow can be restricted, contributing to issues such as mould and condensation.

The principle should be simple: insulate tight, ventilate right. This doesn’t only apply to tackling overheating in summer, but also to preventing mould and condensation in the colder months.

What particular challenges does overheating present in apartments and other residential buildings where opening windows may not always be practical or safe?

Overheating can be particularly challenging in apartments where noise, security, outdoor air quality, window restrictors and high-rise conditions can limit natural ventilation. Single-aspect layouts can also make effective cross-ventilation difficult, while solar gain, internal heat loads and increasingly warm night-time temperatures can cause heat to remain trapped within the home.

This means overheating needs to be considered as part of the overall building design rather than relying on occupants simply opening windows. Individual apartments within the same building can also experience different risks depending on factors such as orientation, position and opportunities for cross-ventilation.

How should Part O influence design and specification decisions, and where do you commonly see projects getting it wrong?

Approved Document Part O is a critical component of the Building Regulations, but compliance shouldn’t be treated as a box-ticking exercise. It needs to inform decisions around orientation, glazing, shading, ventilation and the building’s overall approach to managing heat from the beginning of a project.

One of the challenges we still see is a lack of understanding around what Part O actually requires and how the different measures work together. If overheating is only discovered during compliance checks, it can be difficult and expensive to make meaningful changes retrospectively. Better coordination between architects, engineers, contractors and manufacturers earlier in the process is key.

Are we too quick to see air conditioning as the answer to overheating, and what should designers consider first?

Definitely, but I understand why – it is the cooling technology everyone understands. While it is effective when used in the right way, it shouldn’t necessarily be the first response to a building at risk of overheating.

Air conditioning can be energy intensive and may have implications for refrigerant use, running costs and acoustic comfort. Before specifying it, we should be asking why the building is getting hot in the first place and what can be done to prevent or remove that heat.

That means starting with the building and its surroundings. Factors such as glazing, external shading and orientation should all be considered before moving further up the cooling ladder. Where active cooling is ultimately required, integrating it with the wider ventilation strategy can provide a more joined-up solution.

How should designers think about overheating in the context of future climate conditions, rather than designing only for today’s temperatures?

We shouldn’t be designing buildings for the climate we have today when we know that the climate is changing. A building that just passes an overheating assessment under current conditions may not provide the same level of comfort in 10, 20 or 30 years’ time.

Future climate scenarios therefore need to be part of the conversation at the design stage, particularly for buildings with a long operational life. We need buildings that are resilient and adaptable, rather than simply compliant at the point they are completed.

Is overheating really a ventilation problem, or does it point to a wider failure to take a whole-building approach to design?

Overheating isn’t simply a ventilation problem. It is a whole-building problem and if focus is put too much on one specific factor, you run the risk of missing the underlying causes. Every part of a building interacts, so the design needs to consider how everything from the ventilation system to the building fabric works together in practice.

The broader aim should be to design buildings through the lens of indoor environmental quality, considering air quality, thermal, visual and acoustic comfort together. Buildings are increasingly complex, and the systems within them need to work together if we are to create spaces that are climate resilient, energy efficient and healthy for occupants.