Passivhaus Also Keeps You Cool:
Why Overheating Matters More Than Ever
When we first published an article on Passivhaus summer comfort in 2022, the UK had just experienced its first recorded temperature above 40°C. At the time, the event felt extraordinary. Now just a few years later, extreme heat is becoming a familiar feature of the British climate. We are entering our fourth heatwave of the year with more expected
According to the Met Office, the UK’s climate is warming at around 0.25°C per decade, and temperatures above 30°C are now far more common than they were in previous generations. They now estimates there is roughly a 50% chance of the UK exceeding 40°C again within the next decade.
The challenge is that most of our housing stock was just not designed for these conditions. As more than a third of UK homes were built before 1945, at a time when preventing overheating simply wasn’t considered a priority. Meanwhile, heatwaves are becoming longer, hotter and more frequent, placing increasing pressure on homes, workplaces, infrastructure and our health.
What about El Niño?
Adding to these longer-term climate trends is the emergence of a potentially strong El Niño event in the Pacific Ocean. Although El Niño is thousands of miles away, it influences global weather patterns and typically contributes to higher global temperatures. Forecasts suggest the current event could become one of the strongest on record.
In terms of the UK, this can result in a greater risk of unusually warm periods, more variabile weather and increased pressure on water resources. Research from the Met Office also suggests El Niño can influence UK winter weather patterns long after the initial event, which highlighting how interconnected our climate systems have become.
What is increasingly clear is that designers and homeowners can’t treat overheating as an occasional problem. New homes must be designed and existing homes retrofited to remain comfortable in a future climate that is likely to feature more frequent heatwaves and warmer average temperatures.
This is where Passivhaus excels
Passivhaus in the UK is usually associated with ultra-low energy use and exceptional winter comfort. However, Passivhaus is equally focused on keeping buildings cool during the summer months. This is achieved through careful design, shading strategies, ventilation planning and rigorous analysis. Passivhaus buildings are designed to provide comfortable indoor temperatures throughout the year, and will not achieve certification without managing this risk.
The physics behind Passivhaus hasn’t changed since we first explored this topic. But, as our climate continues to warm, the benefits of designing for summer comfort is becoming increasingly important. Raised internal temperatures are not comfortable, plus they are associated with a rise in mortality rates and increased exacerbation of chronic diseases, especially amongst the frail and vulnerable. Quality of sleep can be impaired, and the sense of general wellbeing is decreased.
Buildings overheat because of uncontrolled solar heat gains through windows or glazing which rapidly warm a space to uncomfortable temperatures. It’s perceived to be more of an issue in well insulated and airtight buildings where the heat can get easily trapped into the house. We’ve all been in those horrid office blocks where the windows were sealed to stop winter drafts, but there hasn’t been a lot of thought given to summer cooling.
The standard defines overheating as when the percentage of hours per year that the interior ambient temperature exceed 25°C with a maximum limit of 10%. If the percentage is higher, it is not possible to achieve certification. However, it is generally considered that an upper limit of 3% of hours is more appropriate, particularly important when considering climate change with many places getting warmer.
Designing Out Overheating
There are several design measures and strategies that can be implemented to minimise this risk, and we will explore these with you. The options include:
- Accurate design of the building aspect, shape and internal layout. This is one of the pillars of bioclimatic design, i.e. a design approach that takes account of the microclimate, geography, and topography of the site to ensure thermal comfort.
- Solar radiation control through:
- The use of shading elements. These devices work by blocking solar radiation before they get into the house and can include shutters, brise-soleil or canopies etc.
- The use of vegetation. Deciduous plants can be a good solution as they guarantee shading in the summer, they then also let solar radiation and light in the winter. Also, Climbing plants on the walls can absorb the heat and prevent this from getting transferred inside.
- Glazing with low g-values, i.e. the percentage of solar radiation that can enter into the glass can help control overheating. However, these will also reduce the useful solar gains into the house in winter.
- Accurate design of the windows and their orientation. Orientation is really important in terms of heat gains and particular attention needs to be paid to the south and west elevations for our hemisphere. Here in the late afternoons in the summer, when the sun gets low and the temperature rises, it might be difficult to find adequate shading. If this is an advantage in the winter months, it might be the cause of overheating in the warmer ones.
Passive and Active Cooling Strategies
Solar control and building orientation reduce heat gains. Materials and building systems also affect summer comfort. These strategies help manage, absorb and remove excess heat.
- Use light-coloured cladding materials. Dark colours absorb more heat. Light-coloured finishes can reduce overheating risks.
- Use thermal mass where appropriate. Masonry, concrete, earth blocks and cob can absorb heat during the day. They then release it slowly as temperatures fall.
Ventilation and Heat Managment
Good ventilation helps maintain comfortable indoor temperatures during hot weather.
- Design for natural cross-ventilation. Place openings on opposite sides of the building where possible. Use height differences to create a stack effect. Openable roof lights above stairwells can improve airflow.
- Reduce internal heat gains. Appliances, electronics and lighting all generate heat. Reducing their use can improve summer comfort.
Furutre-Proofing Homes for the Warmer Climate
UK summers are becoming hotter. Heatwaves are becoming more frequent. Homes need to remain comfortable in future climates.
- Include cooling where necessary. A whole-house plan can help future-proof a home against rising temperatures. Air source heat pumps can provide both heating and cooling. Photovoltaic (PV) panels can power cooling using excess summer generation. This approach can offer a low-carbon or potentially carbon-neutral solution.
If you are considering a Passivhaus project, get in touch here. Alternatively, head over to our contact page so we can find out how we can help you with your plans.
