By Yogini Patel

After last week’s red weather alerts in the UK, the Passivhaus Trust’s Yogini Patel discusses how Passivhaus summer comfort strategies can help make UK buildings more resilient to overheating.

Last week’s extreme temperatures forced the cancellation of a number of London Climate Action Week events, including a number of our own Passivhaus site tours in London. The blistering heat, the hottest ever June in the UK, should be a wake-up call to all working in the built environment of the need to futureproof our buildings.

Extreme heat is becoming more frequent, more intense and longer lasting, particularly in urban areas where the urban heat island effect can further increase temperatures.

Creating buildings that are both low carbon and resilient to a changing climate is one of the built environment's greatest challenges. Passivhaus addresses both. Alongside dramatically reducing energy demand and carbon emissions, it delivers comfortable indoor environments throughout the year, helping occupants stay cool during periods of extreme heat.

Passivhaus works like a thermos flask

It is often assumed that because Passivhaus buildings are well-insulated and airtight they are more likely to overheat. However, this is not the case as Passivhaus works using building physics similar to that of a thermos flask. In winter, the aim is to retain heat, in summer, the Passivhaus envelope helps keep the cool inside. This is one of the reasons why Passivhaus is becoming more popular in southern Europe, as a solution for resilient cooling and heatwave protection.

Summer comfort

All buildings have the potential to overheat if summer comfort is not considered during design. Passivhaus design addresses summer comfort in numerous ways. As a comfort-driven approach, to meet the Passivhaus standard  internal temperatures must not exceed 25ºC for more than 10% of the year. UK guidance suggests a reduced target of 5% or less.

The summer comfort assessment in PHPP (Passive House Planning Package)  is dependent on the assumptions that are made during the modelling process. It is important to ensure that the result remains robust when those assumptions vary, for example as occupant behaviour changes, or as the climate warms. The Passivhaus Trust has developed detailed guidance covering design strategies to reduce risk, limitations, and constraints that may have an impact, key indicators of the likelihood of risk, and a series of stress tests to demonstrate robustness.

Passivhaus reduces overheating risk by combining several complementary strategies. These include optimising building orientation, limiting unwanted solar gains through effective external shading, reducing internal heat gains, designing efficient hot water systems and making best use of natural night-time ventilation. Where passive measures alone cannot maintain comfort, carefully designed active cooling can be incorporated with minimal energy demand thanks to the building's highly efficient fabric.

Summer comfort stress testing is an integral part of PHPP. In addition, Passivhaus certification requires written documentation of the strategy for thermal comfort in summer, signed by the building owner. As heatwaves become more frequent across the UK, designing buildings that remain comfortable without excessive energy use is becoming essential rather than optional. The Passivhaus standard provides a proven framework for delivering resilient buildings that protect both people and the climate. Explore our summer comfort guidance and join us at the UK Passivhaus Conference this November to discover the latest thinking from across the sector.