Passivhaus summer comfort strategies

As UK summers grow hotter, the need to prevent overheating in buildings is becoming increasingly important. All buildings have the potential to overheat if summer comfort is not considered during design. Passivhaus design addresses summer comfort in numerous ways. Find out the key Passivhaus strategies employed to keep our homes and buildings cool, and the situations in which additional active cooling might be needed.

Agar Grove Passivhaus, Camden Council

The Passivhaus ‘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.

The background is physics: Heat flows from hot side to cold side. In winter, from inside to outside. In summer, from outside to inside. In both cases, insulation reduces heat flow and maintains the desired temperatures for the occupants.
In both cases, a small amount of energy (heating in winter / cooling in summer) may be required to keep temperatures as comfortable as possible.

Wolfgang Feist, Founder, Passivhaus Institut

What makes buildings overheat?

If the amount of heat that is entering the building (gains) exceeds the amount that is being dissipated by either passive or active cooling (losses), then the internal temperature will start to rise and, if not addressed, an overheating situation will arise. However, if losses are matched to the gains, then the building remains in balance. In most temperate climates, the primary overheating driver is typically solar energy transmitted through glazing. However, internal gains can also be a significant contributor, particularly in multi-residential or non-domestic buildings.

What makes a building overheat? Image credit: Passivhaus Trust

A warming climate

As global temperatures rise, a cautious and consistent method of assessing the risk of summer overheating is needed to ensure that Passivhaus buildings remain comfortable both today and in a warming climate. The climate data used in PHPP (Passivhaus Planning Package) modelling is historic but allows for easy testing with increased summer temperatures to account for our warming climate.

Passivhaus summer comfort requirements

Plashet Road, Levitt Bernstein. Photo by Kimbo Fidelo Sito.

As a comfort-driven approach, the Passivhaus standard includes a summer overheating criterion which requires that internal temperatures do not exceed 25ºC for more than 10% of the year. In practice, many Passivhaus designers often aim for 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. 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. A short training course on optmising summer comfort in Passivhaus projects is avalable on on demand and is FREE to access.

While Passivhaus is better known for reducing energy bills and keeping homes warm in winter, its performance in extreme heat is just as impressive, if not more critical in our warming climate.
In summer, the goal flips: we’re no longer retaining heat, we’re retaining coolth. If your windows are shaded and you manage solar gain, your home acts like a thermos, keeping indoor temperatures stable even during extreme external heat. During a 40°C+ heatwaves, we’ve kept our home at a steady, comfortable 26°C, without air conditioning!

Marion Baeli, Principal - Sustainability Transformation, 10 Design

Passivhaus design strategies for summer comfort

Optimising solar gains to avoid overheating

Building orientation & glazing

Building orientation should be optimised as far as possible to benefit from the opportunity of solar gains in the winter without the risk of too much gain in the summer. The ideal situation is a north-south orientation with daylight-optimised glazing on the north façade and somewhere between 15 and 25% glazing on the south façade.

External shading

External shading systems are highly effective in reducing solar gain. Most preferred are systems which are fixed and require no movement or occupant action to be effective. These types of systems include brise soleil and overhangs. In situations where fixed shading is not appropriate, deployable shading such as shutters, blinds or awnings are all effective.

Internal shading

Where external shading is not feasible, internal shading can be considered. However, this method is far less effective than external shading. External shading can reduce solar gain by between 80 and 100%. In contrast, even the most effective internal shading will only reduce solar gain by a maximum of 40% and in most cases it will be considerably less than this.

Glazing g-value

A lower glazing g-value will reduce the amount of solar gain. However, this will reduce both winter and summer gains and so the reduction in overheating risk achieved will need to be balanced against the lower winter gains. A lower g-value will also reduce the level of daylighting throughout the year and have an impact on the quality of the views. For individual homes g-values are often the maximum possible, however, using lower g-value glazing in specific locations where there is a particular risk of overheating can be an effective strategy

Old Holloway Passivhaus - Summer comfort

Reduce or minimise internal heat gains

Design of domestic hot water (DHW) systems

One of the primary contributors to overheating is often the domestic hot water system. This is particularly true in multi-residential buildings where there can be long lengths of hot water pipes. In a typical UK dwelling more than half the energy used for hot water can be attributed to wild heat losses within the building. To reduce hot water losses, a number of strategies can be employed. PHPP includes modelling of hot water systems and gives designers feedback on how efficient designs are.

Appliances

There will also be many other devices, including clothes dryers, refrigerators, freezers, dehumidifiers, ovens, computers, printers, within the building that generate heat. Appliances, especially those that are always on, can have a significant impact on overheating risk and so need to be carefully considered and modelled in PHPP.

Maximise passive cooling potential

Once gains have been minimised, there are several design strategies which can be adopted to maximise the ability of the building to achieve effective passive cooling. In the UK’s climate, we can typically expect to be able use cooler air from outside to help cool the building if overheating has started to occur. Even during warm summer days, nighttime temperatures usually drop to 15–20 °C, and ‘tropical nights’—when it stays above 20 °C—are still rare, for now. Thus, moving cooler outside air through the building, particularly at night, is a primary cooling mechanism.

Window ventilation

Moving cooler outside air through the building is primarily achieved by opening windows. Thus, the building’s windows are critical factors in achieving sufficient passive cooling.

Cross-ventilation

The amount of air flow that is achieved by cross-ventilation (i.e. air flow from a window on one façade, through the building to a different window on a different façade) is significantly higher than the air flow achieved, even through multiple windows, on a single façade. Thus, cross ventilation should be included in the design wherever possible.

Overnight ventilation

Overnight ventilation is likely to be the most effective in achieving cooling as this is when the outside air will be at its coldest. However, night-time ventilation is also when there are the most significant limitations on window ventilation – e.g. noise, security (particularly if unoccupied), drafts, insects and closed internal doors. The windows, and the associated openable areas, that can realistically be used for overnight ventilation should be clearly identified as part of the design and reduction factors applied where ventilation is likely to be compromised.

Mechanical ventilation

The MVHR (mechanical ventilation with heat recovery) systems within a Passivhaus building can help with reducing overheating risk. The summer bypass mode will ensure that when the outside air is cooler than the internal setpoint, and the internal temperature is too high, cooler external air is brought directly into the building, without being heated by the outgoing warmer internal air. In situations where window opening is not possible, or likely to compromised by noise, pollution or security, the MVHR may need to provide all the passive cooling required to address overheating risk, and must be sized accordingly.

For more details on any of these strategies see PHT Guidance: Avoiding Summer Overheating

Active cooling

Active cooling has long been part of Passivhaus design in warmer climates but is still a relatively new consideration for the UK. Ordinarily, in the UK's cool temperate climate, the Passivhaus Standard has been able to deliver summer comfort through the passive cooling methods and strategies outlined above. However, as temperatures rise due to global heating, we expect to see more active cooling solutions, to address summer heatwaves. The Passivhaus Trust is currently exploring how these tried and tested Passivhaus active cooling measures can be deployed in the UK.

The Passivhaus Trust's upcoming webinar on 'Summer comfort in a warming world' brings together expert speakers to explore how active cooling is modelled in PHPP (Passive House Planning Package), and share practical strategies and lessons from UK and Mediterranean case studies. The webinar will also seek to gather feedback from UK project teams on overheating and comfort challenges. Attendees will have the opportunity to engage in a Q&A session, share experiences, and to be part of the conversation about staying cool in a changing climate.

Read the Trust's free to download Avoiding summer overheating guidance paper