Are London’s homes ready for a heatwave?

Extreme heat is becoming an increasing problem in London as a result of climate change. The 2022 heatwave saw record temperatures recorded across the UK, with London hitting 40°C that July. The London Assembly Planning and Regeneration Committee is investigating whether London's homes are ready for a heatwave. As part of this investigation, the Committee has launched a Call for Evidence, open to all who would like to respond. The Passivhaus Trust is calling for its members to respond.

Aerial view London. Image credit: pexels-jhuneblue-20427131

The consultation centres on two key questions:

What is the overheating risk currently facing London’s homes and how is this risk evolving?
How should we be tackling overheating?

TAKE ACTION

Respond to the Consultation

Please see Passivhaus Trust responses to 7 key questions below. You can use our key bullet points below to inform your responses but please submit your own response.

A KEY AREA OF FOCUS SHOULD BE RESPONSES TO QUESTION 2(e).

DEADLINE FOR RESPONSES: Friday 1 May 2026.

Consultation Question 2(e)

e. Are there developments in London that manage heat risk well?

PASSIVHAUS TRUST ANSWER

The Passivhaus Trust is currently collating POE data on Passivhaus buildings in London to provide evidence of summer comfort performance. Data collected to date includes:

Harris Academy, Sutton (Passivhaus certified secondary school)

Internal temperatures monitored for 2 years "For the most part, despite heatwaves or very low outdoor temperatures, comfort levels are well maintained year round".

Page 102 Measuring Mass Timber report, March 2025

In addition to responding to this question individually, please get in touch at info@passivhaustrust.org.uk if you are aware of any POE data on summer comfort from (London or other UK) Passivhaus projects.

Consultation Question 1(d)

d. How do different building typologies contribute differently to the urban heat island effect and overheating in London ?

PASSIVHAUS TRUST ANSWER

Passivhaus certified buildings are likely to be less susceptible to overheating and offer better summer comfort than standard buildings.*

Passivhaus design addresses summer comfort in numerous ways. 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 Standard includes 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.

*It is important to acknowledge that early Passivhaus projects in the UK did, in some instances, experience summer comfort issues due to incorrect design assumptions and limited early guidance. These experiences contributed to significant improvements in the Passive House Planning Package (PHPP), certification processes, and UK-specific guidance developed by the Passivhaus Trust and others.This evolution reflects the strength of the Passivhaus approach: a performance-based methodology that responds to operational evidence, continuously refining tools and standards. Far from claiming perfection, it is a system that learns and improves over time to deliver consistently high levels of comfort, including in summer conditions.

Building typologies that might contribute to overheating in London include: .

Highly glazed buildings increase the risk of overheating, and also have a high likelihood and demand for active cooling.
Older buildings lacking insulation, particularly in the roof, are at risk of overheating and a ‘night time radiator’ effect in the bedrooms from the roof.

Consultation Question 1(f)

f. Are homes being built now which will be vulnerable to overheating? If so, which factors contribute to this (planning, building regulations, energy efficiency standards, etc)?

PASSIVHAUS TRUST ANSWER

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.

Dr Wolfgang Feist, founder of the Passivhaus Institut, commented:

“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.”

Two recent academic studies back this up:

Instead of looking at insulation, we should look at the effect of large glazing ratios and gains from services and appliances on overheating.

Glazing: 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.

Internal gains: Internal gains from appliances and hot water storage can also be a significant contributor to overheating, particularly in multi-residential or non-domestic buildings. The Passivhaus standard’s primary energy requirement means that designs manage and model internal gains within PHPP to avoid wasting energy and to minimise overheating risk. Regarding internal gains, it is worth considering the potential overheating impact of district heat networks on homes and buildings. Unlagged HIUs, high temperature pipes etc can all contribute to overheating.

Consultation Question 2(a)

a. What changes would you like to see made to the London Plan to better manage the risk of overheating in homes?

PASSIVHAUS TRUST ANSWER

Passivhaus certified buildings should be encouraged more within the London Plan*. This could be achieved by adopting the recent proposal that the London Plan should increase the proportion of carbon savings needed on site through energy efficiency measures, to reduce demand on the electricity grid. If absolute metrics are used to measure this, delivering to the Passivhaus standard should be an easy way for projects to achieve these savings. The Passivhaus Standard includes 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.
Communal district heat networks can potentially increase the risk of overheating. Even when done well, there are inevitably heat gains from the internal circulation with communal heating. Passivhaus buildings can achieve the carbon energy and affordability goals of the London Plan without the need for communal heating. Another way to reduce overheating risk in the London would be to state within the London Plan that Certified Passivhaus projects are automatically exempt from having to connect to heat networks.
In addressing overheating specifically, it is important that policy supports a balanced, design-led approach. This should follow a clear hierarchy which prioritises fabric efficiency, shading, and passive design measures first, while recognising that a combination of strategies may be required to ensure comfort in all dwellings – see response to ‘2b’ below. Natural ventilation strategies, such as appropriately sized openings, should be considered alongside other key requirements including purge ventilation, daylight, views, and acoustic performance. However, it is important not to assume that increasing ventilation alone is sufficient in all cases, particularly in dense urban environments or under future climate scenarios. In some cases, a limited and carefully considered use of active cooling may be appropriate as part of a wider overheating mitigation strategy. Where required, this should be highly efficient, low-carbon, and justified through a robust overheating risk assessment, with passive measures exhausted first.

Consultation Question 2(b)

b. Is the London Plan’s current stance on air conditioning [Policy SI 4] still appropriate? ?

PASSIVHAUS TRUST ANSWER

The Passivhaus Trust considers that the overall intent of Policy SI4 remains appropriate. The emphasis on avoiding reliance on mechanical cooling and following a clear cooling hierarchy is well aligned with the principles of fabric first design and with the Passivhaus approach to summer comfort.

The Cooling Hierarchy which includes reducing internal gains, limiting solar gains, supporting passive ventilation, and only then considering mechanical cooling closely reflects the Passivhaus methodology. However it is important that this hierarchy is understood as a sequence of design considerations rather than a requirement to avoid active cooling altogether where it may be beneficial or necessary.

There is growing recognition that in some contexts particularly in dense urban housing or under future climate conditions very low energy active cooling may form an appropriate part of a wider overheating strategy. This can include highly efficient systems with very low energy demand used in combination with passive design measures to ensure comfort. Such approaches should not be seen as undermining SI4 but as a complementary evolution where passive measures alone may not be sufficient.

The requirement for Dynamic Thermal Modelling using CIBSE TM59 remains a useful compliance framework but some limitations and unintended consequences should be recognised. In particular concerns have been raised that assumptions within TM59 such as very high internal gains and high occupancy levels may not always reflect realistic operational conditions. In some cases this may influence design decisions towards increased glazing or ventilation areas even where other factors such as solar control or overheating resilience may be more effective.

Similarly, assumptions about window opening behaviour may not fully reflect occupant behaviour during extreme weather conditions where comfort thresholds can limit the effectiveness of increased ventilation alone. It is also important to note that where outdoor temperatures consistently exceed comfort thresholds natural ventilation regardless of opening size has limited capacity to maintain acceptable indoor conditions.

There is, therefore, a case for further refinement of overheating assessment methodologies to ensure they support balanced design outcomes and avoid unintended optimisation towards a single strategy.

The Passive House Planning Package (PHPP) framework remains valuable but should continue to evolve alongside improved understanding of building performance occupant behaviour and climate adaptation.

More broadly, the experience of Passivhaus projects in the UK has demonstrated that while early designs sometimes experienced summer comfort issues due to limited assumptions or modelling approaches subsequent refinements to PHPP certification criteria and guidance from the Passivhaus community have significantly improved outcomes. This reflects an evidence led approach that continuously adapts to operational experience rather than assuming fixed perfection.

Finally, it is important to recognise that cooling hierarchy approaches are most effective when interpreted as cumulative rather than exclusive. Passive measures remain essential but in some cases they need to be complemented by low energy active cooling solutions to deliver safe comfortable and resilient buildings. Where used these systems should be efficient quiet and designed to work in synergy with fabric performance for example through tempering of supply air or low energy radiant approaches rather than high energy space cooling alone.

Consultation Question 2(c)

c. What should developers do to ensure that the homes constructed today will be heat resilient?

PASSIVHAUS TRUST ANSWER

Passivhaus design addresses summer comfort in numerous ways.

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.

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.

Marion Baeli, Principal - Sustainability Transformation, 10 Design has commented:

“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!”

Passivhaus design strategies for summer comfort

i) 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 for primary façades, with daylight-optimised glazing on the north façade and somewhere between 15 and 25% glazing on the south façade. It is also recommended that glazing is adjusted by solar exposure as well, with different window sizes being accepted on an elevation.

External shading

External shading systems are highly effective in reducing solar gain. The simplest solutions 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.

ii) 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. Passivhaus also supports quality control in delivery, to ensure the systems perform as designed.

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.

iii) 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-ventilationThe 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.

An MVHR with a ‘bypass cooling boost’ function has been found to work well in such situations. When the air temperature outside is higher than inside, and uncomfortably warm, the MVHR can revert to ‘recovery mode’ and reject most of the incoming heat into the exhaust air, maintaining comfort indoors for longer. MVHR technical design that enable this to work include: Good summer bypass logic; Units which actively prevent air going through the heat exchanger, rather than just providing an alternative route as well as allowing air to go through the heat exchanger; Allowing the option to boost ventilation rates; Acoustic design of the system to avoid disturbing occupants when the system is operating in this mode.

In Pasivhaus buildings the airtight envelope (combined wtih MVHR) also minimises the escape of heat via infiltration and exfiltration via fabric leaks and vents, while maintaining the required flow of fresh air. By analogy, when outside temperatures are uncomforabely high and exceed the indoor temperature, the Passivhaus airtight envelope conbined with MVHR minimises the ingress of uncomfortably warm air and the loss of more comfortably cool air.

iv) 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 needed, to address summer heatwaves and ensure summer comfort. The Passivhaus Trust is currently exploring how these tried and tested Passivhaus active cooling measures can be deployed in the UK. The Passivhaus standard is used worldwide so already has an energy efficiency criterion for cooling demand and calculates this in PHPP.

Examples of London Passivhaus projects designed with active cooling include:

For Passivhaus standard homes and buildings, the cooling load is minimised, making it easier to deliver cooling discreetly, eg via a small unit. Cooling can also be delivered in a Passivhaus via a wet heating system (single dwelling or communal ASHP) so there is flexibility about active cooling building services options. This approach has been used in Passivhaus with underfloor heating/cooling and ASHPs, and is also possible on ambient loops. Compact units that need no outside plant, can also provide cooling.

Even where active cooling is not deemed ‘necessary’, under for example CIBSE TM59, the London Assembly should be mindful of the increasing frequency with which building occupants will retrofit active cooling. Passivhaus buildings that have a low cooling load will be kept comfortable wtih smaller and less intrusive plant, lower energy use, and less rejected heat.

PHT Guidance: Summer Comfort Statement

PHT Guidance: Keeping Cool: Avoiding Summer Overheating

PHT on-demand course Keeping Cool: avoiding overheating risks

Consultation Question 2(d)

d. What support should be made available to homeowners and tenants to better manage overheating risk?

PASSIVHAUS TRUST ANSWER

If window, cross-ventilation and overnight ventilation strategies have been designed to be part of a Passivhaus building’s summer comfort strategy then the building occupants/ facilities managers would need to be educated as to how to undertake this. Such guidance should be made available as part of the building handover.

The Passivhaus Trust provides guidance on occupant handover in Chapter 10 of its 'How to build a Passivhaus' publication.

DEADLINE FOR RESPONSES: Friday 1 May 2026.