Are UK Retrofit Standards Ready for Extreme Heat?

Why overheating is becoming a retrofit issue

For years, retrofit in the UK has mainly been discussed through the lens of winter performance. Warmer homes, lower heating bills, reduced fuel poverty and improved EPC ratings have rightly been central to the conversation.

But the climate context is changing.

The UK is getting warmer. The Met Office states that the UK has warmed at around 0.25°C per decade since the 1980s, and that recent years have been among the warmest on record. Temperature extremes are also becoming more frequent and more intense.

The 2022 heatwave marked a turning point, with the UK recording 40.3°C for the first time. The Met Office described 2022 as the UK’s warmest year in records back to 1884.

More recently, the Climate Change Committee warned that 92% of existing UK homes could be at risk of overheating by 2050 without adaptation measures.

That raises an important question for the retrofit sector:

Are our current retrofit standards moving quickly enough to address extreme heat as well as extreme cold?

Retrofit is no longer just about keeping heat in

Traditional UK retrofit strategies have focused on reducing heat loss. Measures such as external wall insulation, loft insulation, airtightness improvements and better windows are essential for improving winter comfort and reducing energy demand.

But as summers become hotter, retrofit design also needs to consider how buildings behave during heatwaves.

A home that performs well in winter may still be uncomfortable or unsafe in summer if solar gain, ventilation, shading and purge ventilation are not properly considered.

This is particularly important for:

• flats with limited cross ventilation

• top-floor properties

• homes with large areas of south or west-facing glazing

• urban properties affected by the heat island effect

• vulnerable residents, including older people and those with health conditions

The challenge is not to move away from fabric-first retrofit. It is to make sure fabric-first retrofit is designed for year-round performance, not just winter energy savings.

What PAS2035 already gets right

PAS2035 remains one of the most important frameworks in UK domestic retrofit. It provides a structured process for assessing, designing, installing and evaluating retrofit works.

The whole-house approach is one of its biggest strengths. It recognises that energy efficiency measures cannot be treated in isolation. Insulation affects ventilation.

Airtightness affects moisture movement. Heating strategy affects comfort and running costs.

PAS2035:2023 also introduced stronger emphasis on airtightness strategy. TrustMark guidance confirms that projects involving fabric measures may require an airtightness strategy, including consideration of airtightness targets and air leakage testing.

This is important because airtightness can improve energy performance, but if it is not paired with ventilation and overheating checks, it can contribute to unintended consequences.

PAS2035 also references the need to consider overheating. Available PAS2035 guidance notes that retrofit design should include measures to inhibit overheating during the installed life of energy efficiency measures and refers to CIBSE TM59 as a methodology for assessing overheating risk in homes.

That is a positive foundation. The issue is whether this is being treated as a central design driver often enough in practice.

The gap between regulation and future climate risk

The UK does already have a specific overheating regulation: Approved Document O.

Part O was introduced to reduce overheating risk in new residential buildings and came into force in England in June 2022. It focuses on limiting unwanted solar gains and providing means to remove excess heat from indoor environments.

However, Approved Document O applies to new residential buildings, not the vast majority of existing homes being retrofitted.

This creates a clear gap.

Most of the homes that will exist in 2050 have already been built, yet the main overheating regulation is focused on new-build housing. Retrofit projects may consider overheating through PAS2035, but there is not currently the same direct regulatory requirement for existing homes that Part O provides for new dwellings.

For councils, landlords and housing associations, this matters because much of the housing stock being upgraded today will still be occupied in 2050. If retrofit projects are designed only around today’s minimum requirements, they may not be resilient enough for future conditions.

Can insulation make overheating worse?

Insulation itself is not the enemy. Well-designed insulation can reduce both winter heat loss and summer heat gain through the building fabric. In many cases, good fabric retrofit can help keep homes cooler during the day.

Overheating risk can increase where homes are made more airtight without sufficient ventilation, where solar gains are not controlled, or where purge ventilation is limited.

Government-backed research into overheating risk from domestic retrofit found that, where homes are protected from unwanted solar gains and ventilated to remove internal heat, overheating is generally no worse after whole-house retrofit than before.

In some cases, retrofit can reduce overheating because heat gains through the fabric are reduced. The same research also noted that flats or homes with single-sided ventilation are more at risk under future weather conditions.

This is a crucial point.

The problem is not retrofit. The problem is retrofit that only solves one season.

Ventilation is now a summer resilience issue too

Ventilation has often been discussed in retrofit as a way to prevent condensation and mould. That remains essential.

But ventilation also needs to be seen as part of overheating resilience.

In winter, ventilation must remove moisture without causing unnecessary heat loss. In summer, it must help remove excess heat, support purge ventilation and keep indoor conditions safe during prolonged hot periods.

PAS2035:2023 brought ventilation requirements more closely in line with Approved Document F, and functional ventilation testing is now seen as a core part of ensuring systems perform as intended.

For retrofit designers, this means ventilation strategy should not be treated as a tick-box compliance exercise. It should consider:

• whether rooms can be effectively purge ventilated

• whether cross ventilation is possible

• whether noise, security or air quality issues prevent residents opening windows

• how mechanical ventilation performs during hot weather

• whether residents understand how to use ventilation systems correctly

A technically compliant ventilation system may still fail residents if it cannot be used comfortably in real-life conditions.

Solar gain, shading and glazing need more attention

Overheating is often driven by solar gain, particularly through poorly controlled glazing.

This is especially relevant for:

• flats with large areas of glazing

• homes with limited external shading

• top-floor dwellings

• south and west-facing elevations

Retrofit design should consider simple passive measures before mechanical cooling is introduced. These may include:

• external shading

• improved window specification

• reflective or lower g-value glazing where appropriate

• roof and loft insulation improvements

• ventilation paths for night-time cooling

• careful treatment of balconies, walkways and external features

In large housing portfolios, these considerations should be assessed by archetype. A ground-floor flat, top-floor flat and exposed end-terrace property may all need different overheating strategies, even if they share similar EPC ratings.

Should retrofit standards be updated further?

In our view, the direction of travel is clear.

PAS2035 has moved the industry forward by embedding whole-house thinking, airtightness strategy, ventilation considerations and monitoring. But as the UK climate continues to warm, overheating risk needs to become a more prominent part of everyday retrofit decision-making.

There is a strong argument for future updates to place greater emphasis on:

• future climate modelling, not just current weather assumptions

• overheating risk in existing homes, especially flats and vulnerable housing

• shading and solar gain control within retrofit design

• post-occupancy monitoring during summer periods

• clearer guidance for housing providers managing stock at scale

• resident handover guidance for hot weather ventilation and cooling behaviour

Building performance evaluation is also likely to become more important. PAS2035:2023 references further monitoring and evaluation alongside BS 40101, which provides a framework for evaluating actual building performance in occupied buildings.

This matters because predicted performance and lived experience are not always the same. If we want to understand whether retrofit is working in a changing climate, we need better feedback from homes after installation.

What this means for landlords, councils and housing providers

For housing providers, overheating should now be considered a strategic retrofit risk.

This does not mean every project needs expensive cooling systems. In fact, good passive design should always come first.

But it does mean asking better questions at the start:

• Could this property overheat after retrofit?

• Does the dwelling have adequate purge ventilation?

• Are there solar gain issues on certain elevations?

• Will residents be able to use windows safely and comfortably?

• Are vulnerable occupants at higher risk?

• Should monitoring be included after completion?

For larger programmes, these questions should be built into the design process at portfolio level.

A retrofit strategy that improves EPC ratings but creates summer discomfort is not a successful strategy. The next generation of retrofit must deliver homes that are warmer in winter, cooler in summer and healthier all year round.

A thought leadership challenge for the sector

The retrofit industry has made huge progress in recent years. PAS2035 has helped professionalise the sector and reduce the risk of poorly designed works. But the climate is changing faster than many buildings, policies and procurement models are adapting.

The question is no longer whether overheating should be considered in retrofit. It is how seriously we are prepared to treat it.

At Target Green, we believe retrofit design must evolve beyond short-term compliance and EPC uplift. The homes we improve today need to perform for decades, not just pass current assessments.

That means designing with future summers in mind, not only past winters.

We would be interested to hear from others across the sector: should overheating play a bigger role in retrofit standards, funding requirements and project briefs?

If you are planning retrofit works across housing stock and want to explore how overheating, ventilation and fabric performance should be considered together, we would be happy to continue the conversation.