Passivhaus architect Sean Ronnie Hill explains how simple passive design measures plus AECB-aligned targets could be the key to cutting peak summer loads in UK homes.
For all the attention given to heat pumps, smart home dashboards and active cooling systems, we are still missing the simplest truth about designing for summer comfort: overheating is, above all, a fabric problem. A home that keeps heat out in summer is the same home that uses less energy in winter. A home that can reliably shed heat overnight is one that avoids mechanical cooling for most of the year. Before we reach for gadgets, we should be reaching for the basics: shading, airtightness, insulation continuity and good building physics.
Across dozens of London retrofits and extensions, the pattern is unmistakable. Overheating risk is rising, driven by climate change, denser urban sites, increased glazing ratios and the persistent assumption that ‘north London doesn’t need cooling.’ Many of the homes we assess regularly exceed 28°C in summer without intervention. Yet with measured, fabric-first strategies, peak temperatures can often be reduced by 2-4°C – frequently enough to avoid mechanical cooling altogether.
But where do projects go wrong, and does why aligning with AECB and Passivhaus summer comfort metrics give UK homes a fighting chance in the decades ahead?
The UK’s overheating problem is no longer theoretical. Historically, UK homes struggled far more with cold than heat. That balance has shifted decisively. Met Office data shows that the UK’s 10 warmest years on record have all occurred since 2002. In cities, the urban heat island effect compounds the problem, with night time temperatures in central London often sitting 4-7°C above surrounding rural areas. At the same time, the default architectural response to extensions has become glazing heavy, with floor-to-ceiling glass facing south or west and little or no external shading.
In one recent retrofit, thermal modelling showed a typical 1950s terrace with a full-width glazed rear facade exceeding 28°C for 12% of the year – well beyond the commonly accepted 1-3% thresholds. Crucially, the problem was not the presence of glass itself, but the absence of shading, unmitigated internal heat gains and night time ventilation constrained by noise and security concerns. These issues are entirely solvable, but only if they are addressed early in the design process.
Before you think about technology, start with orientation. One recurring pattern, particularly in London, is that overheating has little to do with how ‘modern’ or well-finished a space feels. It correlates far more strongly with where, and how, the sun strikes the building. South-facing rear extensions typically receive high-angle summer sun, which is relatively easy to control. Well designed external shading – whether brise soleil, fins, deep reveals or modest overhangs – can block 70-80% of unwanted solar gain without compromising winter daylight.
West-facing glazing is more problematic. Low-angle afternoon sun is harder to shade and often coincides with peak internal heat gains. In these situations, we often combine external blinds or shutters with louvred pergolas, carefully placed deciduous planting or, in some cases, a deliberate reduction in glazing area without sacrificing spatial quality.
North-facing spaces are often assumed to be ‘safe,’ leading to unnecessarily large glazing areas. Without insulation continuity and airtight detailing, however, they leak heat in winter and admit diffuse gains in summer. Good fabric performance matters here just as much as on sunnier elevations.
None of these strategies is inherently expensive. What becomes costly is designing a glass box first and attempting to fix it later with mechanical cooling.
The quiet power of insulation, airtightness and continuity, a persistent misconception is that insulation increases overheating. Insulation only traps heat if solar and internal gains are poorly controlled. A well insulated, airtight home with effective shading remains cooler for longer because heat transfer from outside is slowed, cool night time air is retained within the fabric, and hot external air is prevented from leaking in. In EnerPHit and AECB-aligned retrofits, we consistently observe lower daytime peaks, more stable overnight temperatures and a dramatically reduced reliance on fans or active cooling.
When insulation and airtightness are paired with MVHR, summer comfort improves again. MVHR does not cool a home, but it allows controlled night time ventilation without opening windows on noisy or polluted streets, helping homes shed heat passively without compromising security or air quality. Summer comfort emerges from the relationship between envelope performance and ventilation strategy, not from a piece of technology.
Shading & ventilation
If there is one habit UK designers and homeowners should adopt, it is this: design shading before designing glazing. External shading reduces solar gain by orders of magnitude more than internal blinds, which simply trap heat on the wrong side of the glass. Yet shading is still widely seen as optional or unnecessary in our climate.
In practice, shading alone often reduces peak summer temperatures by 1.5-3°C. When combined with night time purge ventilation, reductions of 4°C or more are possible, often eliminating a need for active cooling. Deep timber reveals, orientation specific steel fins, overhangs sized through solar modelling, and integrated external blinds can become architectural assets rather than compromises.
UK homes still rely heavily on opening windows for summer comfort. In dense urban areas, this approach often fails due to noise, pollution, insects, and security.
The most successful projects combine multiple passive strategies: cross ventilation where the plan allows, stack ventilation using stairwells, secure night time purge routes and MVHR systems with summer bypass. High-level openings such as rooflights or clerestories are particularly effective. Each litre of warm air leaving at the top of the house draws cooler evening air in at lower levels, delivering comfort gains disproportionate to their cost.
Why AECB targets matter
The AECB performance standard provides one of the clearest and most practical frameworks for UK residential design. It sets realistic limits for glazing ratios, shading, summer comfort metrics, fabric U-values, airtightness and ventilation, while remaining aligned with typical UK budgets. For many clients, the assumption is that tackling overheating requires cooling. In reality, most projects meet summer comfort targets simply by aligning with AECB benchmarks. This fabric-first approach also supports wider policy goals, from retrofit incentives to the transition away from gas and long term reductions in operational energy use.
Across Victorian terraces, post-war homes, mews houses and 1930s semis, the same lessons repeat. Glazing itself is rarely the problem; poorly designed glazing is. Shading and night ventilation consistently outperform smart thermostats. Fabric-first approaches deliver comfort in both summer and winter, while internal heat gains from appliances and lighting are often underestimated. Most importantly, overheating risk must be modelled before planning, not discovered after construction.
In one EnerPHit mews retrofit, overheating hours were reduced from 12% to under 1% through shading, airtightness, MVHR with summer bypass and secure night time purge paths. Generous glazing remained – comfort and daylight are not mutually exclusive.
Cooler thinking for a hotter future
We are moving towards a climate where homes must remain comfortable at external temperatures of 34-36°C, not the 28-30°C peaks of recent decades. Mechanical cooling will become more common, but it should remain a last resort, not a starting point.
The UK’s best chance lies in homes that keep heat out, dump heat at night, use airtightness and shading intelligently, and rely on simple physical principles rather than complex systems. Fabric-first is not a slogan; it is a survival strategy for a warming climate.
Architects, clients and contractors who embrace it now will create cooler, healthier and more resilient homes – and avoid costly retrofits in the years ahead.
Sean Ronnie Hill is an architect & Passivhaus designer