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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.
F
or 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 retrofi ts 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-fi rst 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 fi ghting 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 Offi ce data shows that the UK’s 10 warmest years on record have all occurred since 2002. In cities, the urban heat island effect
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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 fl oor-to-ceiling glass facing south or west and little or no external shading. In one recent retrofi t, 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-fi nished 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, fi ns, 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
ADF JANUARY 2026
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