Overheating in UK Homes: Research Methods, Evidence, and Practical Implications

Why Overheating Matters 

As UK summers become hotter, the indoor thermal environment of existing housing stock is becoming a significant public health issue. The core recipe for managing overheating has four elements: keeping solar heat out, maximising ventilation at night, minimising ventilation during the hottest part of the day, and reducing internal heat gains. Each of these runs against instinct for most residents. Air conditioning was discussed as a last resort that may become realistic in certain contexts, but passive measures should always come first. 

Research Methods 

The principal field study, the Energy Follow-up Survey, monitored 750 homes with half-hourly temperature data and comfort questionnaires during the hot summer of 2018. It is representative enough to scale to national estimates, but behaviour and building fabric are difficult to separate, and results cannot easily project forward into a hotter climate. Controlled experiments at two paired semi-detached houses in Loughborough allow individual variables to be isolated with synthetic occupancy. Computer simulations support regulatory compliance assessment but have significant accuracy concerns. A climate chamber study examined the effect of cool mattress toppers on sleep. 

Key Findings 

Who overheats most 

Flats overheat significantly more than houses. Risk rises with occupant numbers, presence of children, and lower income. Local authority homes show the highest rates on raw temperatures. Social rented homes, once the higher proportion of vulnerable residents is factored in and the more stringent Category 1 criterion applied, reach a comparable level. 

Older residents are at acute and hidden risk. Measured temperatures show overheating rising steeply with age, while self-reported discomfort falls. Declining temperature perception and thermoregulatory capacity mean older people are genuinely very hot without knowing it. They also tend to prefer hot drinks even when overheated. Relying on resident complaints will systematically miss this group. 

Insulation and overheating 

Higher EPC-rated homes are warmer but not dangerously hotter: there is no statistically significant difference in hours above overheating thresholds between well and poorly insulated homes. Loft insulation actively reduces overheating risk, with 27% of well-insulated homes overheating against 35% of poorly insulated ones, because loft spaces reach extreme temperatures during heat waves and insulation acts as a barrier. External wall insulation helps according to modelling. Internal wall insulation carries a small risk that appropriate ventilation behaviour can fully offset. 

Sleeping in a different room 

In homes with an overheated bedroom, sleeping in the living room resolves the problem in 75% of cases, whether during a heat wave or across a whole summer. This works well in two-storey houses because bedrooms are upstairs in the hottest air, while downstairs rooms benefit from cooler air, ground-floor thermal mass, and distance from the hot loft. It does not work in flats or bungalows, where all rooms reach similar temperatures. 

Predicting overheating from dwelling data 

Overheating can be reasonably predicted from dwelling characteristics in bedrooms in flats, but not in houses, where occupant behaviour varies too much. Flats offer fewer behavioural options, making outcomes more predictable from physical data. A DESNZ-commissioned report on this is complete and due for publication shortly. 

Sleep and bedroom temperature 

The Home, Sleep and Health study of London flat bedrooms found a linear relationship between temperature and sleep duration, with no clear threshold. However, subjective comfort data shows a distinct change point at around 26 degrees Celsius, above which residents describe their sleep as unacceptable. This suggests the experiential threshold is clearer than the physiological one. Proximity to roads and railways was also studied as a barrier to night ventilation. 

What Works: Test House Evidence 

Curtains versus external shading 

Closing curtains helps on south-facing windows but has two problems: curtains absorb heat rather than reflecting it before it enters the space, and they block airflow from open windows. A north-facing bedroom with curtains closed all day was measurably hotter than one with curtains open, purely from reduced ventilation. External shading solves both problems: it intercepts radiation before it enters the building while leaving the window free. Test house experiments comparing external blinds against no shading showed indoor temperatures up to 6 degrees Celsius cooler. The test programme is currently running 13 to 15 different external shading systems in rapid succession to identify the most effective and practical options. 

New window installations routinely omit any discussion of inward-opening windows to facilitate external blind fitting. This is a missed opportunity at every replacement cycle. 

Night ventilation and passive grilles 

Night ventilation is the most effective behavioural tool available. High and low window openings create a convection stack effect that ventilates deeper into a space than a single opening. For single-sided ventilation, effective penetration extends to twice the room height with one opening and 2.5 times with high and low openings combined. A Loughborough passive house office building uses secure louvred grilles with insect screens that can be left open overnight without security or weather risk. Stephen asked whether this approach should be more widely adopted in homes. 

Communal heating pipe runs 

Uninsulated heat network pipes running through ceiling voids and risers in flats are a significant and underappreciated source of internal heat gain in summer, since communal systems continue operating to provide hot water year-round. Both insulation improvements and better controls offer quick wins. 

The Simulation Problem 

Overheating simulation models used for Part O compliance diverge substantially from measured temperatures even in controlled conditions. In a 2019 benchmarking exercise, four experienced modellers produced results broadly similar to each other but all significantly at odds with actual measured data in the Loughborough test house. A subsequent exercise in a new London flat block with six modellers and three software packages showed some predicting peak temperatures up to 10 degrees above measured values. The models appear to have systematic errors, likely because they were designed for energy demand prediction rather than peak temperature assessment. Compliance certificates should not be treated as reliable performance forecasts. 

Air Conditioning 

Air conditioning uses 10 to 30 times more energy than fans. If passive measures and fans achieve adequate comfort, that is strongly preferable. For certain building types, particularly urban flats, passive measures alone may be insufficient as the climate warms. Reversible air-to-air heat pumps providing both heating and cooling in one system are a rational option where heating also needs replacing. The electricity grid concern is mitigated by the fact that a grid sized for heat pump heating in winter should have capacity for summer cooling, provided passive measures are not abandoned in favour of air conditioning. 

Resident Communication

The most effective overheating management behaviours are counterintuitive: closing windows during the hottest part of the day, opening them wide at night, keeping curtains open on non-south-facing windows. Social landlords with smart device contact with residents are already using this channel to send targeted heat advice during heat events. The consensus in the room was that guidance needs to reach the standard of an airline safety video: simple, visual, universally accessible, and standardised across the sector. Language barriers, literacy, and differential information access mean the most vulnerable residents are the least likely to encounter guidance delivered only through conventional channels. 

Key Takeaways for Social Housing Providers 

• Overheating is already serious and will intensify. It is concentrated in the stock social landlords own and among the residents they house. 

• Do not rely on resident self-report to identify risk, especially in older person housing. Physiological changes mean older residents are unlikely to flag overheating even when it is severe. 

• Retrofit insulation does not create an overheating crisis. Proceed with confidence. Prioritise loft insulation and ensure ventilation guidance accompanies every programme. 

• External shading should become a standard retrofit and new build consideration. The evidence base is now strong. Design for it at the point of window replacement. 

• The Part O overheating simulation models are not reliable. Treat compliance as a regulatory minimum, not a performance guarantee. 

• Resident communication on heat management is low cost and high impact. A standardised, accessible sector-wide resource is worth developing collaboratively. Smart device channels can deliver it in real time during heat events. 

• Communal heat network pipe insulation and controls improvements are quick wins for reducing summer heat gain in flats.