RETROFIT & SUSTAINABLE BUILDINGS
Cooling without the carbon: the case for natural night ventilation
To mitigate the recent heatwave phenomenon, WindowMaster is harnessing its expertise as a hybrid-ventilation specialist to advocate natural night flushing as a simple, passive solution to cool down city centre and vulnerably-located buildings overnight
H
eatwaves in the UK are becoming more frequent and more severe. According to the Met Office, the number of days exceeding 28°C has more than doubled since the 1961-1990 baseline, and very hot days above 30°C have more than trebled. The South East of England now averages over 12 such days per year.
As buildings heat up during the day and fail to cool overnight, schools, offices and public buildings are turning into heat stores. WindowMaster’s suggested solution is automated windows that use the cool night- time hours to regulate indoor temperatures, without the need to engage expensive air conditioning systems.
Passively tackling rising temperatures
The UK’s heat problem is worsening. The summer of 2022 saw an estimated 2,985 heat- associated excess deaths in England alone, according to the UK Health Security Agency (UKHSA). The Office for National Statistics (ONS) recorded 3,271 excess deaths across England and Wales during the same period. The UK’s statutory mandated independent adviser, the Climate Change Committee (CCC) warns that without adaptation, heat- related deaths could triple to exceed 10,000 per year by 2050. The West Midlands records the highest rate of heat-related mortality per million population, while London and the South East account for the highest absolute numbers of heat-associated deaths. Schools, offices and public facilities across England are increasingly turning into heat stores as days grow hotter and buildings fail to cool overnight.
Natural cooling instead of artificial air conditioning
Night flushing is a straightforward, sustainable approach to refreshing a building’s ambient temperature. The principle is simple: it draws on the temperature difference between hot days and cooler nights. In the evening, fresh outdoor air flows in through automatically controlled windows, carrying accumulated heat away with it. The heat stored in walls, ceilings and furniture during the day escapes overnight. By morning, the building is already cooler, before anyone has arrived. Mainstream hybrid systems, including those
levels, humidity and weather conditions. When conditions are right, the windows open
from WindowMaster, manage this process automatically. Sensors monitor temperature, CO2
City of Glasgow College; Credit: Edmund Sumner
on their own and close again as soon as wind or rain picks up. Window openings stay within burglar-proof gap widths throughout.
A refreshingly efficient solution
Night flushing works passively – using natural air currents, it saves electricity and is kind to the environment. A peer-reviewed study from London South Bank University found that natural ventilation can reduce building cooling energy use by between 13% and 40% compared to mechanical systems. Reducing reliance on the use of energy- intensive cooling equipment lowers operating costs, while comfort and wellbeing improve at the same time.
The technology works in both new builds and existing buildings: easy to retrofit motorised window actuators fit directly onto existing windows, so there’s no need for complex duct installation. Fitting typically takes just a few days, making it well suited to school refurbishments during the holidays. For example, the urgency for schools is real.
Future-proofed air handling Rhoss ADV-DNAIR AHUs from Klima-Therm
Fully customisable AHUs for every application:
● Infinite variations in componentry and dimensions
● Patented structure for compactness, access, performance and hygiene
● Integral control options ● TB1/T2 thermal performance as standard
● IAQ optimised: up to ePM1 90% filtration and active anti-viral options
● Eurovent certified performance ● Reduced lead times
www.klima-therm.co.uk
DfE-funded research from the University of Southampton found that the average English school currently exceeds 26°C for approximately 59 days per academic year. The Department for Education’s Building Bulletin 101 (2018) sets the ventilation and thermal comfort standard for all new and refurbished school buildings in England, yet many existing schools still lack adequate passive cooling. Night flushing offers a low-cost, low-disruption route to compliance and comfort.
A regulatory tailwind
Regulation is moving in the same direction. Part O of the Building Regulations, which came into force in June 2022, now requires all new residential buildings to limit solar gain and provide passive means of heat removal. Pure- play mechanical cooling is only permitted as a last resort. The FutureBuildings Standard sets out the path to Net Zero ready new homes, with passive cooling at its core. For existing buildings, night flushing is a practical, cost-effective way to meet the spirit of these requirements today.
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12/09/2025 13:28 14/10/25 10:30 BUILDING SERVICES & ENVIRONMENTAL ENGINEER SEPTEMBER 2026 9
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