Nearly Half of Over 400 HDB Flats Found Warmer Than Outdoors in SUTD Heat Study

The study, conducted from October 2024, covered almost all HDB flat types from one-room rental units to executive flats. Researchers measured indoor conditions using the heat index – which combines air temperature and humidity to estimate overall thermal conditions and potential heat stress.

In one home, the heat index was 35.9 deg C even though it was 31.1 deg C outdoors. In 1 per cent of homes, the difference was even wider, with one home recording 36.2 deg C when it was 27.8 deg C outdoors. Around one-third of the households were up to 2 deg C warmer than their immediate void decks, and about 10 per cent of homes were up to 5 deg C hotter.

Close to 60 per cent of the households also had lower airflow than the outdoors, pointing to widespread limitations in natural ventilation when clutter, home layouts, closed windows or building arrangements impede wind flow. Researchers also found that higher levels of indoor clutter and keeping windows closed for long periods were associated with warmer homes and poorer ventilation.

The report noted that seniors, low-income families and residents of smaller homes and rental units are more vulnerable to rising temperatures, and yet have less access to cooling options and air-conditioning. Singapore is in the midst of the hottest period of the year, with temperatures across the region – including Thailand and Malaysia – currently at heatwave levels. The warming El Nino climate phenomenon is expected to return in the second half of 2026.

Most residents surveyed said their outdoor activities and sleep were most affected by the heat. Yet the surveys also showed that residents tended to view heat as a condition to be lived with rather than a problem to be actively addressed – a significant normalisation of discomfort that researchers flagged as concerning.

Seniors, who are more vulnerable to rising temperatures, were found to be less likely to feel disturbed by the heat and preferred air-conditioned environments less. As older people may notice heat stress less clearly and delay cooling measures, this poses particular health risks as the body’s ability to regulate temperature weakens with age.

On coping with indoor heat, electric fans remain the most popular solution at 76 per cent of respondents. This is followed by opening windows and adjusting attire. More than half of respondents turn on air-conditioning at bedtime, but only around 14 per cent use it regularly during the day.

Researchers suggested practical measures including removing clutter to improve airflow, using fans to support cross-ventilation by placing them near windows or doors to draw cooler air in, and applying solar control films to windows facing strong sunlight.

The multi-institute team is planning to trial indoor cooling solutions and develop a cooling toolkit for homes and a renovation guide to help avoid stuffy layouts. A separate project called Staying Cool will focus on specific interventions for vulnerable groups and residents in rental units. Experts also called for housing retrofits in older flats to improve natural ventilation and comfort, and for affordable access to cooling technologies for vulnerable groups.