The Singapore University of Technology and Design (SUTD) has unveiled groundbreaking findings from its Climate Resilient Citizenry project, revealing that nearly half of the 416 Housing and Development Board (HDB) flats surveyed across 10 neighborhoods were warmer indoors than outdoors. This landmark research, conducted over nine months starting October 2024, highlights a critical urban challenge in Singapore's tropical climate, where rising temperatures exacerbate indoor overheating in public housing. Led by researchers from SUTD's Lee Kuan Yew Centre for Innovative Cities (LKYCIC), the study measured heat indices—combining air temperature and humidity—in living rooms and compared them to immediate outdoor void decks, uncovering disparities that affect daily comfort, health, and productivity.

With Singapore warming at twice the global average, projected to see up to 326 days above 33°C by the 2080s, these insights underscore the need for targeted interventions in HDB homes, home to 80% of residents. The multi-institutional effort, involving collaborators from the National University of Singapore (NUS) Yong Loo Lin School of Medicine, Singapore-ETH Centre, and Singapore Management University (SMU), provides the first in-depth, on-site assessment of indoor thermal conditions in HDB flats, from one-room rentals to executive units.

🔥 Methodology: A Comprehensive Door-to-Door Investigation

The SUTD team employed portable Kestrel 5500 devices to capture real-time data on dry-bulb temperature, wet-bulb temperature, relative humidity, wind speed, and heat index during home visits. Surveys reached over 1,000 residents, documenting cooling practices, heat perceptions, and barriers to ventilation. Thermal imaging documented heat retention in concrete structures and appliance hotspots. This rigorous approach revealed nuanced indoor-outdoor divergences not visible from external weather stations.

Flats spanned generations, including older pre-1980s blocks with natural ventilation designs and newer ones with enhanced features. Measurements focused on living rooms, capturing peak afternoon conditions when solar gain peaks. Participant demographics reflected Singapore's diversity, with emphasis on vulnerable groups like seniors and low-income households.

Key Findings: Alarming Indoor-Outdoor Temperature Gaps

Precise data from the technical report shows 37.7% of homes up to 2°C warmer indoors via heat index, 10.1% 2-5°C warmer, and 1% exceeding 5°C warmer—totaling nearly 49% hotter inside. Conversely, 36.3% were slightly cooler. Wind speed was lower indoors in 58%, with 28.4% dropping over 1 m/s, confirming impeded airflow as a primary culprit.

• Extreme case: 36.2°C indoors vs. 27.8°C outdoors.
• 33% up to 2°C > void decks; 10% up to 5°C hotter outdoors.
• 60% lower airflow, linked to clutter blocking 40% of pathways.

Smaller rental units showed highest overheating, while larger executive flats benefited from better layouts.

Root Causes: Why HDB Flats Trap Heat

Poor cross-ventilation tops the list, with furniture, clutter, and partitions obstructing airflow in 60% of homes. Closed windows—due to rain, insects, pollution, odors, privacy, or security—exacerbate stagnation. Concrete walls and floors absorb daytime heat, releasing it nocturnally. Heat-emitting appliances like refrigerators and cookers add 1-2°C locally. Older flats lack modern insulation, while layouts in densely packed estates limit prevailing winds.

Urban Heat Island (UHI) effects amplify this: Singapore's built environment raises city temperatures 2-4°C above rural areas, with HDB corridors and void decks trapping warmth.

Vulnerable Groups: Seniors and Low-Income Households Hit Hardest

Seniors, comprising 20% of participants over 65, reported lower heat disturbance and AC use (38.6% nighttime vs. 52.7% overall), risking heat stress as thermoregulation declines. Low-income and rental residents in sub-50m² units faced 15% higher overheating, limited by voucher ineligibility or costs. Sleep disruption affected 55%, productivity 40%, with cumulative effects like dehydration and cardiovascular strain.

Assoc Prof Jason Lee from NUS Medicine notes: "Seniors grew up without AC, altering comfort baselines, but age compromises cooling responses."

Photo by Joshua Kettle on Unsplash

Resident Behaviors: Normalization of Discomfort

76% rely on fans, 52.7% nighttime AC (set at 25°C), but daytime AC is rare (14%) due to costs. Windows open when home (73.8%), closed unoccupied (64.9%). Barriers: rain (32.9% always close), air quality (high concern). Climate vouchers ($300) saw 60% unused, favoring fans/LEDs over AC. Dr Samuel Chng observes: "Residents endure heat as normal, acting only at peaks—dangerous amid projections."

Health and Societal Impacts in Singapore's Warming Climate

Indoor heat links to sleep loss, fatigue, reduced cognition, and exacerbated conditions like hypertension. With El Niño looming and heatwaves regional (e.g., Thailand 2024), HDB residents face amplified risks. UHI adds 3-4% to city heat via AC exhaust. Economic toll: lost productivity, higher energy bills (avg 346 kWh/month).

The study ties to Singapore's Third National Climate Change Study, forecasting 5°C rise by 2100. For more on government heat strategies, see the Straits Times coverage.

Government Initiatives: Vouchers, Cool Coatings, and Design Upgrades

HDB's cool coatings on all estates by 2030 reduce roof absorption by 20-30%. $300 Climate-Friendly Households Programme aids efficient appliances, but uptake low for AC ($1,000+). NEA heat stress advisories and public cooling centers help, yet indoor focus lags. Newer estates like Jurong Lake District optimize wind corridors.

SUTD's full technical report details audits: WG1 Population Household Study.

SUTD Recommendations: Practical Solutions and Policy Shifts

Immediate: Declutter vents, fan-driven cross-ventilation, solar films on sun-facing windows. Long-term: Retrofits for older blocks, modeling tools for layouts. Dr Zheng Kai's ventilation simulator accounts for rain/insects. Policy: Co-fund AC for vulnerable, rental incentives, HDB design mandates. "Staying Cool" trials target low-income.

• Remove clutter: Improves airflow 20-30%.
• Fans + open windows: Cools 2-3°C effectively.
• Solar films: Blocks 70% solar gain.
• Toolkit: Free guide forthcoming from SUTD.

SUTD's Role: Pioneering Climate-Resilient Urban Research

SUTD's LKYCIC leads transdisciplinary efforts, blending architecture, psychology, and health. Dr Harvey Neo emphasizes infrastructure over individual fixes. Future: AI-optimized designs, community pilots. As Singapore's tech-design uni, SUTD equips grads for resilient cities—explore faculty roles in sustainable urbanism.

Photo by Makarios Tang on Unsplash

Future Outlook: Building Cooler, Healthier HDB Homes

With net-zero 2050 goals, HDB's ventilation enhancements and green roofs promise relief. SUTD's work informs URA Master Plan, prioritizing wind-optimized estates. Vulnerable support via subsidies could cut risks 50%. Ongoing CRC monitors El Niño, ensuring data-driven adaptation.

WG3 report on dialogues: Citizen Dialogue Insights.