Nighttime Cooling for Enhanced Thermal Comfort: Negative Inertia and Radiative Cooling

In hot regions, the question of thermal comfort often relies on a simple equation: how can daytime overheating be limited without increasing air-conditioning needs? Among emerging passive solutions, the association between negative inertia and nocturnal radiative cooling is becoming a credible and technically grounded path. Scientific work published over the past decades now makes it possible to better understand the effectiveness of these mechanisms and their conditions of application.

Where conventional use of thermal mass aims to absorb daytime heat and gradually release it at night, negative inertia reverses this logic:

• voluntary night-time cooling of the mass,

•  use of this cooled mass to absorb daytime heat gains.

This strategy is only relevant if the structure can genuinely lose heat during the night. This is precisely the role of radiative cooling, extensively documented in recent literature.

Although nocturnal cooling is a well-known natural phenomenon, its architectural application is far from straightforward. The true innovation lies in the ability to transform this spontaneous mechanism into a controlled, optimized, and fully integrated design strategy. Negative thermal inertia and radiative night cooling are not simply about “letting the building cool down at night”: they rely on high-emissivity materials, surfaces engineered to maximize radiative exchange, and the deliberate management of thermal mass to create genuine “reservoirs of coolness.” This shift from a natural occurrence to a structured technical system represents the key breakthrough of these modern approaches. It also explains why they are now considered advanced passive technologies capable of competing with far more energy-intensive mechanical cooling solutions.

A High-Performance Physical Mechanism: Radiation Toward the Sky

Nocturnal radiative cooling relies on the infrared emission of a surface toward the sky vault. When the sky is clear, the atmospheric window (8–13 µm) allows a roof or slab to radiate more energy than it receives, which can lower its temperature below that of the ambient air.The review by Guo L. (2024), dedicated to roofing applications, highlights that systems exploiting this principle can generate 20% to 60% energy savings in suitable scenarios.

However, this efficiency depends on parameters well identified in the literature: material emissivity, atmospheric conditions, humidity, cloud coverage, and insulation level. [4]

Concrete Validation in Tropical Climate

The most instructive case remains the study by Khedari et al. (2000), conducted in Thailand in a hot-humid climate. Four configurations of surfaces exposed to the night sky were compared under three weather conditions (clear sky, cloudy, rainy).The results show that:

• under clear or cloudy skies, the tested surfaces exhibited temperatures 1 to 6 °C below ambient air,

• under rainy skies, the effect almost completely disappeared, with surface temperatures staying close to air temperature.

This study demonstrates notably that, even in regions where humidity is high, nocturnal radiative cooling remains operational as soon as the sky offers sufficient opening for radiation.

Articulating Mass and Radiation: How Negative Inertia Actually Works

Negative inertia is based on a precise sequence:

•  the thermal mass is exposed at night, directly (roof, slab) or indirectly (via night ventilation),

•  it loses heat through radiation, enhanced under a clear sky,

• it becomes a reservoir of coolness,

•  it absorbs daytime heat gains, delaying the indoor temperature rise.

Although the Thai study did not explicitly apply the concept of negative inertia, it provides the essential experimental proof: the structure can indeed cool down in tropical conditions. Recent scientific reviews also confirm that interest in these systems has grown significantly: a bibliometric analysis of 175 articles published between 2005 and 2023 [1] shows rapid growth in work dedicated to roof radiative cooling.

Useful Metrics for Design

Data from the studies reviewed help situate observable performances:

• Thermal difference obtained: 1 to 6 °C below air temperature under favorable conditions [2].

• Potential savings: 20% to 60% reduction in energy consumption possible, depending on climate, material emissivity, and sky conditions [1].

• Climate sensitivity: performance is greatly reduced in case of rain or excessive atmospheric humidity [2].

These elements form a solid basis for integration into design or renovation, especially in hot climates.

Usage Perspectives in Tropical Regions

The Thai experience provides a directly applicable case study for Southeast Asian regions. It shows that:

• nocturnal radiative cooling is possible despite humidity,

• the 1 to 6 °C thermal difference is already sufficient to support a negative-inertia strategy,

•  the technology fits well with massive envelopes (concrete, masonry), common in local construction [2].

More recent scientific reviews show that passive radiative roofing technologies are today among the most active research topics, strengthening the credibility of the approach in low-energy architectural projects [1].

An interesting example comes from an experiment conducted in Malaysia, where a system combining nocturnal radiative cooling and thermal inertia was tested in a hot-humid context. The device relied on a loop of water cooled at night through a surface exposed to the sky, then used during the day to supply radiant wall and ceiling panels [5]. The results show:

•  an operative temperature reduced to 26.7 °C in the experimental room, while the reference building reached 31.8 °C;

•  up to 85% energy savings potential according to the simulation performed in this study;

•  performance dependent on the capacity to sufficiently cool the water during the tropical night [5].

This example clearly illustrates the logic of negative inertia: nocturnal coolness storage, passive release during the day, and adaptation to a tropical humid climate comparable to that of Thailand.

An Opportunity for Sustainable Architecture

By combining the results of available studies, negative inertia associated with radiative cooling can be seen as a strategy offering:

• a notable reduction in heat gains,

•  potential reduction in air-conditioning needs,

•  relatively simple architectural integration,

•  a passive approach with no direct energy consumption,

• robustness to disruptions (e.g., power outage).

In a context of growing cooling demand, particularly strong in hot-humid climates, this approach represents a strategic option, supported by real experimental data and an expanding scientific basis.

Conclusion

Recent work shows that the combination of nocturnal radiative cooling + negative inertia is a relevant passive approach for buildings located in hot climates. Experimental results obtained in Thailand demonstrate the possibility of lowering surface temperatures by 1 to 6 °C below ambient air, even in humid contexts. Academic reviews confirm in turn an energy potential of up to 60% savings according to Guo L. (2024) and the conditions necessary for efficiency.

The challenge is no longer to prove the feasibility of the phenomenon, but to perfect its architectural and constructive integration. In tropical regions, this is a concrete lever to reduce dependence on air-conditioning and improve thermal comfort in a passive and sustainable manner.

Sources

[1] Guo L., Li D., Wei H., et al., A bibliometric and systematic review of radiative cooling roof applications, Sustainability, 2024.

[2] Khedari J., Waewsak J., Pratinthong N., Hirunlabh J., Field investigation of night radiation cooling under tropical climate, Renewable Energy, 2000.

[3] Meng X., Davies M., Kolokotroni M., Atmospheric influence on radiative cooling performance, UCL, 2023.

[4] Pirvaram A., et al., Advances and challenges in radiative cooling: A comprehensive review, Renewable & Sustainable Energy Reviews, 2022.

[5] Sopian K., Saadatian O., Lim C., et al., Night-Cooled Radiant Cooling Panel for Sustainable Building Cooling Mode in Malaysia, International Journal of Renewable Energy Research, 2018.