What is passive cooling?

Cooling buildings with natural heat sinks rather than machinery such as chillers. Methods include radiative cooling to the night sky, night ventilation that pre-cools a building's thermal mass with cool night air, evaporative cooling, and shading or highly reflective roofs that block sun. It uses no energy but depends heavily on weather and humidity.

How much can radiative cooling achieve?

On clear nights surfaces emit infrared to space and cool by the difference between that and infrared from the sky. At 25 °C air with a 10 °C dew point (dry), it's about 96 W/m²; with a 20 °C dew point (humid), about 63 W/m². Humid skies emit more infrared, reducing the effect. Daytime cooling requires special materials reflecting nearly all sunlight (95% or more).

How effective is night ventilation?

Large volumes of night air cool exposed concrete floors and ceilings, which then absorb daytime heat. Thick concrete swings in temperature to about 14 cm deep over a daily cycle and stores 279 kJ per m² per K. Cooling 100 m² by 3 K stores 23 kWh, about 80% of 30 kWh of daytime gains, but needs about 2,200 m³ per hour of night ventilation.

Passive Cooling Technology

Category: Thermal Analysis – HVAC | Revised 2026-10-01

CAE visualization for passive cooling theory - technical simulation diagram

Passive Cooling Technologies

Theory: using the sky and the night as heat sinks

Overview

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How realistic is cooling a building without air conditioning?

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There are two natural heat sinks. One is the night sky: space is very cold, so on clear nights upward-facing surfaces radiate infrared and cool below air temperature. The other is cool night air, which pre-cools a building's concrete so it absorbs the next day's heat. Both use almost no energy, but their capacity is limited and depends on humidity and weather. So first cut cooling loads with insulation and shading, then estimate numerically how much of the remainder they can handle.

Radiative cooling

$$ P_{net} = \varepsilon\,\sigma T^4\,(1 - \varepsilon_{sky}) - (1 - R)\,G_{sun},\qquad \varepsilon_{sky} = 0.711 + 0.56\,\frac{T_d}{100} + 0.73\left(\frac{T_d}{100}\right)^2 $$

ε is surface emissivity, ε_sky clear-sky effective emissivity (Berdahl–Martin), T_d dew point (°C), R solar reflectance and G_sun solar irradiance. Convective gain from surrounding air is excluded.

Night ventilation and thermal storage

$$ \delta = \sqrt{\frac{\alpha P}{\pi}},\qquad E = \rho c\,\delta\,A\,\Delta T,\qquad \dot V = \frac{E}{\rho_a c_{p,a}\,\Delta T_{air}\,t_{night}} $$

δ is the depth reached by daily (P = 86,400 s) temperature swings. Concrete: conductivity 1.4 W/mK, density 2,300 kg/m³, specific heat 880 J/kgK. V̇ is the airflow needed to remove the heat over t_night with an air temperature rise ΔT_air.

Coffee Break Trivia

Daytime radiative cooling

Radiative cooling to the night sky has long been known; in arid regions it was used to freeze water overnight. Daytime cooling was thought impractical because solar heating dominates, but in 2014 Raman and colleagues at Stanford made a multilayer film that reflected 97% of sunlight while emitting strongly in the atmosphere's transparent infrared band (8–13 μm), cooling about 5 °C below ambient in direct sun. Daytime radiative-cooling materials have been an active field since.

Worked examples

Example 1: net radiative cooling (air 25 °C, emissivity 0.95)

Dew point	Sky emissivity	Night	Day (95% reflectance)	Day (85% reflectance)
10 ℃	0.774	96 W/m²	56 W/m²	−24 W/m²
15 ℃	0.811	80 W/m²	40 W/m²	−40 W/m²
20 ℃	0.852	63 W/m²	23 W/m²	−57 W/m²

Daytime sun is 800 W/m². Negative values mean net heating. A 25 °C blackbody emits 448 W/m², so net cooling is only 10–20% of that.

Example 2: night ventilation (100 m² exposed concrete)

Item	Value
Daily penetration depth	0.138 m
Storage per m² per K	279 kJ
Stored cooling at 3 K	23.3 kWh
Daytime gains (30 W/m² × 10 h)	30 kWh
Fraction absorbed	78%
Night airflow needed (8 h, 4 K air rise)	about 2,170 m³/h (7.2 ACH for 300 m³)

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Even for white paint, 95% versus 85% reflectance flips cooling into heating.

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Sunlight is 800 W/m², so a 10% difference in absorptance is 80 W/m² — about the size of night-time radiative cooling. Ordinary white paint looks white but absorbs some ultraviolet and near-infrared, often reflecting only 80–85%. Cooling below ambient by day needs a material that reflects sunlight almost completely while emitting strongly at 8–13 μm. Absorbing even a little sun backfires, so roof materials should be chosen on both solar reflectance and infrared emissivity. In humid regions, higher sky emissivity also shrinks the effect.

Planning and evaluation

First reduce cooling loads with insulation and shading.
Estimate radiative and night-ventilation effects from local temperature, dew point and cloud cover.
For night ventilation, expose concrete indoors and secure ample airflow.
Run hourly simulations of room temperature and check comfort.
Plan backup cooling for humid periods when effects drop.

Coffee Break Trivia

“An office where night cooling failed”

An office added night ventilation, but next-day temperatures barely fell. The ceiling slab was concrete, but a suspended ceiling hung below it and carpet covered the floor. The concrete to be cooled was nearly isolated from room air, so it couldn't be cooled at night or absorb heat by day. Replacing the suspended ceiling with a highly perforated one cut next-day peak temperature by about 2 °C. Thermal mass must not only exist but be exposed to the air.

Common mistakes

Mistakes and fixes

Mistake	Effect	Fix
Ignoring humidity	Radiative cooling overestimated	Sky emissivity from dew point
Judging reflectance by eye	Heats up by day	Measure solar reflectance
Covering the concrete	Storage ineffective	Expose it to the room
Too little night airflow	Insufficient pre-cooling	Calculate required flow
Ignoring convective gain	Cooling overestimated	Account for wind or covers

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