In urban environments, apartments exposed to direct solar radiation can record indoor temperatures significantly higher than outdoor temperatures during heat episodes. In the absence of air conditioning, two complementary passive levers can contain this overheating: solar protection of glazing and night ventilation. Combined according to a precise chronological protocol, they form a coherent passive thermal regulation system, applicable without mechanical equipment.
Glazing, the Main Vector of Daytime Overheating
Solar heat gains transit primarily through glazed surfaces. Unprotected glazing allows a significant fraction of short-wave infrared radiation to pass through, contributing to raising the temperature of walls and indoor air. The effect is particularly pronounced on east, west and south-west orientations, exposed to the sun during the hottest hours of the day. The thermal mass of walls (concrete, tiling) absorbs this heat and releases it progressively, even after sunset, which explains why indoor temperatures remain elevated in the evening.
Lever 1: Solar Control Film – Reducing Daytime Heat Gains at Source
Applying a solar control film to glazing acts directly on the incident solar energy flux. The film reflects and absorbs a portion of radiation before it passes through the glass, thereby reducing thermal gains towards the interior. The Total Solar Energy Rejection rate (TSER) is the reference performance indicator: as an example, the SOLAR 50 C by Solar Screen presents a TSER of 43% in interior installation (source: Solar Screen technical datasheet).
Unlike blinds or shutters, solar control film does not obstruct the view and maintains natural light transmission. Its action is permanent, passive and requires no daily intervention.
For a comparative analysis of thermal protection solutions according to glazing type, consult our detailed article on insulating glass against heat with adhesive film.
Lever 2: Night Ventilation – Evacuating Stored Heat
Night ventilation exploits the thermal differential between the indoor thermal mass (heated during the day) and the cooler outdoor air in the second part of the night. By opening windows when the outdoor temperature drops below the indoor temperature, a transverse airflow is established that carries away the heat accumulated in the walls and ambient air.
This technique relies on two structural conditions:
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A cross-ventilated configuration: the dwelling must have openings on two distinct façades (ideally opposite or perpendicular) to allow effective cross-airflow. An apartment with windows on a single façade does not generate transverse airflow.
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A sufficient thermal differential: night ventilation is only effective if the outdoor temperature actually drops below the indoor temperature. In areas subject to a strong urban heat island effect, this differential may be insufficient on certain nights.
Chronological Protocol: Coordinating the Two Levers
The effective articulation of these two approaches relies on a strict alternation:
Daytime (approximately 7:00 to 22:00):
The solar control film limits solar gains through the glazing. Windows remain closed. Complementary protections (blinds, thick curtains) can reinforce the effect on the most exposed orientations (west, south-west).
Night (approximately 22:00 to 6:00-7:00):
When the outdoor temperature becomes lower than the indoor temperature, opening windows on opposite façades allows accumulated heat to be purged. Effectiveness is greatest in the late night, when the thermal differential is at its highest. Windows are closed again early in the morning, before the outdoor temperature rises again.
Rigour in applying this cycle directly conditions the effectiveness of the system. A window left open during the day in hot weather introduces warm air into the dwelling and cancels out the protective effect of the film.
Impact of Window Orientation on Device Effectiveness
The orientation of glazing determines the hierarchy of treatment priorities:
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West and south-west: exposure in the late afternoon, during peak outdoor temperature hours; priority treatment with high-TSER film.
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South: exposure concentrated around midday; intense but time-limited gains.
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East: morning exposure, with heat accumulated in the first part of the day.
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North: negligible direct solar gains in metropolitan France; less critical protection.
For night ventilation, the orientation of openings relative to local dominant night winds determines the transverse airflow rate and the effectiveness of thermal purging.
Conclusion
The combination of solar control film and night ventilation constitutes a passive and economical response to summer overheating in cross-ventilated apartments. It does not replace air conditioning in extreme heat conditions, but significantly reduces dependence on it by maintaining a favourable thermal cycle: limitation of daytime gains through the film, evacuation of stored heat through night ventilation.
The Solar Screen range of solar control films groups several TSER and light transmission configurations, adapted to each type of glazing and exposure. Access the Solar Screen range