The repeatability of overheating is no coincidence. When a room consistently reaches its heat peak at the same time each day during summer, three physical variables are responsible: the cardinal orientation of the glazed facade, local shading conditions, and the building's thermal mass. Understanding how these three factors interact allows for precise diagnosis and helps identify the right course of action for each configuration.
Glazed Facade Orientation: The Primary Driver of Overheating Time
The sun follows a predictable path in the northern hemisphere — rising in the east, reaching its peak in the south, setting in the west. A glazed facade's orientation therefore directly determines when it is exposed to direct solar radiation.
| Orientation | Direct Sunlight | Approximate Overheating Peak |
|---|---|---|
| East | 7 am – 12 pm | 8 am – 11 am |
| South-East | 8 am – 2 pm | 10 am – 1 pm |
| South | 10 am – 4 pm | 12 pm – 2 pm |
| South-West | 12 pm – 6 pm | 2 pm – 5 pm |
| West | 2 pm – 8 pm | 4 pm – 7 pm |
For a west-facing facade, overheating typically occurs in the late afternoon, when the sun drops low and penetrates rooms nearly horizontally. This configuration is particularly problematic: low-angle radiation bypasses horizontal roof overhangs, and solar intensity remains high at the end of the day. Conversely, an east-facing facade is exposed from sunrise, with temperatures rising sharply in the morning before cooling off from midday onward.
In almost every case, identifying the orientation is the starting point for a room's thermal diagnosis.
[Image placement suggestion 1: solar diagram showing the sun's path across cardinal orientations and corresponding sunlight windows]
Shading: Modulating Direct Exposure Time
The actual overheating time can deviate significantly from the theoretical pattern when nearby elements partially block the window. Three types of obstacles matter:
- Roof overhangs and horizontal canopies: effective at blocking overhead radiation (south-facing facades in midsummer), they become ineffective against low-angle radiation, such as for east- or west-facing facades in early morning or late afternoon.
- Vegetation: a deciduous tree provides natural seasonal protection — maximum in summer, none in winter. Its geometry — height, canopy width, distance from the window — precisely determines the shading window.
- Neighbouring buildings: depending on their height and angular distance, they can delay the start of direct exposure or cut it short, shifting the heat peak by one to several hours.
Partial shading doesn't eliminate overheating — it reshapes its timing. A west-facing window blocked by a nearby building may not receive direct sunlight until 5 pm, whereas it would be exposed from 2 pm on open ground.
Thermal Mass and the Phase-Shift Effect
Thermal mass refers to a material's ability to absorb and store heat, then release it gradually over time. The greater the mass and thickness of the walls, the longer the delay between solar exposure and the rise in interior temperature.
In a high-mass building — thick concrete walls, heavy floors, stone masonry — heat absorbed during sun exposure is slowly radiated inward. A room with a south-facing facade exposed between 11 am and 2 pm may not reach its peak temperature until 4 or 5 pm, with heat persisting into the evening. Conversely, a lightweight structure — steel frame, thin partitions — reacts almost instantly to solar flux: the temperature peak coincides with peak exposure, and the room cools as soon as shade reaches the facade.
This explains why two rooms with the same orientation in buildings of different types can reach their maximum temperature several hours apart.
Cross-Diagnostic Method
Reading these three variables together makes it possible to estimate a room's overheating time without complex instrumentation:
- Determine the main glazed facade's orientation (compass, sun exposure app, or digital cadastre).
- Identify obstacles likely to cause partial or full shading: type, angular height, sun direction at different times of day.
- Assess the building's thermal mass: old stone or concrete construction (high inertia, significant phase shift), modern or lightweight building (low inertia, fast thermal response).
This method distinguishes two common profiles: direct overheating, where the indoor peak coincides with peak solar exposure; and delayed overheating, where temperatures continue to rise for several hours after the sun has moved on, sometimes persisting into the evening.
Glazing: The Main Entry Point for Solar Heat Gain
Glazed surfaces are the primary entry point for solar heat gain in a building. Standard glazing transmits most of the incident radiation as thermal energy into the interior. Applying a solar control film reduces this heat gain at the source. Certain Solar Screen solar control films reject up to 80% of incident solar energy while preserving visible light transmission (source: Solar Screen technical documentation). The film acts upstream of the overheating phenomenon: thermal mass absorbs less heat during exposure hours, reducing both the intensity and duration of the indoor heat peak. This solution complements external passive shading and requires no structural modification to the building envelope.
For a comprehensive look at strategies to reduce building overheating in hot weather — insulation, night ventilation, external solar shading and window films — a dedicated article covers the various options by building configuration.
[Image placement suggestion 2: illustration of a window fitted with a solar film, showing the proportion of solar energy rejected versus transmitted]