When the Thermometer Is Not Enough to Explain Discomfort
In summer, a common situation in offices and homes with large glazed surfaces: the displayed temperature is acceptable, often between 23°C and 26°C, and yet occupants near windows or glazed walls feel intense heat. This phenomenon is not subjective. It is explained by a physical parameter distinct from air temperature: mean radiant temperature (MRT).
Lowering the setpoint of the air conditioning system does not resolve this problem. Understanding why requires entering into the physics of thermal comfort.
MRT: A Fundamental Parameter of Thermal Comfort
Mean radiant temperature represents the uniform temperature of a fictitious enclosure in which the radiative exchanges between the occupant and surrounding surfaces would be equivalent to those occurring in their real environment. Concretely, it is the weighted average of surface temperatures perceived by the human body in all directions.
This quantity enters directly into the calculation of operative temperature (To), the thermal comfort indicator most representative of real physiological experience. Under conditions of low air velocity (below 0.2 m/s), operative temperature is calculated as the arithmetic mean of air temperature (Ta) and MRT (Tr):
To = (Ta + Tr) / 2
Thus, an air temperature of 24°C combined with an MRT of 38°C, a value measurable near a sun-exposed glazed wall, gives an operative temperature of 31°C, well above the comfort thresholds defined by standards ISO 7730 and EN 15251 (subsequently incorporated into EN 16798-1).
The Thermal Behaviour of Glazing in Summer
Ordinary glazing, single glazing or standard double glazing without energy treatment, transmits a significant fraction of incident solar radiation, particularly in the visible and near-infrared wavelengths. Under direct sunlight, the interior surface of a glazed wall can reach elevated temperatures. These heated surfaces then emit thermal radiation towards the interior of the room, contributing to raising the local MRT.
This phenomenon generates radiation asymmetry: the side of the body facing the glazed wall receives a significantly higher radiative flux than the opposite side. Standard ISO 7730 (Ergonomics of the thermal environment — Analytical determination and interpretation of thermal comfort, ISO 7730:2005) sets a discomfort limit for asymmetric radiation at 10 K for warm vertical surfaces. This threshold is commonly exceeded in the presence of untreated glazing exposed to direct radiation during summer periods.
It should be emphasised that this mechanism is independent of air temperature regulation by air conditioning. Lowering the cooling system setpoint does not act directly on MRT, which explains the partial effectiveness of HVAC installations in spaces with a high proportion of glazed surfaces, particularly those with south or west orientations.
Diagnosing Radiant Discomfort: Practical Indicators
Before undertaking a technical intervention, it is useful to objectively characterise the source of discomfort. Several indicators allow radiant discomfort to be distinguished from discomfort related solely to air temperature:
-
Differential measurement: compare air temperature at the centre of the room and at 0.5 m from the glazed wall, using a black globe thermometer (reference instrument for estimating local MRT).
-
Temporal observation: does the discomfort appear only during periods of direct sunlight? A correlation with sun position clearly points towards a radiant cause.
-
Perceived asymmetry: an intense sensation of heat on the side of the body facing the glass, in the absence of perceptible air movement, is a characteristic sign of asymmetric radiation.
For tertiary, institutional or educational projects, a comprehensive thermal study integrating solar film performance parameters allows expected gains in MRT reduction and operative temperature to be quantified before any installation.
Solar Film as a Lever for MRT Reduction
Applying an adhesive solar film to the glazed surface acts directly on MRT by reducing the fraction of solar radiation transmitted towards the interior. By limiting the incident flux, the film reduces the heating of internal room surfaces and consequently lowers the thermal radiation re-emitted towards occupants.
The effectiveness of this intervention is directly dependent on the energy rejection rate of the selected product. By way of illustration:
-
The Solar 80 C rejects 82% of total solar energy, representing a substantial reduction in incoming flux. Its visible light transmission is 15%, suited to environments where radiation control takes precedence over transparency.
-
The Solar 50 C offers a rejection of 42% while maintaining a light transmission of 60%, preferred when natural light intake requirements are high.
The Solar Screen thermal control solar film range groups several product families, silver, neutral, transparent, one-way mirror, covering a broad spectrum of thermal and light performance, adapted to the architectural, regulatory and usage constraints specific to each project.
Conclusion
The sensation of heat near glazing in summer results primarily from a localised increase in mean radiant temperature, a phenomenon not directly corrected by air temperature regulation. This mechanism, formalised by standards ISO 7730 and EN 16798-1, is measurable and quantifiable. Treating glazing with solar film constitutes one of the most direct interventions for reducing incident radiation, lowering local MRT and improving the real thermal comfort of occupants, whether in residential, tertiary or institutional spaces.