14 January 2026
When we design classrooms or homes, comfort is usually measured through air temperature, humidity, and air circulation. Yet one of the most influential components of human thermal comfort, radiant heat exchange, often goes unnoticed and, more importantly, it is still modeled as if everyone were the same size.
Our bodies exchange heat with surrounding surfaces through longwave radiation. This process is governed by geometric relationships known as view factors, which describe the portion of each wall, floor, or ceiling a person’s body “sees.” View factors determine how much radiant heat a person’s body will receive from each surface.
Most of the data used to calculate view factors comes from research conducted in the 1970s on college-aged students. The resulting measurements were adopted in standards such as ASHRAE 55 and ISO 7726, forming the foundation on which engineers predict the effect of radiation from indoor surfaces through the mean radiant temperature (MRT) — a key variable in thermal comfort models.
But children are not simply scaled-down adults. Their shorter stature, rounder proportions, and higher head-to-body ratio alter their radiative interaction with the surrounding space. Until now, no study had quantified the significance of these differences.
Modeling the Problem
To address this gap, ÉTS researchers Nour Youssef and Katherine D’Avignon used detailed digital models of a typical adult male and a 5-year-old child. These manikins were then placed in a simulated room and analyzed using computational tools that traced virtual “rays” between each manikin’s body and the surrounding surfaces. This process made it possible to calculate the projected area factors and view factors for the adult and child manikins under identical conditions.
In simple terms, the researchers measured how much of each surface (floor, ceiling, or walls) the human body could “see,” and how those views changed with body size and shape.
The Differences are Significant
The analysis revealed notable differences between the adult and child models. Depending on the angle of the surface concerned, the child’s radiation factors — called projected area factors — differed by as much as 22% from those of the adult. This is a much greater variation than that observed in previous studies between adults of different body sizes or nationalities.
To understand what this means in practice, the team ran two test scenarios:
A scenario where the child and adult manikins were in a room with a radiant floor heating system at 26 °C, and a scenario in a room with a chilled ceiling at 17 °C.
In the first scenario, the mean radiant temperature felt by the 5-year-old was calculated to be ~1 °C higher than that felt by the adult. In the second scenario, it was 0.6 °C higher. These differences may seem modest, but in the thermal comfort models used today, such as Fanger’s Predicted Mean Vote, even a 0.5 °C shift in mean radiant temperature can significantly alter the perception of comfort — for example, from feeling “neutral” to “slightly warm.”
Why the Child Feels Warmer
These findings make intuitive sense when viewed from a geometric point of view. A child’s center of gravity is lower, closer to the floor, and its body is more compact. In a classroom with radiant floor heating, the child’s body “sees” more of the warm surface relative to their total surroundings, which increases the radiant heat they receive.
Conversely, with cooling coming from the ceiling, a smaller portion of the child’s body is exposed to the cooler ceiling, but their larger head and upper body surface area amplify its effect. In both cases, the difference in radiation balance is enough to alter perceived comfort.
Implications for Building Design
This research highlights an important limitation in current comfort standards: they rely on the same data for all populations. In spaces frequented mainly by children — such as schools, daycares, and pediatric hospitals — this assumption can lead to systematic biases in thermal comfort predictions.
In real-world terms, this means a classroom designed to be “thermally neutral” for adults could feel too warm for the children in it. This aligns with previous field observations showing that children often report higher warmth sensations than predicted by current comfort models.
Incorporating child-specific radiation data could improve both the accuracy of comfort assessments and, consequently, the energy efficiency of HVAC systems. Knowing that children experience a slightly higher mean radiant temperature under radiant floor heating might justify, for instance, lowering the floor temperature by a degree. Far from compromising comfort, this energy efficiency measure would, in fact, improve comfort for the majority of occupants.
Toward More Inclusive Comfort Models
Youssef and D’Avignon’s study represents the first detailed quantification of child-specific view factors and their influence on thermal comfort. It provides foundational data for developing new comfort models that account for differences in body geometry across age groups.
Further work is needed to determine at what age children’s radiant exchange patterns begin to resemble those of adults and how other postures — such as sitting or playing on the floor — modify the results.
In the broader picture, this research reminds us that thermal comfort is not universal. Just as building designers consider accessibility or ergonomics, thermal environments may also need to be adapted to the diversity of their occupants.
A room that feels “just right” for adults may not feel the same for its smallest users — and now, we have the data to explain why.
