Daylight in a room can influence our thermal comfort

A pioneering study carried out at EPFL shows that the amount of daylight in a room can influence our thermal comfort and how well we tolerate heat or cold.

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Understanding the elements that influence human thermal responses is important to appropriately design and operate low-energy buildings. It has been recommended that factors not identified with the thermal condition can influence thermal responses of occupants, yet these factors have not been integrated in thermal comfort models because of an absence of learning of indoor factor connections.

While some studies have investigated the effect of electric light on thermal responses, no study exists on the effect of daylight.

In a new study, scientists at the EPFL sheds light on how variations in daylight influence our thermal response. They also discovered an important psychological factor that is associated with daylight and alters how we perceive the thermal environment in a room.

For the study, scientists recruited 42 men and 42 women aged 18–25. Each participant was asked to spent three hours in a room at one of three ambient temperatures (19°C, 23°C and 27°C) and three daylight illuminance levels (low, medium and high).

The illuminance was regulated using color-neutral filters and set randomly for the participants. Participants’ body temperatures were additionally estimated persistently during the experiment.

Giorgia Chinazzo. © Murielle Gerber / EPFL
Giorgia Chinazzo. © Murielle Gerber / EPFL

Scientists also checked the cognitive performance (logical reasoning capability, written comprehension, etc.) of the participants and their overall comfort in the room.

Participants reported that they felt more comfortable in the 19°C room. The temperature more acceptable when the room was filled with daylight – as opposed to having little daylight – even though their body temperatures were the same in both cases.

And when the room was warmer, participants felt more comfortable when the room was not as bright, although, once again, there was no difference in body temperature. That implies the effect is purely psychological.

When scientists compared their findings with a thermal comfort model developed in the 1970s, they found, relative to the model’s predictions, participants in their study reported a lower thermal sensation – they felt less warm – in the 27°C room when the room was filled with daylight.

The main difference between the scientists’ study and the model’s calculations was the type of lighting involved, these results indicate that the key factor behind participants’ lower thermal perception may be the presence of daylight.

The participants’ thermal sensations were comparable to a perceived temperature of 1.7°C lower than the actual room temperature. This is a statistically significant difference, and it means that building operators could use less energy to cool their buildings, especially if the buildings are designed to let in a lot of sunlight.

Giorgia Chinazzo, the lead author said, “Our findings suggest that we may be using too much air conditioning – particularly in buildings with glass walls, since the natural light makes the heat more tolerable. If our hypothesis turns out to be correct, buildings could be made more energy efficient by creating additional space for natural light during either the construction or renovation phase. That would also make buildings more comfortable for their occupants.”

The study highlights the importance of taking the psychological effects of daylight into account in practice, such as by updating existing building standards. The paper published in Scientific Reports.