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Radiant Heat

Homes and ever-deepening workplaces are constructed with lots of glazing. Why? Because natural light is the holy grail of light sources and we enjoy a view to the outside world.

BS EN 17037 sets out standards that new workplaces must attain in terms of daylight provision, a quality view out and glare. These factors are fundamentally important for our psychological wellbeing both in the workplace and at home.

However, lots of windows create all sorts of problems in managing comfort. Radiant temperature has a much greater impact than air temperature on how the body gains and loses heat to its environment.


Good examples of sources of radiant heat are the sun shining through a window, a wood-burning stove or infra-red heaters. I am going to add you, you’re a walking heat source, to that list and I will explain why later.

We associate comfort with air temperature, but how could we get it so wrong. Even heating and cooling systems use air temperature to determine comfort.

The air temperature may be set for your comfort, but the experience when you are in close proximity to a large window is a totally different story.

On a bright day the sun’s radiant heat shining through the window makes us too hot. The variations in the sun’s heat caused by clouds passing across the sky causes fluctuations in the radiant temperature. The human body takes about 10 minutes to adapt to its environment. During this time we tend to be distracted and uncomfortable. Our temperate UK climate means that this is very common. Note that the air temperature has hardly changed.

Conversely, in winter the glazing is the coldest part of the home. Radiators were traditionally positioned beneath windows to compensate. However, modern building may have patio doors, bi-fold doors, conservatories, glass roofs and even floor to ceiling glazing. These areas do not benefit from a compensating heat source. We feel uncomfortable because now our body radiates heat asymmetrically, from the side closest to the window, to the cold surface of the large expanse of window. We have, in turn, become the radiant heat source for the window. The situation is often confused with being a draft from the window, but it is actually our body losing heat.

I filmed all the double-glazed windows of my home with a clear, insulating and solar control combination window film. The purpose was originally to just protect my new wooden floors from sun damage. However, I have been able to adjust the thermostat down by 2°C, as I am no longer compensating the for the feeling of cold near windows. The saving on my heating bills is just over 20 percent.


The image shows before and after the application of window film to a glass window. The image on the left (before filming) shows the glass absorbing body heat. The image on the right (after filming) shows the window film reflecting body heat back into the room.

  • The Health and Safety Executive

    Radiant temperature has a greater influence than air temperature on how we lose or gain heat to the environment