The physics behind roof windows.

Posted by Lawrie Fraser on 22nd February 2023

Roof windows are an increasingly popular architectural feature in modern homes. They provide natural light, ventilation, and a unique view of the sky. However, there is more to these windows than just their aesthetic appeal. The physics of roof windows plays a crucial role in their performance and efficiency.

In this blog, we will explore the physics of roof windows and how it affects their functionality.

Solar Heat Gain

The sun's rays can heat up a house and cause discomfort. This is especially true in hot and sunny climates. Roof windows can mitigate this problem by allowing natural light to enter while blocking excessive solar heat gain.

The physics behind this lies in the glass used in the window. Most modern roof windows use low-emissivity (low-e) glass, which has a thin, transparent coating that reflects the sun's rays while allowing visible light to pass through. This reduces the amount of solar heat gain without blocking natural light.

Thermal Insulation

Heat loss through windows can account for a significant portion of a home's energy consumption. Roof windows are no exception. In cold climates, poorly insulated roof windows can lead to a significant increase in heating costs.

The physics behind thermal insulation lies in the properties of the glass and the frame. Double or triple-glazed windows are common in modern roof windows. These windows have two or three panes of glass with a layer of insulating gas between them. This reduces heat loss through the window.

In addition to the glass, the frame of the window also plays a crucial role in insulation. Most modern roof windows use uPVC or aluminium frames, which have excellent insulation properties.


Roof windows are an excellent source of natural ventilation. They allow warm air to escape from the house, which can help cool it down in hot and humid climates. The physics behind ventilation lies in the principle of convection.

As warm air rises, it creates a natural flow of air through the window. This can be further enhanced by installing roof windows on opposite sides of the house, which creates a cross-ventilation effect.


Condensation is a common problem in roof windows. It occurs when warm, moist air comes into contact with a cold surface, such as a windowpane. This can lead to mould and mildew growth, which can be harmful to your health.

The physics behind condensation lies in the difference between the temperature of the air inside and outside the house. To reduce condensation, modern roof windows often use warm edge spacers between the glass panes. These spacers reduce heat transfer between the panes, which can reduce the temperature difference and prevent condensation.

In conclusion, the physics of roof windows plays a crucial role in their performance and efficiency. Solar heat gain, thermal insulation, ventilation, and condensation are all factors that need to be considered when designing and installing roof windows. By understanding these principles, you can make informed decisions about the type of roof windows to install in your home.