Windows are essential for a house and the comfort and well-being of its inhabitants, as they let natural light and fresh air into the building and enable views.
Appropriate window design, size, location and glazing treatment, combined with shading and internal covers, can significantly reduce the energy required for heating and cooling. Maximum solar access for north-facing windows can reduce winter heating bills up to 25%. External shading can block up to 80% of summer heat gain through windows. Double glazing and internal coverings can reduce heat loss in winter up to 40%.
Glass is the potential weak point of a building in terms of energy efficiency. A single glazed window can gain or lose up to ten times more heat than an insulated wall. The main heat gain through windows is due to thermal radiation. Windows receive direct solar radiation when the sun strikes the glass, but also diffuse radiation reflected from the sky and the ground. Between 30-40% of total radiation to north windows is diffuse, depending on the weather conditions. Radiation from the sun travels through glass to the inside of a house. This radiant heat is absorbed by thermal mass, building elements and furniture, which when warmed up, re-radiates heat to the room air. This re-radiated heat is trapped inside, resulting in convective heat build-up within the room. This process is called ‘glasshouse effect’. In order to hinder direct rays from the sun entering the building in summer, glass needs to be shaded appropriately. On the other hand, it is also important to ensure valuable winter sun can shine into the house, as heat gains in winter can reduce the requirements for mechanical heating.
Read more about how to design and orientate your windows and the importance of avoiding thermal bridges in our other articles.