Unprotected glazing and single glazing in particular means the surface of the glass is noticeable colder than the warm air in the room. This lowers the room temperature and produces draughts. The Relative Air Velocity ends up too high and occupants will feel winter discomfort. For this reason, all windows require protection from heat loss in winter. To minimise winter heat loss, it is important to trap a layer of insulation still air between the window and the room. This can be achieved for instance by using internal coverings, such as drapes, Holland blinds, Roman blinds or Australian blinds, and thin or lace curtains combined with pelmets.
Effect of window treatments on winter heat loss
(According to Sustainable Energy Authority Victoria 2002)
- Unprotected single glazing: 100%
- Vertical or venetian blinds: 100%
- Unlined drapes or Holland blinds, no pelmet: 92%
- Heavy, lined drapes, no pelmet: 87%
- Unlined drapes or Holland blinds, pelmet: 79%
- Standard double glazing: 67% (the higher the U-value the less the heat loss can be)
- Heavy, lined drapes, pelmet: 63%
- Double glazing with Low-E coating: 57%
- Double glazing, heavy drapes, pelmet: 46%
The most effective way to protect windows against heat loss in winter is a combination of double glazing and internal window coverings. However, if internal coverings are inappropriate or not desired, for instance in highlight or clerestory windows, in kitchens or simply where unobstructed views are wanted, double glazing is an indispensable measurement in order to prevent heat loss in winter. Yet double glazing won’t prevent sun coming into the building, which means that the windows need to be protected from harsh summer sun by means of external shading.
Another, often underestimated roll in the energy efficiency of a window, is the frame itself, as it can effect negatively on the overall performance. As we talked about in the blog “Adequate Insulation”, some materials, such as metal, glass or aluminium, allow heat to pass through them more easily, therefore they shouldn’t be used for windows frames if at all possible. If metal frames are used, such as aluminium, they should have thermal breaks to reduce the heat transfer. Generally speaking, PVC and timber frames perform better than metal frames.