Old-fashioned windows, such as box or coupled windows, generally do not feature edge spacers, meaning they are not airtight. In cold weather, when we sit by such a window, it feels draughty, which in turn makes us cold. Additionally, draught also occurs in case of poorly installed windows and old-fashioned types of roller blind headbox assemblies. There are gaps in the joint between the window and the wall that have not been sealed. However, we also feel cold by windows with proper edge spacers, i.e. by airtight windows featuring glass panes with poor thermal insulation properties. For example, box and coupled windows with only two separate windowpanes or integrated windows with ‘thermal pane’ glazing still have a very high U-factor: U = 2.9 – 3.1 W/m2K. On cold days, such windows and/or glazing still let a great deal of radiative heat flow outside. Consequently, the temperature of the inside of the glass pane remains quite low even though the ambient temperature in the room is 22 °C and the internal walls are comparatively warm. The wide temperature gap between the warm internal wall and the comparatively cold inside surface of the windowpane can cause the air to circulate, i.e. move from the warm internal wall to the cold glass. The result is that this invisible air circulation is felt as a draught and cold even though the windows and doors are closed! This ‘draught’ is proportionate to the abovementioned temperature difference.

Generally, we counterbalance the effect by bringing the ambient temperature up to 26 °C or more, when the air circulation is no longer felt as a cold draught. Unfortunately however, that also results in a significant increase in heat losses through the windows and peripheral walls as well as the consumption of energy required for heating. It has been established that by raising the ambient temperature by one degree, the energy consumption increases by 5 – 7%!An economic and ‘healthy’ solution to that invisible draught is provided only by modern airtight, i.e. energy efficient, windows and thermal insulation of external walls. What actually needs to be achieved is to make the temperature on the inside of the glass pane as high as possible. Measurements have shown that at outside temperature -10 °C and ambient (room) temperature +20 °C, the temperature of the inside windowpane primarily depends on the quality of glazing. In case of ordinary ‘thermal pane’ glazing with U-factor U = 2.9 – 3.1 W/m2K, the temperature on the glass is only + 8.4 °C, while with low emissivity glazing with U = 1.1 – 1.6 W/m2K, the glass temperature is already + 13.8 °C to 15.5 °C, and in case of insulation glazing with U = 0.4 – 0.8 W/m2K, it can even reach +16.8 °C to +17.3 °C.

Low-e glazing usually features a 4-16-4 mm structure and one low emissivity coating on the inside of the internal windowpane with low-conductivity inert gas argon hermetically sealed in the insulating glass unit. The industry recommends that existing buildings with old-fashioned windows be retrofitted with energy efficient windows, which should also be installed in case of new construction. A comparison of heat losses in an average single-family dwelling shows that when ‘thermal pane’ windows are replaced with low-e glazing e.g. where U = app. 1.1 W/m2K, heat loss through the windows can be halved. Additional heat savings can be achieved by fitting the windows with modern blinds that also increase safety and keep the room dark as well as provide protection from the scorching sun. Furthermore, the industry has established that over 20% of occupants are planning to replace worn out and old-fashioned windows, in flats and single-family dwellings, in order to improve their living environment (ambient conditions, noise protection, additional safety, energy savings, the look of the building and the room, etc.).

Public Relations Services, Inles d.d. Ribnica, 30 July 2004