While heat losses vary depending on the nature of a building and its aspect, in a typical uninsulated house about l5 percent of house heat is lost through the windows. If all the windows in such a house are double-glazed, this heat loss will be halved to give a seven and a half percent saving on fuel bills. There are many factors which can affect this figure – for example, the type of system used and how well it is fitted. Installing double glazing in an old cottage with just a few small windows would not obtain this saving whereas there will be higher savings in a modern ‘goldfish bowl’ type of property.
Double glazing is not a money saver on the scale of other forms of insulation such as glass fibre laid in the loft or cavity wall infill; however, there are a number of reasons why you will find the necessary expenditure worthwhile to add to the comfort of your home. An efficient system will eliminate cold, draughty areas round window making the whole floor area of a room usable on cold days, and rooms will seem larger without the need for occupants to cluster round the fire or radiators.
When rooms are properly heated and ventilated, condensation will be reduced and possibly eliminated by double-glazed windows, since the inner panes of glass will be warmer and less susceptible to misting. With some double glazing systems, interpane misting may occur; this is usually slight and can be wiped away provided the new window is hinged or sliding. Alternatively you can place silica gel crystals between the panes of glass; these absorb moisture and, when saturated, should be temporarily removed and dried in a warm oven.
Misting on the room side of the window indicates the temperature of the glass is too low, given the water content of the room’s atmosphere; by a process of trial and error, you should carry out adjustment until there is a proper balance between heat and ventilation in the room. Condensation on the cavity surface of the outer glass is usually a sign that moist air is leaking into the cavity from the room. Make the seal round the new double glazing as airtight as possible, using a tape form of draught excluder, and seal any gaps in the joints of a timber framework with a matching wood filler, making sure the filler penetrates through to the full depth of the joint. If this fails to cure the problem, drill ventilation holes through the primary frame to the drier air outside. In a 1m wide window, two 10mm diameter holes set 500mm apart should be sufficient.
More will be needed for larger windows; you can decide the exact number by a process of trial and error – drilling an extra hole and waiting to see if this cures the problem. Pack the holes with glass fibre to act as an air filter. With hermetically sealed units the air in the cavity is dried, so condensation between the panes is not possible as long as the seals remain sound; failure of the seals is a rare occurrence, but reputable manufacturers give long-term guarantees to cover the possibility.