Glass is not a very good material for keeping in heat—for a given area, you will lose more heat through the windows of your house than through the walls or roof. The way to reduce heat losses via windows, and so cut your heating bills, is to double-glaze them—instead of having a single pane of glass, the windows have two parallel panes separated by a trapped layer of air. But do be careful when you install double-glazing that you can easily open the windows in the event of fire.
There are many ways of providing two parallel sheets of glass in a window —from sheets of polythene taped to the inside frame of an existing window, to professionally-installed, purpose-made replacement windows. With existing windows in the UK, double- glazing usually takes the form of an additional window of some sort mounted on the inside of the existing frame— this is called secondary glazing. In the US and Canada, double-glazing often takes the form of an additional window mounted on the outside of the existing one—these are usually called storm windows. Storm windows, and indeed internal secondary glazing systems, can be either permanent or removable for the summer; storm windows might also double as fly screens, or you might swop removable ones for similarly-shaped fly screens in the summer.
The benefits of double-glazing
The most obvious reason for double-glazing is to reduce fuel bills. In the UK, however, the amount of fuel saved is not likely to make anything but the very simplest and cheapest form of double-glazing system worthwhile— but this is not true in many parts of Canada, and some parts of the US: indeed even the additional cost of < triple-glazing may be cost ‘s effective in some areas.
I Double-glazing does have other < benefits, though. Particularly with large windows it reduces the cooling downdraught effect that can make a room feel chilly even when heated properly, and can severely restrict the area of the room that is comfortable to sit in. It reduces condensation and, if the right system is chosen, can dramatically improve sound insulation. It is also worthwhile working out the cost-benefit of double-glazing if you are installing a new central heating system: with double-glazing, you will need a smaller heating system, so you can offset some of the costs against the savings here.
Double-glazing often has a aramatic effect on the level of draughts through the windows, but this is mainly because the new windows are fitting better than the old ones: in effect, you are draught proofing. And if you think this is the main benefit you will get from double-glazing, you will find that weatherstripping materials are a lot cheaper.
There are a huge variety of double-glazing systems available, especially in the UK. Simple systems are at the cheapest end of the scale, and the cheapest of these are based on plastics sheeting, which come in many thicknesses, ranging from that used for food-wrapping to acrylic sheet such as Perspex. In between there are numerous other plastics forms, including the popular middle-of-the-road choice for double-glazing on a budget: acetate sheets, available in various thicknesses.
The disadvantages of thin plastics film, such as food wrap, is that even when fitted well it tends to look terrible. Polythene sheeting is little better, as it tends to whiten and become more brittle when exposed to sunshine. Acetate sheet, which keeps its clarity well, is a better choice—even if there are problems over the dust-attracting static that tends to form, especially near curtains.
A further disadvantage of all forms of plastics sheeting is that it usually has to be discarded when removed for window cleaning. You may also risk damaging the paintwork when you pull away the double-sided adhesive tape normally used to fix the sheet or frame in place.
It is possible to incorporate thicker plastics sheeting within a frame arrangement constructed from wood battening or plastic channelling, perhaps using components from a double-glazing kit. The frame can clip into place for easy removal when cleaning or storage is required.
For thicker plastics sheet or, even, another sheet of glass, you can make a more substantial frame. But this or any other frame and the window frame which supports it, must be capable of taking the weight of the glazing material used.
If you choose to use glass, 4mm float is normally sufficient but your glass supplier will be able to advise on local requirements if you are in any doubt. For small windows and thin ‘horticultural’ glass, a clip and frame arrangement is normally adequate providing you can achieve a good seal with the window.
Secondary-sash glazing describes several methods of providing a more permanent form of double-glazing. With these, the second sheet of glass is supported against the window frame by a hinge arrangement , or in channels which permit horizontal sliding for opening and closing. Some of these frames are supplied ready-glazed and complete with all fittings, including the seals so important to these designs. Alternatively, you may prefer to construct your own secondary-sash arrangement using components from some of the many kits available. All forms of secondary-sash double-glazing permit access for easy cleaning of inner window surfaces, and enable you to open the window for ventilation in dry weather if required.
Extruded aluminium frames are most commonly used for secondary-sash systems, particularly those which are supplied ready-glazed.
Coupled glazing is an even more ‘permanent’ variation where, in effect, two window frames are employed in place of one. This arrangement is particularly suitable for windows with a narrow sill which cannot take a sliding secondary sash or other form of glazing frame. Both windows open and close as one, but the additional one is hinged and clipped to enable easy separation for cleaning. A form of coupled window can be used for vertical sash windows, though the counterweights on the cord may need to be increased slightly.
Although coupled windows are available factory-made in a variety of standard sizes, making your own should not present too many difficulties.
Storm windows, although also mounted on the outside, are not often coupled to the original window, but are rigidly fixed to the frame.
Sealed units consist of two layers of glass, with an air gap between, sealed together in the factory. You install the complete unit in the window frame opening in place of the existing single sheet of glass. These are covered in the next section.
Width of air space
The heat insulation property of double glazing varies with the width of the enclosed air space. The optimum space depends very much on the temperature difference between the inside and outside surfaces, but is usually taken as 20mm in temperate zones when heating is up to normal standards. For easier fitting, this gap is reduced to 12mm in factory-sealed insulating glass and the difference in heat loss can be considered negligible. A gap of less than 12mm leads to progressively greater heat losses.
The heat loss, or thermal transmit-tance, of a single pane of glass under average conditions of use and exposure is taken to be 5.6 watts per square metre of glass, for each degree Centigrade temperature difference between the inner and outer surfaces. This is expressed as 5.6W/m2 ‘C. A 20mm air gap between two sheets of glass almost halves the heat loss to 2.9W/m’-°C. Insulating glass yields figures of 3.0 and 3.4W/m2 °C for air gaps of 12mm and 6mm respectively.
Sound insulation is often one of the most important considerations when planning a double-glazing scheme, particularly if your home suffers greatly from the noise of a busy road or nearby airport.
Any double-glazing reduces noise on account of the trapped airspace between panes and as this gap increases, so sound insulation improves. However, the gap needs to be at least
I / / / 100mm—and preferably 150mm with 4mm glass—for the best combination of sound and thermal insulation. While a bigger gap of up to about 400mm improves sound insulation, thermal insulation is greatly reduced because of the convection currents that then have space to form between the two panes of glass.
Whatever type of double-glazing you use, fitting it to allow a gap of 100mm or more is often difficult on most types of domestic window. So, if thermal insulation is important, you may have to make suitable modifications to the window frame surrounds.
Factory-made sealed units usually have an air spacing of between 5mm and 12mm and the sound insulation properties these may have are due only to the weight of glass involved. However, if used as a secondary unit this insulating glass can be positioned to provide the necessary gap, in which case further benefits are provided by the additional trapped layer of this multiple glazing set-up.
Where the gap width permits, you can achieve additional sound insulation by using acoustic tiles to line the soffit, reveal and sill of the window. Bear in mind that this insulation should not impede drainage arrangements which may have to be provided.
A frequent problem with single-glazed windows is condensation, which can reach severe proportions if the atmosphere indoors is very humid. Over a period of time this ruins fixtures and fittings and may even cause wood framework to rot, replacement of which must be the first priority in any double-glazing project.
Although double-glazing does not always eliminate condensation, it goes a long way towards curbing the problem—especially if fitted properly. Secondary-sash and coupled double-glazing present a slightly different problem in that it is difficult to prevent slight condensation on the inner surfaces, although you can take measures to reduce this effect.
Drilling a series of holes, at an angle downwards through the window frame, enables the air gap to ‘breathe’—but no more—and so prevents a build up of too much moisture. Or, you can place silica gel, a moisture absorber, in the air gap on shallow trays made of up-ended channelling. The silica gel has to be removed and dried from time to time.
Insulating glass normally has a filling of inert gas sealed within the unit. Condensation between the panes should not be a problem and there is usually some form of replacement guarantee to this effect, providing the seal remains intact.
Condensation is likely to occur if there is a poor seal between a secondary-sash and the supporting frame, a very real problem with some of the cheaper DIY double-glazing kits as adequate precautions cannot normally be taken. Always provide a generous and effective seal if you can, but make sure you install your glazing on a cold, dry day as the moisture content of the air is at its lowest then. Even with removable double-glazing arrangements such as secondary-sash systems, it may be worth adding a ‘bedding seal’ of temporary or permanent nature, particularly for opening windows.
Draughts are an unwanted feature of every house, and many originate from badly-fitting windows or doors. Most problems of this nature are easily put right and must not be allowed to go unchecked when double-glazing is installed as many of the benefits may then be lost. Draught-proofing between an air gap and the inner glazing sheet is especially important if condensation is to be minimized or eliminated.
If a frame, window or door is in a particularly bad state of repair, it may be easier and cheaper in the long run to consider fitting a double-glazed replacement from the outset of any improvement scheme.
Where to glaze
In most cases, double-glazing is an improvement job which is undertaken in stages as both weather and funds permit. Deciding quite where to start can be a problem, so base your choice on those rooms which you use a lot, those kept at the highest temperatures, and those which have the largest area of glazing.
The living room, sitting room and kitchen are obvious starting points and as these are likely to account for much of the groundfloor glazing, it is a good idea to choose one, matched system for all of them. Double-glazing upstairs is a luxury by comparison, but should be considered in your long-term plans. Temperatures here are usually kept lower, and even if heating is turned off at night, this hardly matters when you are snugly asleep.
Take care to match the likely strength of a glazing kit with the area of glass that it has to support.
Most forms of double-glazing in kit form are easily installed by the home handyman. Of these, the various forms manufactured from aluminium section tend to be more durable and better looking than their plastics counterparts which often cost as much. With all forms, particular care should be taken to ensure clean, square cuts as errors here can greatly mar the finished appearance. Take care to avoid scuffing aluminium which readily shows this ill-treatment. Buy height and width kits oversize.