Using cling film
Before discussing double-glazing with glass you might consider a very cheap method. This uses ‘cling film’ kitchen wrapping material instead of glass as the inner pane. Double-glazing works partly by trapping a layer of still air against the window and it is this layer of air that insulates: the insulating effect of the glass or other material is not great by comparison. Tests have shown that cling film double-glazing is at least 80 per cent as good as proper double-glazing and, of course, it is extremely cheap.
The glazing works because the film sticks to clean paint, so the first step is to clean the window surround with washing-up liquid and water to remove any grease, and to clean your hands as well. The film is available in rolls either 300mm or 450mm wide which means that it cannot be used for windows wider than this. If you have wide windows you could consider making up a frame of 25mm x 50mm planed softwood to fit into the window surround and support the film. If you do this the frame should be sanded carefully to remove any rough spots on the side that will face into the room and then given a coat of primer, a coat of undercoat and two coats of gloss paint. Similarly, if the paint of the window surround is in poor condition it should be sanded down and repainted to give a smooth surface. Allow the paint to dry for at least twenty-four hours before trying to apply the film.
When the paint is ready take the roll of film out of its box and allow about 50mm to unwind from the end. Put one of your little fingers inside each end of the cardboard roll on which the film is wound and hold the corners of the piece that you have unrolled between thumb and forefinger. Take the roll to the window and make sure that the film is unwinding from the side of the roll nearest the glass. Press the film that you are holding in thumb and forefinger against the top of the window frame or surround, making sure that the side edge of the film always overlaps the sides of the window frame. Using your thumbs, press the film against the painted window frame and smooth it down to exclude air bubbles.
When the film has stuck down firmly use one hand to hold the roll while you smooth down the rest of the film at the top of the window. Then let the film unroll slowly down the window and press it against the sides of the frame, doing about 100mm at a time. When you reach the bottom of the window press the film firmly against the frame and trim it off with a single-edged razorblade (to prevent cutting your fingers with a conventional blade), or a Stanley knife.
At this stage the window will not look very impressive as the film will be rather wrinkled, so the next step will be to remove the wrinkles. To do this peel back one of the top corners of the film for a distance of about 100mm. Pull the film gently so that it is taut and press it back again. Then gradually go round the window unpeeling the film, pulling it tight without splitting it, and pressing it back into place. With any luck this process will remove the wrinkles and you will be left with a clear taut film covering the window.
If you have added a wooden frame to the window to make it narrow enough to take the film, make sure that the film does not come more than halfway across the width of each vertical framing piece. Use the razorblade to trim it if necessary. If you do not do this you may find that the previously tightened film will be loosened by the removing and refixing of the subsequent piece of film. The film should last for at least a year if you are careful with it, and can then be replaced with a new roll.
Because of the problem of width limitations the film is best suited to windows that are divided into small panes. On Victorian windows these dividing bars are too narrow for the film to stick to them; but modern mass produced ‘Georgian’ windows use a wide flat bar which is ideal for sticking on the film. If you need to use a wooden frame to divide up the window area into smaller widths the cost will be increased, but this method will still be much cheaper than any other double-glazing techniques.
If the film extends across the whole window area it will help to reduce ventilation heat loss by acting as a draught seal, but if you use a nonflued combustion heater such as a paraffin stove or portable gas heater or if you have a gas fire be sure enough air for combustion can enter the room, otherwise you will run the serious risk of being overcome by fumes. If you use a coal fire with a chimney you will know if there is not enough draught because the chimney will not draw.
If you do not want to replace cling film every year, the next stage up in double-glazing is to use real glass with a simple edging to hold it in place. This type of glazing is not openable so you will have to take it down in the spring and store it somewhere until the autumn, hoping you do not break it in the meantime. The simplest type uses a plastic edging strip which you press on to the edges of the glass, cutting 45° angles at the corners with a sharp Stanley knife to make it fit neatly. Small plastic clips are then fixed to the window surround and they are used to grip the edging strip that you fixed to the glass. The edging cushions the glass and provides a seal to keep out draughts and noise.
The cavity between the existing window and the new sheet of glass should be ventilated to the exterior. If the old window is loose fitting you need take no further action, but if it is fairly tight you should drill 6mm holes through the top and bottom of the window at 300mm centres. Slope the holes downwards to reduce the chance of rain coming through them and plug them with small bits of glass fibre to keep out insects but allow the air to permeate. The purpose of the holes is to allow sufficient air to nte r the cavity to evaporate any moisture that forms in there as a result of condensation.
If you are adding a fixed pane of glass to a window, remember that building regulation K4 stipulates that, in any room, there must be a ventilation opening with an area at least a twentieth of that of the floor area. You will need the ventilation anyway if you are using a paraffin stove, calor gas heater, or gas stove. If you plan to use any of these do not seal up the windows D r you may fail to wake up one morning.
It is easier if you can open your double-glazing so that you can open the existing window for ventilation, and it is very helpful not to have to take down the double-glazing and store it each summer when you want the windows open. There are various do-it-yourself systems of permanent hinged and sliding double-glazing. Points to look for when choosing one of these systems are that it should be made of long-lasting materials and that it should be easy to assemble. Most types of hinged double-glazing are made of aluminium sections with plastic gaskets to hold the glass and plastic or longer lasting neoprene rubber seals to fit against the window frame. The hinges and fasteners are usually nylon.
The types that are easiest to assemble use moulded corner pieces which eliminate the need to cut 450 mitres at the corners of the frame sections: it is much easier to cut a piece of frame to length at right angles rather than trying to cut to length at a mitred corner. The sliding systems are more expensive because there is a separate frame in which the framed pieces of glass slide. The seal between the sliding part and the fixed frame is maintained by pile seals which are like small brushes that brush along the sliding section. The sliding frames themselves are made like the hinged frames described above. Most of these systems have clear instructions and it is best to follow these carefully, reading them at least twice before starting to cut into the pieces of framing. The instructions will also tell you what size to have the glass cut to fit the chosen glazing system.
For many of these systems you will have to fit a simple wooden frame into the existing window opening. If you have sliding sash windows you will probably be able to screw the double-glazing directly to the surround of the window but many more recent houses have metal windows and the sides of the window openings are of plastered brick. In this case a frame of ex 50mnn x5Omm planed softwood should be screwed to the window opening using 75mm no 12 screws, preferably zinc-plated, and plastic plugs. At the top of the opening there will be a lintel to carry the wall above; if it is made of concrete you will have great trouble drilling into it. The easiest solution is to hire a
percussion drill which delivers hammer blows to the drill bit while it rotates; this used with a masonry bit should make suitable holes. Try to make the surface of the frame level all the way round so that the glazing will fit flat against it. Before fixing the glazing, the timber frame should be primed and painted or else treated with a preservative stain.
Before deciding to build double-glazing you should think whether it is really necessary. If your windows are fairly small (10-15 per cent of the total external wall area of your building), and if you can draught seal them effectively, you may not need double-glazing at all. Other ways to prevent more heat being lost through windows than absolutely unavoidable include the use of thick lined curtains which touch the ceiling or else have a pelmet to reduce the chance of air circulation at the top. Ideally the curtains should be long enough to brush along the floor so as to form a draught seal against the cold air that will tend to flow down the window. If you shut the curtains as soon as it becomes dark you will save a worthwhile amount of energy.
Another saving is to contradict the universal practice of putting radiators under the windows if you install central heating. The idea is to use the heat from the radiators to counteract the down draughts of cold air from the surface of the glass and try to warm up the draughts before they enter the room. What happens is that the heat from the radiator is almost immediately lost through the windows and you receive little benefit from it. The best place to put radiators is on internal walls, so that they heat the fabric of the house, rather than on the colder outside walls.
If you already have radiators under your windows and do not want to move them you can fit a shelf above them to deflect the hot air from the window; arrange the curtains so that they touch this shelf. You can also fit shiny foil insulation behind the radiators to reflect heat into the room and away from the outside wall. Finally, do not sleep with your window open in a heated bedroom, either leave the heating on and open your door, or turn off the heat and have the window open. When you do want to ventilate the house choose a warm day when the heating is not operating at full blast and remember to close the windows when the air has cleared.