Double Glazing Buying Guide

There are two ways to obtain double glazing. The most costly is to get a specialist company to install hermetically sealed units - generally the most efficient. Installation involves taking out the existing window frames and installing new ones. The outside appearance of the property will consequently be altered - slightly or drastically, depending on the type of replace­ment window frames you choose.

The second, much cheaper method is to fit secondary glazing, which is available as a kit. The advantage here is that the existing win­dows remain in place so that the outside appearance of the house is unaltered. This type is fixed on the inside, either to the existing window frames or to the surrounding walls of the window reveal.

The simplest and cheapest kit system uses a clear, durable sheet of film, which you can fix in minutes. Cut the film to the overall size of the window, with an allowance of 2in all round. Then fix double-sided adhesive tape or pvc strip around the frame and secure the film to this around the edges. Finally heat the film with a hair drier to remove all the wrinkles, leaving a perfectly smooth surface. Trim off any excess film with a sharp knife.

A slightly more sophisticated type involves a magnetic system that uses clear rigid plastic instead of glass, but still requires no clips, screws or other similar fixings. You cut the plastic sheet to the size of the window frame and then fix steel strip, which comes in rolls, around the outside of the frame. Apply the matching magnetic strip around the perimeter of the plastic sheet and position this so that the two strips meet. The magnetism will hold the double glazing in place.

Conventional double glazing kits are similar in principle and vary only in detail. A framework of pvc or aluminum accepts the glass and this assembly is then fixed either direct to the existing window frame or to the walls of the windows reveal. The glazing panels can be fixed, hinged or sliding. Hinged or sliding panels will give access to any windows you may want to open, whether casement or top hung.

You will have to buy the glass separately, since this is not supplied with the kit. The correct thickness - either single or double thickness - will be specified by the manufactur­er in his literature. Details for measuring the overall dimensions of the glass required will also be given and you must follow these instructions very carefully. Measure the windows accurate­ly and then refer to the literature to establish exactly what you need.

Often you will find horizontal channels come in one pack and the vertical ones in another, with all the fittings in a third. It gets more complicated with sliding units, where an addi­tional frame pack is supplied.

In terms of materials, the choice is between pvc and aluminum frames. Since both perform equally well, your decision is likely to be purely an aesthetic one.