There are two reasons to fit double glazing – to keep noise out and heat in. Since about 20% of the heat in your home is lost through windows and glass doors, double glazing can translate into considerable savings on the heating bills. Professional double glazing can be costly, as it involves removing existing windows and frames and replacing them with a double frame and two sheets of glass. Secondary double glazing can provide a solution, but it is by no means the ideal solution it is made out to be.

Double glazing creates a layer of insulating air between the two sheets of glass that prevents heat transfer and, if the gap is greater than 100mm, prevents noise coming into your home as well. Secondary double glazing creates the same effect, without having to remove the existing windows. An additional frame is fitted around the window recess to hold the second sheet of glass, thus doing away with the need to fit a whole new frame.

Unfortunately, many secondary double glazing windows are made from aluminium or uPVC. Not only are these materials decidedly unfriendly to the environment, they are not particularly attractive. Although they are now available with various finishes, the cheapest option is plain white PVC, which can look rather tacky, particularly in older, more traditional homes. When fitting this type of secondary double glazing, make sure that the windows can be opened so you still have ventilation when you need it.

The cheapest type of secondary glazing is a DIY option where a plastic film is fixed to the window recess with double-sided tape and shrunk to fit with hot air, as from a hairdryer. The plastic film is not completely transparent, so it is quite obvious and rather unsightly. This is not a long-term solution either, as it will only last one season. Because it cannot be opened, you won’t want it on your windows during the warmer season any way.

Secondary double glazing is not permitted on many listed buildings, in part due to the nature of the materials used, and because of the detrimental effect it has on the appearance of the building. There are some companies that will fit secondary double glazing with proper wooden frames, but they are not easy to find, and often the cost is the same as professionally fitting double glazing.

Proper double glazing can be found at a reasonable price if you shop carefully. Try to get referrals from people who have had double glazing installed, and make sure you compare a few quotes before selecting a glazer. Often smaller, local glazers will be prepared to negotiate on price, unlike bigger, national franchises.

Professional double glazing will look better, last longer, and work better. Secondary double glazing is not ideal, as it can be difficult to seal the secondary frame properly because the window recess is not perfectly straight, which can lead to heat loss and defeat the purpose of double glazing.