Secondary Glazing

The installation of secondary glazing inside your existing windows is an effective way of reducing heat losses in the winter. Unlike shutters and curtains, which are usually open during the daytime, this method is effective 24 hours a day. In traditional buildings, the installation of secondary glazing is sometimes preferable to complete window replacement because of the historic importance of the existing windows. Secondary glazing can reduce heat losses through the window by over 50% and will all but eliminate the draughts through your windows. Noise from outside will also be dramatically reduced.

Although secondary glazing preserves the existing windows, it is vital to ensure that there is no visual conflict between the original and secondary glazing. This can happen if the secondary windows have glazing bars that do not align with those of the original windows. Secondary glazing is usually designed with a minimum of glazing bars to avoid this problem.

If you have shutters, you need to ensure that the addition of secondary glazing does not obstruct their use. This typically requires installation within the architrave between the closed shutters and the existing window.

Temporary options

Secondary glazing can be installed on either a temporary (seasonal) or permanent basis. The temporary options tend to be a lot cheaper. The cheapest option of all is a proprietary film, available from DIY stores, that is stretched across the window architrave at the beginning of winter. With the aid of a hair-dryer, this film tightens and sets fast. You have no access to the window but your draughts are completely removed. The film is torn down in spring and replaced anew the following winter.

Another seasonal approach is to use sheets of acrylic plastic, cut to size, with a magnetic edge. These are held in place by discreet plastic carrying strips that are stuck to the window frames using double-sided tape which can be easily removed if need be. Come winter, the plastic sheet is simply held up to the window and locks into place against another magnetic band on the carrying strip (See case study). As with the cling-film, you no longer have access to the window during the winter months, though the sheet can be taken down temporarily if need be.

Secondary glazing of this kind can be almost invisible if installed with care and is entirely reversible. Furthermore, as the carrying strips sit within the window architraves, there is no conflict with shutters which can still be used as normal. It is an extremely low cost way of cutting out draughts and reducing heat losses in the winter.

Integrated secondary glazing

Secondary glazing can also be installed on a semi-permanent, integrated basis. Although this is more expensive, as it effectively involves the specification of a new set of interior made-to-measure windows, it is much more durable than the temporary, seasonal options and will also be more effective in cutting heat losses. Ideally, use timber, slim-profile double-glazing for the secondary unit as this will dramatically reduce your heat losses, and specify low emissivity coatings, gas-fill and warm edge details (see related opportunity on double glazing) .

If secondary glazing is to stay in place throughout the year, it must be openable to allow summer access to the original windows. This is normally achieved by dividing the window into two panels and allowing one panel to open. The glazing bar between the two panels should align with the centre bar of the original window. In the winter, it is best to keep the secondary glazing fully closed to minimise heat losses and reduce the risk of condensation on the primary window.

Historic England have prepared specialist guidance on this subject (see Resources).