How to install secondary glazing

Old windows are beautiful, but they can often allow in draughts, especially during the winter. Helaine Clare shows how to reduce any problems at once by installing secondary glazing

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Single window glazing encourages condensation, lets in rain and wastes energy as warm air leaks out through the gaps, making for a chilly winter. So if you have wonky windows in your house it may be time to invest in some secondary glazing – you’ll recoup the initial financial outlay as you save on heating bills year after year.

As well as saving money on heating, secondary window glazing makes life less noisy if you live near a busy road, airport or railway line, as long as you leave a gap of at least 150mm between panes. If you plan to revamp the whole house and choose a system that effectively seals windows in winter, leave one or two windows free in case of a fire.

You will need…

There are various systems available on the market. The sheets can be delivered to your home already cut to size and including fittings. They are simple to install and can be easily removed when spring and summer arrive. For extra cosiness in the winter months it’s a wise idea to hang heavy lined curtains at the windows and draw them as dusk descends.

Adding the window glazing

1. Measure up window: Using a steel tape measure take measurements of the glass (not the frame). Add 25mm to each measurement to give you the size of plastic sheeting required. When using the magnet fixing system there must be at least 19mm of flat mounting surface around each side of the window glass.

2. Trim sheets to fit the dimensions: If your window is out of true and you have enough mounting surface around all sides you may make the sheet over-size to allow for the discrepancy. If this isn’t an option, you will need to take measurements and trim the sheet to size. To shape the sheet to follow the curve of the window make a template of card.

3. Cut carefully by hand: Protect the cutting table with a cotton sheet. Clamp the plastic firmly along the line of the cut and ideally cut by hand using a padsaw or hacksaw. A power saw can be used but it must be set on a slow speed to avoid melting the plastic. Apply light pressure at a shallow angle letting the saw do the work.

4. Apply magnetic strip: Once the top has been shaped, smooth the edge with abrasive paper if required. Use a cloth to wipe away any bits of plastic. Now take the coil of self-adhesive magnetic strip and stick a length to each edge, peeling off the protective paper as you work. The corners can be mitred or simply butted up to the adjacent strip.

5. Position the glazing panel: Hold the sheet up to the window so that it overlaps the window frame evenly on all sides. Two pairs of hands are best but if you’re doing it on your own you will need to prop up the bottom. Once it’s adjusted so the sides overlap evenly draw a pencil line around the sheet to mark the correct outer edge for mounting.

6. Fix the adhesive strip: Take down the sheet. Fit the self-adhesive white metal strip to the window frame matching the outer edge exactly to the pencil lines. Again, peel off the protective paper as you go. At the corners we marked the strip with pencil and used tinsnips to cut the metal. Alternatively you could use heavy-duty scissors.

7. Line up the new pane: Before lining the plastic pane up at the window and fitting the two parts together, mop up any condensation and clean the glass. Peel away the protective coating. Keep it crystal clear by cleaning with a soft lint-free cloth and mild soap in lukewarm water. Abrasive cleaners may damage the plastic.

8. Use extra support for big windows: For larger window glazing (1200mm x 1200mm or more) it is recommended that plastic securing clips are used for extra support. This will stop the possibility of the sheet slowly slipping downwards and coming away from the frame. Remove the secondary window glazing at the end of winter.

A note of caution

Don’t be tempted by the special offers and cost-saving promises of double-glazing contractors. Many of these involve ripping out old wooden window frames and replacing them with modern PVC ones, which can seriously affect the period quality of your home. It might also mean that you could face legal proceedings if you live in a conservation area or if your property is a listed building.