How to install secondary glazing

Love your old windows but tired of the constant draught? Time to install some secondary glazing with our simple step by step guide

How to install secondary glazing
(Image credit: Getty)

While old windows in period properties do look beautiful, they aren't always the most energy efficient but before you consider replacing your old windows, try installing secondary glazing. Fitting secondary glazing is relatively straightforward (more often than not the sheets can be delivered to your home already cut to size and including fittings) and can be tackled even by a DIY novice...

Find out more about renovating a period house, and about all things windows on our dedicated page.

You will need:

1. Measure up the 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. Carefully cut the sheet

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.

How to choose windows for period properties

(Image credit: Getty)

4. Apply magnetic strips 

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.

How to install secondary glazing

(Image credit: Getty)

7. Line up the sheet with the window

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.

Top tip: For larger window glazing (120cm x 120cm 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.

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