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• Impact from a stone or other object smashing the glass in a ‘bullseye’ effect
• The result of a break-in
• Extreme weather or changes in pressure causing a crack
• The sealed unit being ‘blown’, meaning that it’s no longer energy efficient due to air leaking out

All new windows come with an energy efficiency rating from A to G. A is best, while G is the lowest rating. Anything above C is considered good, but these days most companies will be able to supply you with A-rated windows to maximise your energy efficiency.

Whether you’re hiring a professional to replace your double glazing or are considering DIY, it’s a good idea to know how to fit a double glazed window.

You need to start by assembling your window. Fit any handles and cut the outer sill to size by measuring the outer wall depth. Then, screw the outer sill to the window frame and attach the frame fixers. Screw the ventilation grilles onto the inside of the frame and fix the vents on.

Next, you need put the window into place, using spacers to wedge it in. Use a spirit level to check it’s level horizontally and vertically, adding extra spacers if needed. Then, fix the frame into place at each fixing point using a drill, wall plugs and screws. Measure and cut the interior sill, and check that it’s level, adding spacers if needed. Then fix it using screws, masonry nails or adhesive.

Finally, fill in any wide gaps with foam filler and small gaps with sealant. You can get interior and exterior trims to cover these up, making your window look neat and professional.

Unfortunately, you can’t fix the glass on one side of a double glazed window. That’s because they’re sealed units – once one pane of glass is broken, that will break the seal and its thermal insulation properties will be lost.

The good news is that if your window frame is still looking great, there’s no need to replace that. You can simply replace the glass in the window, which will cost much less. You can look to spend around £250 on a sealed unit, whereas a whole replacement single window may cost up to £600.

When an installer comes to replace the glass in your double glazed window, they will survey the inside and outside of your home around the window, then move any furniture by the window so they’ve got access. They should then pop a dust sheet down to stop any debris from getting on your floor.

Then, the installer will carefully remove the beads from the frame. They will then lift the glass out of the frame and put it somewhere safe before cleaning out all of the dust and any debris that has found its way into the frame. Using spacers at the bottom of the frame, the installer will make sure that the sealed unit is square in the frame to get a good fit. Once they’re happy, they will replace the beading and make sure it looks perfect before cleaning up their tools.

You could have a go at replacing the glass on a double glazed window yourself, but it’s not recommended. Leave it to the professionals who will know exactly what to do and will be able to do it quickly and safely.

As well as keeping your home warmer and reducing noise, you’ll notice a saving on your energy bills from installing double glazing. If you install A++ rated windows in your home after having single glazing, you could save up to £120 year on your energy bills. The Energy Saving Trust has more information.

By replacing the single glazed windows in your home you stand to make savings of around £170 each year. It’s not just the financial gain that makes double glazing attractive though and you can also save up to 680kg of carbon emissions annually. Over 20 years this is a saving of around £3,400 and 13,600kg of CO2.

You can only repair blown double glazed windows in the sense that you can replace the sealed unit (the glass part of the window) instead of having to replace the entire window.

When a double glazed window is blown, it means that the seal around the window has failed and is now letting air in and out. Often the first sign that the sealed unit has blown is condensation on the inside of the window, which is annoying since there’s no way to get rid of it! When your sealed unit has blown, you’ll be losing heat out of the window, meaning you could be spending more on your energy bills.

As a temporary fix, you could see if you can buy a misting repair kit online, but the condensation will eventually occur again as the air gets in. The best thing to do is ask a window installer or glazier to replace the sealed unit for you.

It's simple to replace the glass in a window – the specialist will remove the beading around the window, remove the blown glass from your window frame and set it to one side. They will then clean up around the frame before adding spacers in, then fitting the new sealed unit into the frame. They’ll make sure it’s a snug fit before replacing the beading back around the window to secure it.

There's no reason to put off repairing a blown double glazed window since the cost is generally much less than replacing the entire thing. A sealed unit with installation costs an average of £100, whereas a completely new window will cost around £400. You’ll soon notice that it’s less draughty and will be able see out of your window once you’ve had the blown glass replaced.

On the grand scale of things there isn’t a huge difference between B-rated windows and A-rated windows. Consumer watchdog Which? have calculated an extra 6.5% would be saved on energy bills with A rather than B rated panes. Be aware that A rated windows are slightly more expensive, however.