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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.

uPVC is unplasticised polyvinyl chloride. It is a favourite amongst homeowners and manufacturers for its cheap production cost and excellent benefits. uPVC is used for window frames, conservatories and doors. In fact, uPVC can be three times cheaper than wooden frames and is also the most energy efficient option.As well as this, uPVC is now available in a range of colours other than the typically seen bright white finish. It can also be recycled, is durable and resistant to changing weather conditions.

Aluminium windows have a range of benefits that make them a great alternative to uPVC windows. Developments in technology have meant that modern aluminium windows now have the same energy efficiency levels as uPVC. Plus, their strength and flexibility make them a more attractive choice to many homeowners, particularly for more modern buildings.

Aluminium frames are typically much thinner than uPVC frames, allowing more light in and wider views. The metal can also be moulded to suit custom shapes, making it ideal for unusually shaped windows. Aluminium windows are available in lots of different colours, so can be designed to suit a wide range of interiors and exteriors.

To install one small double glazed window, it will take anywhere between two hours and half a day. It will depend on whether one installer is fitting your window or two. And if you’re doing it yourself and don’t have any professional experience, it will take even longer.

When the window fitters arrive, they will make sure they’ve got safe access to the window and move any heavy furniture if they need to. Any good installer will lay down a dust sheet, but it’s a good idea to check that they’ve got one laid down before they start.

The first step any window installer will take is to double check the measurements of the new frame against your existing one. Hopefully it should match up, so then they’ll take the old window out. Then, they’ll insert the new frame into the aperture and bolt it into the concrete. They'll check it with a spirit level to ensure it’s even before moving onto the glass sealed unit.

The installer will place the glass into the aperture, then fit the beads to secure the glass in the frame. They’ll then use silicone and trim on the inside and outside to ensure the window looks perfect. You won’t be able to see where they’ve used sealants or trims because they’ll colour match them so it’s not noticeable. Then, they’ll tidy up all of their tools and the working area and let you know what needs to be done in terms of proving that the window meets building regulations. Most good installers are registered with a self-certification scheme, so will be able to issue a certification once the work is complete.

Double glazed windows are better than single glazed generally because they are more energy efficient. The clue’s in the name – double glazed windows have two panes of glass while single glazed windows only have one. In double glazed windows, there is a space between the two glass panes. This space is filled with inert gas or a tight vacuum, which helps to provide an extra level of insulation. That means that heat won’t escape your home easily, nor will it creep in, so you can keep your home at a more comfortable temperature.

The extra pane of glass also helps to increase your home’s security. Single glazing is pretty easy to smash through, whereas it will take much more effort to smash double glazing and will create more noise, so burglars might be put off.

Another reason why double glazed windows are better than single glazed ones is that they help to reduce noise. Of course, if you live by a busy road you’ll still hear some traffic noise, but it can be significantly reduced buy double glazing. In the same way that the inert gas in the middle of the two panes keeps heat in and the cold out, it also helps keep noise out.

With double glazing, you’ll also notice much less condensation. With single glazing, it’s common to see condensation on the inside of the windows, especially when the heating is on. This is because the outside of the window is cold, but the inside is warm. With double glazing, the insulation between the two panes stops there being such a difference in temperature between the inside and outside of the window.

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.

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.

• The glass has smashed from an impact such as a stone hitting it from a lawnmower
• You have been broken into and suspects used the window to gain entry
• The glass has blown (lost its airtightness) from constant water build-up around the seals, which is common in bathrooms
• Extreme weather or pressure changes have caused a crack

1. Including the beading (the plastic and rubber seals around the windows that can be removed), measure the width of the window frame.
2. Do the same for the height of the window, making sure you include the beading.
3. Then, take 10mm off each measurement, and you have the approximate size of your glass.