Window insulation and double glazing
How much heat is lost through windows?
A typical home loses around 10% of its overall heat through the windows. However, if your home only has single glazing or poor window insulation, then you could be losing much more than this.
The amount of heat lost also depends on the number of windows in the house and how well they're maintained.
What can I do to prevent heat loss through windows?
One obvious measure is to replace windows and doors, including frames, with double or triple glazing. This reduces heat loss through the windows and resolves any issues with draughts as well. This can be expensive, especially if you get the whole house done.
According to the Energy Saving Trust, a set of A-rated double-glazed windows for a semi-detached house will typically cost around £12,000. However, along with making your home more comfortable, it could save you up to £140 a year on your energy bills.
How does double glazing work?
Double glazing reduces heat loss by having two panes of glass about 16 millimetres apart. The space between the panes of glass will either be filled with air or a specific gas like argon. This reduces heat being conducted from inside the house to outside because the air or gas cannot circulate, which means it's a poor conductor..
Some homes might have the option of getting double glazing with a deeper cavity of up to 100mm or more. The larger gap protects against external noise such as aircraft and traffic. Some suppliers also offer triple glazing sealed units.
What types of window frame are available?
You can find energy-efficient windows made with any of the following main frame materials:
- uPVC, or plastic, windows generally contain steel for added strength. There's no need to paint uPVC window frames, which saves on maintenance costs. They can look quite basic though.
- Composite windows and doors consist of an inner timber frame coated with a protective layer. They’re generally available in a variety of colours but can be more expensive than uPVC.
- Wooden frames are the greenest option but require a fair bit of maintenance. They’re often used in conservation areas where the original windows had timber frames.
Aluminium or steel frames are sleek and long-lasting, but they can be prone to condensation.
Does the energy efficiency rating apply to windows?
There's an A to G energy efficiency rating system that applies specifically to windows. Look for the BFRC energy efficiency label and ensure you use an authorised retailer.
It's worth remembering, however, that some uPVC doors and window frames are no more energy-efficient than wooden ones. The inefficiency of old windows and doors has as much to do with the glazing and draughts as with the material that the frames and non-glazed panels are made of.
If you want completely new window frames, consider triple glazing or low-emissivity (Low-E) double glazing. This is glass with a special coating which enables a better thermal performance than that of normal glass.
How do I know my double glazing is working?
From time to time, sealed units can fail and will need repairing or replacing. You can usually tell when there's a problem as condensation or droplets will appear between the two panes of glass. Most suppliers offer a guarantee - usually 10 years - and will replace windows free of charge if they fail within the guarantee period.
Can I fit double glazing if I live in a conservation area?
If you live in a conservation area or in a listed building, there may be restrictions on what you can do to your windows. uPVC replacement windows are typically not allowed. Consider these options instead:
Slim double glazing
You may be able to replace your existing pieces of glass with sealed units if your frames can accommodate them. Slim units are now available that are suitable for use in many original window frames without the need for replacements. These can have an overall thickness of as little as 12mm with a gas-filled cavity and a 5mm perimeter seal designed to fit into Georgian-style windows to maintain the visually slim glazing bars. A traditional glazer should be able to fit them.
Secondary glazing
Secondary double glazing is another option, and one which is often overlooked. It's also a potential DIY option, and a strong option for use in conservation areas, as well as locations that suffer from noise pollution, such as under an aircraft flight path or near a main road. The basic concept is to put up a second wall of glazing on the indoor side of the existing pane.
Is double glazing my only option to prevent heat loss?
Double glazing is just one option. There are alternatives and some are cheaper. For many of the options, the services of a joiner or handyman might be needed. But, to save money, you can carry out many of them as DIY solutions. Options include:
Window film
Putting one of the temporary but purpose-made transparent films - a polymer membrane - on your windows is one way to create a very cheap form of double glazing. Alternatively, products like Ecoease secondary glazing attach to your windows with strong magnetic strips, so you can remove them when the weather gets warmer.
Silicone sealant
If you have small cracks or gaps around the window, you can fill these with a waterproof sealant. It’s durable, long-lasting and cheap to buy from DIY stores, but it can be a bit tricky to apply if you’re a novice.
Thermal curtains
Curtains lined with thick material can act as a barrier between your window and the rest of the room, reducing heat loss. Hollow blinds, installed with a sealed frame, can also help to improve room insulation.
Blown windows: what are they and how do they occur?
A blown window is a double-glazed window where the seal that keeps air from entering inside the double-glazed unit has failed. This reduces the window’s thermal efficiency – its ability to prevent heat loss in the home – resulting in draughts, more noise and increased energy bills.
The main reason for a blown window is a damaged seal or pane of glass, which can be caused by:
- Age – double-glazed windows are rated for 15-20 years and over time the glass can weaken and cracks can appear in the sealant around the panes of glass
- Faulty installation – sealant can fail if the window hasn’t been fitted properly
- Weather damage – extreme weather can cause damage to windows over time
- Harsh cleaning materials – using strong chemical cleaners on your windows can also lead to a failed seal.
How to spot a blown window
The most visible signs of a blown window are condensation and mist building up on the inside of the two panes that make up the double-glazed unit.
This shouldn’t be confused with the normal build-up of condensation on an indoor pane caused by moisture in a room from, for example, drying clothes, a boiling kettle or hot shower.
Other signs of a blown window include:
- Glass becomes cloudy or dull
- Water droplets or watermarks indicating water is leaking through the frame
- Physical distortion resulting in windows becoming hard to open or shut
- Room feels colder and draughty
- External noise is more noticeable
- Unexpected increase in energy bills.
Do blown windows affect insulation?
Yes, a blown window basically breaches the double glazing’s insulation – namely the pocket of trapped air between panes – that helps prevent heat loss. It can also result in air leaks that lead to draughts, further reducing the window’s thermal efficiency. This means the room requires more energy to heat and keep it warm, increasing bills and reducing its EPC rating.
Repair vs replacement – what’s best?
There are three options when it comes to fixing blown windows:
| Option | Description | Cost |
|---|---|---|
| Reseal the window | Can involve simply replacing any damaged seals or may involve drilling holes into the unit from the outside to ventilate the air space to remove any existing moisture | £50-£120 for simple resealing |
| Replace an individual pane | The affected pane is removed and a new one fitted in its place in the existing frame | £55-£145 for a single pane, £100-£850 for an entire window, plus £45 cost of disposing of existing pane |
| Replace entire window | If the frame is in poor condition or the window is over 15 years old, it’s potentially more cost-effective to simply replace the whole window | Depends on the type of window – from £500-£1,250 for a casement window to £1,500-£5,000+ for bow windows |
Costs include professional installation and come from (top to bottom in the table) Replace Windows, Checkatrade and Checkatrade.
Things to consider when choosing which option to pursue:
- Is the window still under guarantee? If so, contact the fitter to discuss options for fixing the problem
- Check the condition of the frames – if they’re in good order, then replacing panes is more cost-effective than replacing the entire window. However, if there’s evidence of structural damage, they’re proving difficult to open and shut, the frames are soft, or you simply don’t like the frame, consider replacing the entire window instead
- How old is the window? If it’s over 15 years old, then it’s likely coming to the end of its natural life. This might be the time to consider replacing the entire window with a more energy-efficient alternative that will better insulate the room and help to cut energy bills.
Choosing a reliable installer
While some blown window repair jobs (specifically resealing windows) can be attempted by yourself or your local handyman, getting a professional installer is usually recommended. Benefits include:
- Blown double glazing can be caused by poor quality installation, so choosing a reputable installer should reduce or even eliminate this risk
- Repairs follow a standardised process, including a thorough assessment of the damage, to provide an effective and long-lasting repair
- Most reputable installers should back up the window or double-glazed unit replacement with a guarantee for extra peace of mind.
When looking for a reputable installer, look for an installer that offers:
- FENSA accreditation – although this is only required when replacing an entire window, it indicates the installer is properly compliant
- an IBG (insurance backed guarantee) for the work – this ensures your windows are guaranteed even if the installer ceases trading for any reason
- a full written quotation – so you can see exactly what you’re paying for
- access to independent customer reviews – so you can gauge their reputation.
Can you fix blown double glazing without replacing the whole window?
Yes. When examining how to fix blown windows without replacing the entire window, you have two options:
- Renewing the seals: while this can restore the air pocket, it doesn’t remove any existing misting or discolouration inside the unit. Avoid DIY misting repair kits, which involve drilling holes in the window in an attempt to clear the problem. Some housing associations employ the professional resealing solution Crystal Clear if you wish to explore this approach further.
- Replacing the blown double-glazing unit: if only one pane is affected and the window (including frame) is in otherwise a good state of repair, you can look to simply replace the blown unit.
Do blown windows make a home colder?
Yes, blown windows make your home colder because you lose the double-glazing’s insulation layer, leading to an increase in heat loss from your home.
How much does it cost to repair a blown window?
Blown double glazing repair can cost around £50-£120 to simply reseal a blown window. When it comes to replacing individual double-glazing units, it can cost from £55 for a single pane to £150-£600 for all the panes in a window. If the entire window needs replacing, it can cost anywhere from £500 to over £5,000, depending on the type and size of window being replaced.
Costs may or may not include extras like new frames, finishing trims, handles, sealant and the cost of disposing of the blown unit or failed window – check with your installer. Before paying for a repair, check your windows’ existing guarantee (typically covering you for up to 10 years) and your home’s building insurance to see if either will cover the cost of the repair.
Can blown windows cause condensation or mould?
Yes, condensation and/or mould can occur if moisture gets inside the frames and the problem isn’t tackled quickly.
FAQs
What about temporary 'glazing' in winter?
Putting one of the temporary but purpose-made transparent films - a polymer membrane - on your windows is one way to create a very cheap form of double glazing. Alternatively, products like Ecoease secondary glazing attach to your windows with strong magnetic strips, so you can remove them when the weather gets warmer. Generally, you don't need many tools to do this, although a hairdryer is usually one of them - this creates the heat to make the film shrink until it's taut and smooth. You will also need double-sided tape - which is normally provided with the product - to adhere the film to the indoor side of the window frame.
Should I change the size of the opening casements in my windows?
While you may want to save money by reducing the number of opening casements, or by specifying fewer and larger opening casements, this isn’t a good idea. It will create security and ventilation issues.
Larger opening casements make it harder to have a suitably low level of ventilation, and if they are accidentally left open, they create more of a security risk. Having said that, it's a good idea to have at least one larger opening casement in each room, as this can be used as a means of escape if there is an emergency.
What about keeping cool in summer?
Insulation isn't just about keeping your home warm when it's cold outside; it's also about keeping it cool when it's hot outside. External shutters are rare in the UK, but popular in France and other southern European countries. These provide added security and protect against overheating in summer.
