Around 10% of the heat in our homes is lost through poorly insulated windows.

Outside view of a tenement flat with sash windows

Upgrading your windows lets you keep the heat in for longer. This makes your home warmer whilst saving you money on your energy bills.  

Energy efficient windows also have the added bonus of reducing condensation and insulating against noise from the outdoors. 

In this article we’ll go through what you need to look out for when upgrading your windows. 

U-values for windows 

When choosing new windows, one of the things to consider is the window’s u-value.  

The u-value for windows measures how easily heat moves through the window. The lower the u-value, the less easily heat passes through the window.

Whilst u-values are helpful, they don’t give a full picture of the window’s energy efficiency. As with appliances like TVs and freezers, windows should come with an energy efficiency rating. The energy efficiency rating considers the u-value, but also looks at other factors which affect the window’s overall performance.  

The energy efficient rating runs from A++ (most energy efficient) to E (least energy efficient).

Double glazing vs triple glazing 

Double glazed windows have two panes of glass separated by a gap. Triple glazed windows have three panes and two gaps.  

As a general rule, triple glazing will be more energy efficient than double-glazing, but this will depend on: 

  • The materials used 
  • The size of the gaps between the panes 
  • What kind of gas (if any) the gaps are filled with  

Comparing the energy efficiency ratings for double and triple glazed windows should give you a good indication of the difference in effectiveness.  

Although triple glazing is generally more energy efficient than double glazing, it’s not simply a case of automatically choosing triple glazing. This is because: 

Triple glazing is more expensive than double glazing 

Depending on your budget, it might not be feasible to upgrade all your windows to triple glazing. There are currently no window grants in Scotland for upgrading to triple glazing, which may further impact affordability. 

If your budget allows, you might consider installing a mix of double and triple glazed windows. You could choose triple glazed windows for the coldest rooms in your home, and double glazed for the rest. 

Triple glazed windows have deeper frames than double glazed windows 

When replacing your windows, you’ll need to make sure the new window frames aren’t too deep for the existing window space. 

Standard double glazed windows have a depth of 28mm. Standard triple glazed windows have a depth of 36mm. The greater depth of triple glazed windows means they might not fit correctly in an existing property.  

Window materials 

From wood to UPVC, to aluminium, it’s possible to get very energy efficient windows in several different kinds of materials. Remember to check the energy efficiency rating when making comparisons.  

As well as budget, you may want to consider how the window frame material compliments the character of your home, or if it complies with local planning permission.

UPVC (unplasticised polyvinyl chloride) is a type of plastic – usually white – that has been used in window construction since the 1980s.  

Nowadays, UPVC is available in a range of colours. UPVC’s relative low cost, high durability and low maintenance make it the most popular type of window frame material across the UK. 

Although they’re made from plastic, UPVC windows can be recycled.


Metal window frames are usually made from aluminium or steel. Like UPVC window frames, metal frames are strong, light, durable, and available in different colours. 

Producing steel and aluminium is carbon-intensive, but metal window frames can be recycled.


Wooden window frames are usually the most environmentally friendly option when upgrading your windows. 

For wooden frames to last, they need to be properly looked after. They are more likely to be damaged by weather or temperature if they’re not well maintained. 

In conservation areas or listed buildings you may need to choose wood for your window frames to match the character of your property.


Composite window frames are wood frames which have a layer of aluminium or plastic coating on one (the exterior-facing) side. This means your windows are protected from the weather and extremes of temperature, but you still get the character of wooden frames inside your home.

New windows and planning permission 

When thinking about replacing your windows it’s good to check if there are any restrictions on the changes you can make to your property. A good starting point is your local authority’s office for building standards 

There are also likely to be limits on what you can do with your windows if you live in a conservation area or a listed building. In conservation areas, any changes to your windows might need to be in keeping with the look of the building or the locality. In listed buildings, you may not be allowed to replace the windows. In cases like these, secondary glazing is often an option. 

The website has a helpful tool which can help you work out whether you live in a conservation area or listed building. It also tells you who you’ll need to contact for different types of planning permission.

Secondary glazing 

Secondary glazing can be an option if your budget or building restrictions mean you can’t install double or triple glazing.  

Secondary glazing adds an extra layer of glass inside your property, directly behind the existing window.  

It’s possible to put in secondary glazing yourself with a DIY kit, or you might opt for a professional to install a more energy efficient and bespoke system. 

Budget options are also available for secondary glazing. You may have seen cling-film like material being applied to the inside of windows to create an extra layer of insulation. This type of secondary glazing has some impact on energy efficiency, but the material doesn’t tend to last very long.

Window grants in Scotland 

Whilst there are currently no window grants in Scotland for double or triple glazing, you can still take advantage of other funding options.  

Home Energy Scotland offer an interest-free loan of up to £4,500 to upgrade single glazing to double glazing. To find out more about the Home Energy Scotland Loan, simply call their freephone number on 0808 808 2282. An advisor will be able to talk you through the process. 

Scottish grant funding for secondary glazing may be available through the Warmer Homes Scotland scheme. Information about the Warmer Homes Scotland scheme – including the eligibility criteria – is available on Home Energy Scotland’s website. Alternatively, you can call Home Energy Scotland on 0808 808 2282 and an advisor can let you know whether you qualify.

Register your interest in our retrofit service

Register your interest in EcoCosi from Changeworks to find out more about how to retrofit your home and to be sent information about the service.

Home Energy Scotland logo

Changeworks delivers Home Energy Scotland in the south east and Highlands and Islands on behalf of the Scottish Government and Energy Saving Trust.

As well as providing free, impartial expert advice to thousands of people every month to help them to keep warm in their homes for less, they identify funding opportunities for households seeking to install energy efficiency measures.

For more information, give Home Energy Scotland a call on 0808 808 2282 or email and the team will be happy to help you.