What Is the Difference Between Double Glazing and Secondary Glazing?

If you’re considering installing new windows in your home, the choice between different types of glazing can be somewhat confusing. This hasn’t always been the case.

Up to the 1970s, the only real option was single glazing, but glazing advancements emerged leading to the development of secondary glazing and double glazing. While these terms sound as if they mean the same thing – and both types of glazing perform better than single-glazed windows – there are important differences between double glazing and secondary glazing. 

This post explores these differences and why they matter when choosing new replacement windows.

What is Double Glazing?

Most homes in the UK now have double glazing, which lasts decades, improving energy efficiency and cutting heating costs. Double-glazed windows had become a popular alternative to single glazing by the 1980s, and the norm in new-build homes.

Double glazing consists of an insulated glass unit, with two panes of glass. Originally the gap between the panes contained just air. In modern double glazing, the space is filled with an inert gas such as argon or krypton. This results in better insulation, with improved energy efficiency – stopping cold air from getting into your home while preventing warm air inside from escaping.

As well as improving thermal performance and cutting heating costs, modern double glazing is also effective in absorbing external noise and reducing condensation. Today’s double-glazed windows can last around 30 years, or longer if they’re not subject to harsh weather.


What Is Secondary Glazing?

Secondary glazing refers to installing an additional, thin pane of glass on the internal side of an existing single-glazed window. It may be particularly suitable for properties in a conservation area or in listed buildings.

Secondary glazing can be considerably cheaper than double glazing because the glass is thinner and there’s less of it, and installation is simpler.

If you get double-glazed windows, it involves installing new insulated units. Fitting secondary glazing to update an existing set of single-glazed windows doesn’t require the same technical know-how and isn’t as time consuming. In some cases, you may be able to cut the cost of secondary glazing even further if you’re a DIY enthusiast and can do the job by yourself.

One of the main benefits of secondary glazing is that it’s highly effective in reducing noise. However, secondary glazing can’t provide the same level of heat insulation as double-glazed windows – an important consideration if you’re concerned about high energy prices. This is why it’s often regarded as a temporary solution.


Types of Double Glazing

Double glazing has advanced significantly over the last decade, with numerous types of double-glazed windows now available. These include:

  • Double-glazed casement windows. Single- or dual-panelled double-glazed casement windows are hinged from the side, top or bottom. They add an elegant touch in both modern and older properties and allow as much daylight as possible into the home. They cost less than many other types of double glazing. 
  • Double-glazed flush windows. These windows are a contemporary twist on the standard casement window, with glass panes that sit flush with the window frame. Offering the visual appeal of timber casement windows found in traditional cottages, flush double-glazed windows can add a subtle, classic style to both period properties and modern homes.
  • Double-glazed cottage windows. Double-glazed cottage-style windows give homes an old-world, rustic charm. They’re designed with glazing bars on the inside and outside of the window to give the appearance of smaller, individual panes of glass, resembling windows found in period cottages.
  • Double-glazed sash windows. Instead of opening on hinges, double-glazed sash windows have two units that slide smoothly up or down or from side to side. Resembling traditional wooden windows, double-glazed sash windows are especially popular in older properties.


Types of Secondary Glazing

Many types of secondary glazing are available, including:

  • UPVC secondary glazing. This is probably the most reliable and thermally-efficient type of secondary glazing because the permanent uPVC secondary glazing panel has its own, sealed frame.
  • Horizontal sliding secondary glazing. These glazed panels slide along a nylon track or on wheels. When the window is closed, the secondary-glazed panel clicks and fastens onto the outer frame.
  • Lift-out secondary glazing. This type of secondary glazing is often used for large windows It can be lifted out of the frame for ventilation or cleaning. 
  • Hinged secondary glazing. Secondary glazing hinged units can have different types of hinges opening to give either full or limited access to the window. They can be used in combination with lift-out secondary glazing.
  • Magnetic secondary glazing. These removal secondary glazing panels are held in place by magnets, which means they’re only suitable for smaller windows.
  • Fixed secondary glazing. With no subframe or opening mechanism, fixed secondary glazing simply attaches directly onto the existing window and allows no access to it.
  • Temporary secondary glazing. This refers to any simple lightweight covering, such as window insulation film, that can be attached directly to a window as and when required.


Is Single Glazing Still an Option?

If you’re looking for replacement windows on a really tight budget, the least costly option is single glazing, with just one layer of glass. Single-pane windows are cheaper than double glazing because they’re simpler to manufacture and, with a lighter weight, easier to install. 

However, these windows are unlikely to provide a cost-effective solution in the long term because they leak heat. This can send heating bills soaring, especially in an era of high energy costs.

Studies have shown that single glazing can lose up to 70% of the heat generated in a home. Double-glazed windows, on the other hand, account for only 10% of heat loss.

You could use window film to give single glazing an extra layer of insulation, but this is a short-term solution that won’t raise its performance to double glazing standards. Single-glazing also accumulates condensation, lets in draughts, compromises security, and has poor sound absorption.



Households with single glazing often wake up in the morning to find their windows covered in condensation, particularly in colder weather.This occurs through excess moisture when warm air inside the home hits the cold surface of the window pane and quickly cools to form liquid droplets.

Besides having to constantly wipe down your windows so you can see out of them, window condensation can have more serious consequences.It can result in growth of toxic mould on the window frame and sill and surrounding areas of wall. Spores released by this mould can cause respiratory problems and trigger allergic reactions.

Mould can also cause structural issues by breaking down plaster and timber. 



Draughts are common in single-glazed windows, allowing cold air into the home as warm air escapes. This means you need to use more energy to keep your home at a comfortable temperature, which increases your heating bills.


Security Issues

A further problem with single glazing, which window insulation film won’t address, is security. The single pane of glass is much thinner than double glazing and therefore easier to break into. 


External Noise Pollution

If you live in a busy urban area or near a road, one thin pane of glass in your windows will do little to block the noise.


Which Type of Glazing is Right for Your Home?

Despite the disadvantages of single glazing, it may be the preferred choice to preserve the original character of an older property. However, for most of us the choice comes down to secondary glazing or double glazing.

Secondary glazing provides an option if your main concern is noise reduction rather than saving money on energy bills.

Double glazing costs more but will in the long term pay off in terms of thermal efficiency and reduced heating costs. You could save hundreds of pounds on installation costs just by recycling your old uPVC window frames.

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