Triple glazing vs double glazing

It has come out of the niche into mainstream consideration in the UK glazing market within the last few years. High energy prices as well as stringent building regulation requirements have both played their role in this regard, and these products themselves have come quite far since their inception. This means that they are being specified increasingly often for projects in London and even sometimes where there is no compelling reason for this to happen. Whether or not it makes sense to pay extra money will depend entirely on what you’re replacing, what sort of property it is, and what you want to accomplish.

The purpose of the extra pane

In a double glazed window, there is a combination of two panes of glass with an empty space sealed between them. The empty space will normally contain a gas such as argon while a spacer bar separates the two panes from each other. With a triple glazed window, there are three panes of glass together with two sealed spaces in between the glass panes. This creates an additional insulating layer that makes heat and sound energy pass through three air pockets instead of just one. Both effects are real; the issue is their magnitude and whether they are worth the cost in London.

Triple glazing usually offers 40 to 50 percent improved heat resistance over double glazing in experimental tests. How much of a difference you will find in a real-life situation depends greatly on how both units have been designed. A properly designed double glazed window that has argon gas, a good low E coat and warm edge spacer bars will always outperform a poorly designed triple glazed window filled with air and aluminum spacers.

 Identifying areas of true thermal performance

The U-value describes how much heat loss occurs via the window assembly in W/m²K. The Building Regulation requirements for Part L dictate that the minimum whole-window U-value is 1.4 W/m²K in replacement windows installed in existing dwellings in England. Older double glazing that was fitted before 2010 and features an aluminium spacer bar and no low-E glass will be around 2.0 – 2.8 W/m²K. Modern double glazing with an argon fill, quality low-E and warm-edge spacer will be 1.2 – 1.4 W/m²K.

What gets overlooked in most product comparisons is the fact that the difference between old double glazing and modern double glazing represents a much bigger upgrade than the difference between modern double glazing and triple glazing. When an older home uses its original single glazing or early double glazing installed in the 1980s or 1990s, a high-quality modern double glazed window accounts for the bulk of potential energy savings. This can be achieved more economically without increasing load on the window frame.

Cost and Payback in 2026

The triple glazed system usually costs 20% to 35% more than a double glazed window of the same make and design. Based on individual windows, the cost of installation of an aluminium double glazed window ranges between £900-1200; for the triple version, this can be £1100-1600. Considering a full semi-detached house in London consisting of 12-15 windows, changing from double to triple will mean £3000-£6000 more investment. The savings in annual energy consumption for such a change would range from £50-120, implying that the payback period could take up to 25-60 years. This is way beyond the warranty period on the glazing and is more than the normal occupancy period.

Where triple glazing makes a compelling argument

At Citi Homes Windows, we saw many cases where the installation of triple glazing makes sense in its own right rather than based on the payback from reduced energy bills. In London, one of the most common scenarios relates to the property located close to a main road or a train line. The acoustic performance of the triple glazing usually results in a rating of 38-42dB, while double glazing normally provides around 32-36dB of acoustic insulation. When it comes to a bedroom on the ground floor of a Victorian terraced house located close to an A-road, the improvement can make a lot of difference.

The faster cooling rate associated with north-facing rooms that do not have any solar gain through their windows makes the benefits of extra insulation more noticeable and applicable to such rooms than in south-facing ones. The contribution that triple glazing provides to the energy efficiency of a building is particularly beneficial when considering new constructions with a high EPC rating. Triple glazing becomes mandatory for any construction seeking a Passivhaus or similarly efficient design rating. Moreover, triple glazing provides a sufficiently significant improvement to justify its relative higher cost when the initial glazing standard is low or double glazing from earlier times.

 In the case of double glazing being the correct specification

Double glazing will not provide financial benefit over a period of time that makes it worth doing if a house in London needs new windows following the installation of double glazing that occurred after 2005. Properly specified double glazing will comply with building regulations and be comfortable to live in, leaving sufficient funds for more beneficial improvements.

It is also important to realize that the glazing specifications of the double glazed window have much greater importance than just adding a third pane. Changing from aluminum to warm edge spacers, verifying the use of argon instead of air filling, and making sure a good quality low-E coating is used – these improvements combined will bring a lot closer the overall U-value difference between the two types of windows much closer, without spending the extra money required for the change from double to triple glazing. Always request the whole window U-value instead of the center pane when comparing quotes; the latter value is always lower, and is irrelevant to compliance and comparisons.