Two panes of glass in your windows are usually better than one. But are three always better than two?
In this article, we’ll explain how to compare the heat and sound insulation, solar gain, and light transmission of double and triple glazing so you can decide which is the best choice for your home.
Glazed windows use a gap between the glass panes to create an insulating barrier that separates the inside of your home and the outside world. Filling the gap with air, which is a poor conductor, decreases the amount of heat that’s lost through the window. Using an inert gas such as argon or krypton, which is now standard in most modern double and triple glazing, offers an even better insulation level.
The difference between double and triple glazing is as the names suggest. Triple glazing has an additional pane of glass, thereby creating two gas-filled gaps instead of one. The extra glass pane and gap increase the window's heat and sound insulation properties compared to double glazing.
Based on the information above, you might think that triple glazing is the obvious choice. After all, it offers better energy and sound insulation. However, there’s a bit more to it than that.
Let’s look at each of the main factors worth considering when weighing up the pros and cons of double vs triple glazing.
Not all windows are equally energy efficient. The amount of heat a window conducts varies based on:
Frame material and construction
Fitting of the frame and the seals
The spacer bar system
Number of panes
The gap between the panes
The type of gas between the panes
The glazing is rated according to its ability to prevent heat from escaping, called its Ug-value. Higher Ug-values equal more heat loss, while lower Ug-values mean less heat loss.
High-quality double glazing offers low Ug-values (e.g. 1.2 - 1.5 W/m²K), thereby helping to keep your energy bills down.
If you want the best possible thermal insulation, triple glazing with a Ug-value of less than 1 W/m²K is the ideal choice.
Solar gain (g-values)
Glazing is also rated according to its solar gain (g-value). This describes how well the glass transmits heat from the sun. A value of 1.0 means that 100% of solar heat enters the building (i.e. no glass at all), whereas 0.0 (0%) means no solar heat gets in (e.g. a brick wall).
Double glazing typically has a solar gain (g-value) of around 0.7-0.8, while triple glazing is 0.6-0.7.
A higher g-value is generally useful in cooler weather and for north-facing windows because more solar heat can enter the room through the glass. A lower g-value is beneficial in warmer climates and for south-facing windows to avoid the room overheating.
Glazing’s energy efficiency can be improved by opting for low-emissivity glass (aka Low-E glass). Low-E glass has an invisible coating that dramatically reduces heat transfer and reflects interior heat into the room. Low-E glass can make a big difference to a window’s thermal performance, whether it’s double or triple glazed.
In fact, the Low-E glass used in all JELD-WEN windows can make your home more than twice as energy-efficient compared to older double glazing with no Low-E coating.
You might notice that Low-E glass looks a little darker than regular glass, but the tint effect is minor and only noticeable if you have Low-E and non-Low-E glass next to each other or view light-coloured net curtains through the glass.
Keeping it simple with Window Energy Ratings
If comparing U-values and g-values sounds too complicated, you can instead focus on the Window Energy Rating (WER). WER is a standardised, widely recognised score from A+ to E that sums up the window’s overall energy performance (including the glazing). A+ rated windows offer the best energy performance.
High-quality double glazing with a Low-E coating should be more than sufficient for most situations. However, triple glazing can be a good choice if you want maximum thermal insulation, such as for an ultra-low-energy modern new-build house.
A Sound Transmission Class (STC) rating represents a window’s level of sound insulation. The higher the rating (ranging from 20 to 65), the better the acoustic insulation.
Our triple glazed Stormsure Energy+ window has an STC rating of 40db, compared to 37db for a double-glazed window and 26-28db for a standard single-glazed window.
To put that into context, sound reduction increases by around 50% when you switch from a single-glazed to a triple-glazed window. It’s only when the STC rating reaches 40 that noisy train lines, lorries, road works, and other significant noise issues are solved.
Whether you should opt for a higher or lower STC rating depends on your situation, e.g. whether you live near a train line or a busy road. If you want the best chance of a good night’s sleep, consider triple glazed windows with an STC rating of 40db+. Or, for sound insulation that’s suitable for most other situations, opt for double glazed windows with an STC rating of 37db+.
Each additional layer of glass that’s added to a window reduces the amount of light that passes through it (which is why quadruple-glazed windows haven’t caught on). You can compare the amount of light that windows let in by looking for their Light Transmittance value (LT).
An LT of 0.5 means 50% of the light gets through. The LT value of a triple-glazed window is usually slightly lower than a double-glazed window (e.g. 70% vs 80%), but this isn’t generally very noticeable.
The LT value is also affected by the thickness of the glass coating. For example, one pane of laminated glass may have a lower LT value than three panes of glass designed with very high light transmission.
The amount of light your windows allow in is a matter of personal taste. You might prefer glazing with a higher LT value for a room that doesn’t get much direct sunlight or a lower LT value for a space that’s south-facing and in the sunlight for much of the day.
It’s up to you to decide which factor is more important depending on the room in question: conserving heat or letting in lots of light.
One of the benefits of Low-E glass, which we use in our double- and triple-glazed windows, is that it minimises the amount of heat energy that passes through the glass while maximising natural light transmission.
Improved energy efficiency vs double glazing.
A warmer home due to less heat loss.
Could help reduce condensation.
Better noise insulation.
Increased security thanks to an extra layer of laminated glass.
Could add extra value to your home.
Still undecided? Compare the costs and features of our double glazed and triple-glazed windows.
Or use our window quote tool for a free, bespoke estimate based on your choice of style, size, colour and glazing.