Single, double or triple glazing: How to choose the right windows

by | Mar 1, 2022 | Designs, Performance

While it may have seemed like a good idea in the past to use only one pane of glass to protect houses from the outside environment, climate change and the quest for more sustainable energy sources are paving the way for double and triple glazed windows.

After all, windows are a big hole in your wall – and as such, they are the biggest source of heat loss and gain in buildings.

That’s why we want to take some time and look at the most common types of double glazed windows and dive a bit deeper into triple glazing and the cost for passive house windows

In this article, we will look at:

  • Why windows are so important
  • How to measure window performance
  • The different types of windows
  • How double glazed windows work
  • The price of double glazed windows
  • Secondary glazing as an option for existing houses
  • Triple glazing
  • Triple glazing vs double glazing

WHY ARE WINDOWS SO IMPORTANT?

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Windows are the biggest source of heat loss and gain in all buildings. That is not surprising, as they are basically big holes in your house. What is surprising though, is thinking that one thin layer of single glazing will help in protecting your house from the weather conditions outside.

In fact, the Australian Government estimates that windows are responsible for up to 40% of a home’s heat loss and up to 87% of heat gain.

But that’s not the only reason why windows are important: A recent study by the University of Sydney has found that where you live has a huge impact on your mental health, citing noise or housing flaws as major predictors of depression.

So, the quality of your window can not only impact your home’s efficiency and insulation, but also your mental well-being, as the thicker the glass that lays between you and the outside, the less noise will travel inside.

HOW TO MEASURE WINDOW PERFORMANCE

Alright, now that we know why windows are so important, let’s look at how we determine what is a good—or rather high-performing—window.

A valuable resource is the Window Energy Rating Scheme (WERS), managed by the Australian Glass and Window Association (AGWA). The WERS rates the performance of window products in regards to their energy efficiency using a star system that is based on the window’s U-Value as well as the solar heat gain coefficient (SHGC).

U-VALUE

In building design, the U-value (or thermal transmittance) indicates how effective a material is as an insulator. The higher the number, the more heat will be lost by passing through. The lower the number, the better insulated the material.

On average, single glazed windows have a U-value of 5.6 whereas double glazing can have a rating of up to 2.8. This means you will get 50% better insulation simply by adding one pane of glass.

And it gets even better: Triple glazing can offer a U-value of 0.8, keeping your home comfortable and warm at all times.

SOLAR HEAT GAIN COEFFICIENT (SHGC)

Another way to measure the energy performance of a window is through the solar heat gain coefficient (SHGC). This describes the fraction of solar radiation that is admitted through a window, door, or skylight and released as heat inside a home.

A high SHGC rating means the window will be more effective at collecting solar heat during the winter. A window with a low SHGC rating, on the other hand, is more effective at reducing cooling loads during the summer.

The optimal SHGC is ultimately determined by the individual characteristics of your home, such as climate, orientation, and external shading.

THE DIFFERENT TYPES OF WINDOWS

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We have already mentioned the varying amount of glazing, but let’s make sure we are all on the same page in understanding what single, double, and triple glazing means.

SINGLE GLAZING

As the name already suggests, single glazed windows are constructed using only a single pane of glass. Still the most common option you’ll find in your standard Aussie home, a single glazed window can be up to 90% less efficient than an insulated wall in terms of energy loss. This means that even if you have some sort of insulation in your wall, your single glazed window is letting cold and hot air pass through with almost no barrier.

DOUBLE GLAZING

Double glazed windows are created with two panes of glass and a space between the outer and inner surface of the window. This gap reduces the convection of cold and heat while also acting as an additional element for sound proofing.

TRIPLE GLAZING

Triple glazed windows are basically double glazed windows but with an additional pane of glass to ensure extra insulation.

HOW DO DOUBLE GLAZED WINDOWS WORK?

Let’s get down to the details: What exactly is double glazing? As we said, double glazed windows consist of two panels of glass. They are not pressed right against each other, but rather have a small space in between the panes.

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As these two panes are perfectly sealed, you will not have any condensation or fog build up between the panes. If you do see any moisture appear, your glass is probably damaged and should be replaced.

However, simply having two panes of glass doesn’t equal high performance. In fact, there are a number of factors that determine how efficient a double glazed window actually is:

  • The type of glass used
  • The thickness of the glass
  • The size and gas content of the space between the glass panes
  • The material of the frame

THE TYPE OF GLASS

The color of the glass can have an effect on solar heat gain. Depending on their tone (which is usually bronze, grey, blue and green), they will have a higher or lower SHGC and can therefore improve the efficiency of the house.

Additionally, some double glazed windows use low emission, or low-e, glass, which has a coated surface. This reduces the heat radiation from the sun on the glass. The emissivity (say that word three times fast) is measured from 0 to 1, with 1 being the most radiant. There are hard and soft coatings that differ in their durability and U-value.

Finally, some double glazing manufacturers apply window films to their glass, which helps to reduce the solar heat gain by absorbing or reflecting radiation from the sun.

THE THICKNESS OF DOUBLE GLAZING

The actual thickness of the glass panes doesn’t actually have that much to do with controlling heat loss and gain, but rather with noise reduction. The thicker the glass, the less noise will come in.

Most glass panes are about 4mm thick, but it is also possible to vary the thickness of the two panes. For example, if you live next to a busy street, it might make sense to have a thinner inner glass than the outer glass, as this can help reduce low frequency noise, such as traffic.

THE AIR CAVITY BETWEEN THE GLASS PANES

As we mentioned, double glazed windows have a small space between their two glass panes. This space is anything from 6mm to 20mm thick and is filled either with air or insulating gases with low conductivity. In the majority of cases, argon is used for this.

How thick your air cavity is depends on different factors, such as your available space for the window or the climate zone you are in. In Victoria or Tasmania, you will probably want to increase that gap a little bit to reduce the heat loss in winter, while Queenslanders might be able to survive the wet season with a smaller gap between their window panes. But generally speaking, a gap of at least 12mm is best to increase your energy efficiency.

THE STYLE AND MATERIAL OF THE FRAME 

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There are two things to consider when talking about frames: Their material and their style.

Double glazed window frames are generally made out of 4 different materials:

  • Aluminium
  • Timber
  • A composite of the aluminium and timber
  • uPVC

Each material has its own benefits and can impact your thermal performance. Aluminium is a very durable and practical material, keeping costs and potential maintenance to a minimum.

Timber, on the other hand, holds more natural insulating properties and is therefore considered to be more energy efficient than aluminium. It is also quite pleasing to look at, but can be more expensive than the aluminium counterpart.

Composite windows combine the best of both worlds, with timber on the inside and aluminium on the outside. They are a good compromise if you can’t decide which option to go for.

UPVC windows are our preferred option. UPVC stands for unplasticised polyvinyl chloride, a lightweight plastic building material, and has been around since the 1930s. It does not transfer heat or cold, but rather matches the internal room temperature. And all of this at a low price point.

Finally, you should also look at the style of the window frame, especially in regard to how they open and close. You want the window to have multiple hinges, as this increases their insulation.

Casement windows are therefore a better option than awnings, because hot air rises up and if you want to let the steamy air from inside out of the house, you won’t have much luck with this type of window.

And then there are tilt and turn windows, featuring a hinge mechanism that allow them to be opened in two ways: By leaning in at the top and by turning the handle and fully opening the window. They are highly popular in Europe because of their versatility and overall safety. Here’s a video explaining how they work.

DOUBLE GLAZED WINDOWS PRICE

It is hard to generalise the cost for double glazed windows, because there are many factors that influence the overall price tag. As we explored before, not only does the material of the glass and window frame play a role in determining its price, but also the thickness of the glass and the size of the air cavity.

But you shouldn’t just think of the initial cost of a window when making a decision for your home. It’s worth taking a look at the bigger picture and calculating how much heating and cooling cost you will actually save by investing into better windows. Finally, the benefits from noise reduction and increased security are hard to calculate, but we will go out on a limb here and say that your health is invaluable.

SECONDARY GLAZING

What do I do if I have an existing house and want to pimp my single glazed windows a bit, you ask? Well, of course you can consider exchanging the windows and investing in some high-quality double glazed windows for your home. But there are some double glazing alternatives if you can’t afford a full exchange yet want to increase the efficiency and insulation of your home.

Secondary glazing generally means adding another layer to your existing windows. This can be either high-quality acrylic or a full secondary window, added on the inside of your windows.

This allows for some gains in insulation as well as noise reduction at a relatively small price point. However, if you are looking at passive or passive solar design, secondary glazing will not be enough to reach the desired performance levels.

TRIPLE GLAZING

Let’s amp it up a bit, shall we? How about adding a third pane of glass to your window? Triple glazing can be found all throughout Northern Europe, thanks to its increased insulation value as well as noise reduction.

It basically works the same as double glazing, but with an additional sheet of glass. This means that there are also two instead of just one spaces between the panes that help reduce temperature conductivity and therefore create better insulation.

And we are not just saying that! Triple glazing has a U-value as low as 0.8, keeping your home comfortable and warm at all times.

But that comes at a cost. For an average house in Victoria, you are likely looking at a 30-35% price increase by going from double to triple glazed. This is based on our own experience and valuable input from our friends over at Ultimate Windows.

TRIPLE GLAZING VS DOUBLE GLAZING

At the end of the day, every building project is unique and choosing the right window option for your home comes down to more than just one factor. If you are working with a smaller budget, double glazed windows are probably the way to go. But if your new home will be located in a rather cold climate and you want the most performance out of your windows, triple glazing can be a very smart investment.

To give you a quick overview of the pros and cons of triple glazing vs double glazing, we have created a little infographic.

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CONCLUSION: WHAT IS THE BEST WINDOW FOR MY HOUSE?

In some Australian climates, double glazed windows might be sufficient, while in others, triple glazing is the way to go. Either way, on average, passive house windows transmit just one-sixth of the heat compared to a typical Australian single glazed window.

At OUTLIER, we prefer to work with double glazed uPVC windows with a tilt and turn operation, as they offer the best energy efficiency at an affordable price point. In addition to that, they are recyclable, require no maintenance, and offer low VOC (volatile organic compounds-or off gassing). For clients with a slightly larger budget, we are working with composite timber alu clad windows.

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