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Double glazing and glass options

The right glazing choices can help make your home more comfortable in hot and cool weather.

Windows – much more than a view

For a more comfortable home, choose double glazing or fit secondary glazing to your existing windows. The investment will be worthwhile – a warmer, healthier, quieter home with lower power bills.

There are several glazing options available. Some keep heat inside your home. Others keep heat, noise, light and glare out.

To get the best performance out of your windows also choose the right joinery.

Glass options

Most New Zealand homes use clear glass in their windows and doors. Heat, light and noise can easily pass through this type of glass. Clear glass also won't keep your furniture from fading. When used as single glazing, it won't keep your home as comfortable or quiet. If broken, standard clear glass shatters into sharp fragments which can be dangerous.

There are several other types of glass available:

  • low-emissivity glass (also known as low-E glass) lets light and heat in, but helps prevent heat from escaping
  • reflective glass, tinted glass and spectrally selective glass reduce the amount of heat and light that can get in. The type of glass treatment determines whether infrared light (radiant heat), visible light or ultraviolet light is filtered out (ultraviolet light causes fading)
  • laminated glass consists of two sheets of glass bonded with a plastic or resin layer. It absorbs ultraviolet light and some heat and reduces noise. If it's hit, the glass is held together by the resin layer
  • toughened glass is much stronger than standard glass. It's designed to withstand direct impacts or shatter into small chunks if broken.

Building Code clause F2.3.3 requires that glass that people are likely to come into contact with shall either break safely, resist impact or be protected by a barrier. New Zealand Standard NZS 4223 Glazing buildings Part 3 Human impact safety requirements has information on safety glazing materials and where they are required to be used.

Laminated glass is the best option for noise control, as well as being safe and secure. Reflective, tinted and spectrally selective glass are options for keeping summer heat out.

All of these types of glass can be used with double glazing and high performance frames, which is the best option for winter comfort. For a further performance boost you can even ask for triple glazing.

Double glazing

What is double glazing?

A double-glazed window uses two panes, separated by a gap which is filled with air or an insulating gas such as argon. It is also known as an insulating glass unit (IGU).

Although gaps can vary between 6 and 12mm, a 12mm gap has the optimal insulation performance and is therefore most common. Argon gas is a better insulator than air, though slightly more expensive, and in combination with low-E glass and high performance frames can give an excellent thermal performance.

You can vary the glass you use for the two panes in a double glazing unit, and you can choose two different types of glass for each pane.

If you are retrofitting double glazing to your existing windows, there are three main options:

  • replacing the whole window frame with new frames and glass
  • installing double glazed inserts into your existing window frames – this is a cheaper option if your window frames are still in good condition
  • installing temporary or permanent secondary glazing.

Low-E and tinted glasses can be included in these retrofit options.

Diagram of a typical double glazed aluminium window

Typical double glazed aluminium window

How effective is double glazing?

Double glazing can make a big difference to comfort levels in your home. Combined with insulation in the walls, ceiling and floor, double glazing your windows and doors will help keep temperatures stable, reduce condensation on the glass and reduce heating and cooling costs.

By double glazing a window with clear glass, you can halve the heat loss compared to a single-glazed window of the same size. You can reduce the heat loss further by using different glass types (such as low-E glass) or by choosing units which use argon gas to fill the space between the two panes of glass.

The frames you use will make a difference too.

Window frames has more information.

Double glazing and noise reduction

Double glazing can also help to reduce noise levels, particularly in the medium to high frequency range (including voices). However, double glazing on its own won't filter out loud or low-frequency noise such as the sound of traffic.

More effective noise reduction can be achieved by using thicker glass or by using laminated glass (some laminated glass comes with a thick acoustic inter-layer). The gap between the panes can also be increased for more effective noise insulation, but this reduces the effectiveness of double glazing as a heat insulator.

Alternatively, consider secondary glazing. It can be a better option for blocking external noise.

Secondary glazing, solar film and other options

Secondary glazing

'Secondary glazing' means inserting a second pane of glass or acrylic in or on to an existing window frame. It's an alternative to retrofitting new double-glazed windows, especially for old wooden houses and generally, timber reveals and other structural features of the window can be kept. Check with a professional installer to see if this option is for you.

You can choose tinted or low-E glass for your secondary glazing, and with some systems, you can swap the extra pane for insect screens (fitted to your windows in the same way) in summer.

Secondary glazing is generally a cheaper option, and the most recent research indicates that glass or acrylic secondary glazing systems perform as well as some types of double glazing.

However, the appearance can be unattractive to some people, and with double glazing now a common way of meeting the energy efficiency requirements of the Building Code, you may get better market value in the long term from double glazing.

Diagram showing primary and secondary window glazing

Primary and secondary window glazing

Solar films

An alternative to using tinted and reflective glass is to use a thin, flexible, transparent film to enhance a single pane. Film can be applied to almost any glazed fixture that has a frame.

Solar films can reduce glare, reduce solar heat gain, reduce fading, and reduce the likelihood of injury from breakage.

Check the cost and effectiveness of solar films for the comfort and safety level you want. If you need to keep your home warm in winter, some low-emissivity films could help reduce heat loss. Films are an option for privacy, and for sun control in summer.

Shrink wrap secondary glazing kits

These are a great low cost DIY option if you are renting or if you cannot afford double or secondary glazing. They can be an effective way of improving your window performance in winter – they help with condensation as well as reducing heat loss.

Clear plastic film is attached to your window frame with double-sided tape, and then shrunk to fit using a hair dryer. You can buy these from a hardware store or online. Generally they will only last one year though, so you need to redo your windows each autumn.

Triple glazing

Three panes of glass are separated by air gaps to provide very high levels of heat retention and noise reduction. Triple glazing is an option for very noisy or very cold locations and is very expensive.

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Note that this document is published by the Ministry of Business, Innovation and Employment Chief Executive as Guidance under Section 175 of the Building Act 2004. This is a guide only and, if used, does not relieve any person of the obligation to consider any matter to which the information relates according to the circumstances of the particular case.