Double glazing and glass options
The right glazing choices can help make your home more comfortable in hot and cool weather.
There are several glazing options available. Some keep heat inside your home. Others keep heat, noise, light and glare out.
Most New Zealand homes use clear glass in their windows and doors. Heat, light and noise can easily pass through this type of glass. When used as single glazing, it won't keep your home as comfortable or quiet and it won't keep your furniture from fading. If broken, standard 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 ultraviolet light that can get in (ultraviolet light causes fading).
- Laminated glass is two sheets of glass bonded with a plastic or resin layer. It absorbs ultraviolet light and reduces noise. If it's hit, the glass is held together by the resin layer.
- Toughened safety glass is much stronger than standard glass. It's designed to withstand direct impacts or shatter into small chunks if broken. Under the Building Code, safety glass has to be used in some areas.
Laminated glass is the best option for noise, 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, which is the best option for winter comfort.
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 is assumed in most R value calculations. Argon gas is a better insulator than air gaps but these units are very expensive and you can get similar R values from using low-E glass.
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 inserts into your existing window frames – this is a cheaper option if your window frames are still in good condition
- installing secondary glazing.
Low-E and tinted glasses can be included in these retrofit options.
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 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 and shape. 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 can make a difference too (See Frame options for more information).
This table compares the performance of some glass and frame options.
|Single glazing||Standard double glazing||Double glazing with low-E glass||Double glazing with low-E glass plus argon gas|
Thermally broken aluminium frames
Timber / uPVC
Double glazing also helps to reduce condensation by keeping the house warmer.
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 that from 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' 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 result can look ugly 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.
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, but may not be the most effective option for thermal insulation. 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.
Glass blocks can be used as an alternative to double glazing, for glazed areas that don't need opening. The blocks are joined and mounted into strengthened frames.
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 cold locations.
From Smarter Homes
From other sites
The Energywise website has information about window design and placement.
The Window Association of New Zealand website has information on the Window Efficiency Rating System and window systems. You can buy a BRANZ Ltd bulletin on Window Efficiency Rating System from the BRANZ website (click on the link to the BRANZ shop).
Designing Comfortable Homes is a guide to energy-efficient design using glass, thermal mass and insulation, produced by EECA and the Cement and Concrete Association of New Zealand (CCANZ). The booklet can be downloaded for free from the Energywise website.
The Design Navigator website has a table which shows how different combinations of glass and frame will perform in the three climate zones.
You can buy DIY window insulation online from the Community Energy Action website.