What is glazing?
Glazing simply means the windows in your home, including both openable and fixed windows, as well as doors with glass and skylights.
Glazing actually just means the glass part, but it is typically used to refer to all aspects of an assembly including glass, films, frames and furnishings. Paying attention to all of these aspects will help you to achieve effective passive design.
Why is glazing important?
Glazing lets in light and fresh air and offer views that connect interior living spaces with the outdoors. Energy-efficient glazing makes your home more comfortable and dramatically reduces your energy costs.
However, inappropriate or poorly designed glazing can be a major source of unwanted heat gain in summer and significant heat loss and condensation in winter. Up to 87% of a home’s heating energy can be gained and up to 40% lost through windows. Improving your glazing’s thermal performance will reduce energy consumption, therefore lowering costs and greenhouse gas emissions.
Glazing is a significant investment in the quality of your home. The cost of glazing and the cost of heating and cooling your home are closely related. An initial investment in energy-efficient windows, skylights and doors can greatly reduce your annual heating and cooling bill. Energy-efficient glazing also reduces the peak heating and cooling load, which can reduce the required size of an air-conditioning system by 30%, leading to further cost savings.
Achieving good glazing
Glazing decisions are usually made when you are buying or building a home. But you can quickly and easily improve the thermal performance of your home by replacing your windows. This is one of the most effective methods of renovation to achieve improved thermal comfort.
There are thousands of types of glass and frames to choose from. Choosing the right ones is important to improving the energy efficiency of your home. Specific products have been designed to keep heat in or out and have varying impacts on daytime lighting, noise control, maintenance and security.
There are many different types of glass products to choose from.
Single glazing uses a single pane of glass. Single glazing with clear glass is not very efficient when it comes to heat loss or gain. To improve performance, you can use single glazing with a more energy-efficient type of glass such as low emissivity (low-e) glass.
Double or triple glazing
Double or triple glazing (also known as insulated glass units or IGUs), is the combination of 2 or more layers of glass sealed into a frame with a gap between the layers. Multiple layers can be assembled with sealed cavities between each sheet of glass.
IGUs generally offer better energy performance than single glazing, because they transmit less energy. However, the energy performance of IGUs also depends on:
- the properties of each layer of glass. Different glass types (for example, clear and low-e glass) can be put together in an IGU. Different combinations of clear, toned and low-e glass can deliver a wide range of SHGC and VLT values to suit your performance needs
- the contents of the cavity. IGU cavities can be filled with air or a more inert, low-conductivity gas such as argon
- the width of the cavity. Cavity thickness is usually 6 to 18mm. Wider cavities provide lower (better) U values, with 12mm normally accepted as the preferred gap
- how well the cavity is sealed. Cavities must be dry and well sealed to prevent moisture getting in. Manufacturers have special dry rooms for IGU assembly. If argon is installed to the cavity in place of air, moisture is reliably excluded
- the level of desiccant (drying agent). The spacer (metal or polymer strip) that separates the glass layers contains a desiccant to absorb any moisture. Inadequate desiccant may cause moisture to condense on the glass surface in cold conditions, reducing thermal performance.
It is sometimes wrongly assumed that insulated glazing is only for cold climates. In fact, IGUs can deliver better energy performance for all climates, especially in heated and air-conditioned homes.
Low emissivity glass
Low emissivity glass (commonly known as low-e glass) reduces heat transfer. Low-e glass may be either high or low transmission:
- High transmission low-e glass has a coating that allows daylight from the sun to pass into the house to achieve good solar heat gain, but reduces the amount of the long wavelength infrared heat that can escape back through the window.
- Low transmission low-e glass has a coating that reduces the amount of solar heat gain while still maintaining good levels of visible light transmission.
Low-e glass has either a pyrolytic coating or a vacuum-deposited thin film metal coating. Pyrolytic coatings are durable and can be used for any glazing; vacuum-deposited coatings are soft and are only used within IGUs.
Low-e coatings can significantly improve both U value and SHGC; however, they must be used correctly or they will either deteriorate or fail to perform as required. They are often more susceptible to surface damage than standard glass. Low-e coatings can be used in combination with clear, toned or reflective glass.
Toned glass has colouring additives included during manufacture. It is available in various colours, usually bronze, grey, blue and green. Different colours will change the amount of visible light transmitted (VLT) and the SHGC; however, the colours do not change the conduction (U value) of the glass.
Toned glass options include ‘supertoned’ glass, which has heavier colouration that transmits visible wavelengths while filtering out solar near-infrared wavelengths. This provides improved energy performance by lowering solar heat gain but does not affect light levels.
Standard glass will readily break into long shards and small sharp slivers. Laminated glass has a plastic glazing layer, called an interlayer, which is adhered permanently between 2 sheets of standard glass. This reduces the danger of the glass breaking, and if it does break, keeps all shards in place so they do not form loose dangerous shards.
Laminated glass is often used in areas in the home most prone to injury from human impact such as bathrooms, doors, around staircases and in areas close to the floor (it meets the requirements of ‘safety glass’ that is mandated for use in these areas by Australian Standard AS 1288 Glass in buildings).
Careful selection of different interlayer types can also address noise concerns and energy efficiency requirements to some extent, but it is not a substitute for double glazing.
Toughened glass has been ‘tempered’ by being reheated and quickly cooled again. This process makes it much stronger than standard glass – it can resist higher impact loads before breaking. It also makes it safer because, when it does shatter, it breaks into many small cubic pieces rather than dangerous shards. It can be used as ‘safety glass’ as mandated in Australian Standard AS1288, and may be mandated in bushfire-prone regions. However, toughened glass has no thermal or acoustic benefits over other glass of the same toning or thickness.
Secondary glazing is where single-glazed windows are retrofitted with a transparent acrylic or glass sheet attached to the inside of the frame or openable sash with a secondary frame or with magnetic strips. This creates an air space between the 2 layers, which reduces the U value and air infiltration. Secondary glazing will not perform as well thermally as a manufactured IGU, since it is impossible to totally seal the perimeter, but it can provide good noise control.
Window films are a thin polymer film containing an absorbing dye or reflective metal layer, with an adhesive backing. They stick to your glazing to change its colour or make it reflective. They can be a cost-effective way to improve the thermal performance of existing windows or doors.
Applied to existing glass, some window films can halve the overall SHGC of the window by absorbing and/or reflecting solar radiation. This can be particularly beneficial in hotter climates where cooling is the main concern, or on east and west elevations directly exposed to long periods of sunshine.
However, window films may also reduce visible light transmittance. In addition, glass panes with applied films exposed to direct sun become hotter than untreated glass and industry guidelines must be followed to avoid thermally induced cracking. For this reason, it is generally best to use an accredited installer of window film.