Double Glazing
In a well insulated home, up to 70% of the heat loss is through single glazed windows. Double glazing will more than halve the heat lost when compared to single glazing.
How does Double Glazing work?
Double glazing works by trapping dry air or argon gas between two panes of glass. This trapped air creates an insulating barrier which provides three functions:
1. It prevents heat loss or gain through the window because the barrier insulates the room from differing air temperatures at either side of the glazing unit.
2. Double glazing helps keep noise pollution down.
3. Helps reduce condensation. Condensation commonly occurs on the inside of single glazed windows due to the warm air inside the house being cooled by the cold outside air. With a double glazed window the insulating barrier prevents the transmission of air temperature and therefore prevents condensation occurring.
Although double glazing insulates against differing air temperatures, it does not insulate against radiant heat. This makes a double glazed unit a good choice for permitting the winter sun to warm the inside of the house.
 Double Glazing Unit.
There are many of benefits using double glazing:
Warmer in winter
Cooler in summer
Reduces condensation problems
Conserves energy
Reduces costly energy bills
Replaces thermal drapes
Reduces noise
Minimal maintenance
Improves security
Adds value to your property
Improves your lifestyle
New Homes and Existing Homes
Double glazing is best fitted when building a new home. This is because the window glass consists of 2 glass panels with a sealed air gap in between, making it considerably thicker than standard glass. The timber or aluminium joinery has to be wide enough to accept the thick glass panels, and existing single glazed joinery may not be able to accept the thicker panels.
Some systems have been developed that combines your existing joinery with some new parts to enable double glazing to be fitted at a cost cheaper than replacing the whole joinery unit.
Glass Selection:
There are many glass options depending on what you want to achieve with the double glazing:
Low-emissivity glass (also known as Low-E glass) lets light and heat in, and then 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). Best option for keeping summer heat out.
Laminated glass is two sheets of glass bonded with a plastic or resin layer. It absorbs ultraviolet light and reduces noise. If the glass gets broken, the glass is held together by the resin layer. Best option for noise reduction.
Toughened safety glass is much stronger than standard glass. Its designed to withstand direct impacts, or shatter into small pieces if broken. Under the Building Code, safety glass has to be used in some areas such as low windows and sliding doors. Your glazier will know where safety glass is required.
Tinted glass can reflect UV light and solar energy while letting in visible light. Many different options of tinted glass are available with varying degrees of reflectiveness.
Low emissivity glass (Low E), allows the suns warmth in, and wont let the heat out.
All of these types of glass can be used with double glazing, which is the best option for winter comfort. The air gap between the sheets of glass in a double glazing unit can be filled with argon gas to improve its insulating properties even further.
Frame Selection:
The general options for window frames are:
Aluminium
Aluminium with thermal break
Composite timber and aluminium
Timber
uPVC
Aluminium is a poor insulator (metal conducts heat). Despite this, it is the most common framing material in New Zealand because it is light, strong, durable and low maintenance.
To improve the insulating properties, aluminium framing is also available with thermal breaks, which has insulating material between the interior and exterior part of the frame.
Thermally broken aluminium frame, the black material in the diagram is polyurethane, which is poor conductor of heat thereby thermally separating the inside and outside pieces of the aluminium frame.
Composite timber-aluminium windows are better insulators than aluminium. Aluminium sits outside to provide a low maintenance surface, and is joined to the internal timber facing with a moisture barrier. They combine durability with a timber look for inside areas.
Timber is a good insulator, improving with frame thickness. Although a traditional framing material, it is less durable and should be treated and sealed for weather tightness to reduce swelling and shrinkage when exposed to rain and sun. It needs regular maintenance.
uPVC (sometimes also called PVCu or vinyl) window framing have a good insulation value and are made from vinyl plastic. It is light, and low maintenance, and may be aluminium or steel reinforced for strength if required. Because uPVC is affected by ultraviolet light, durability needs proving in NZ conditions due to our extreme ultra violet light conditions. The use of uPVC window frames is growing in New Zealand.
More information on double glazing is available on these websites:
Smarterhomes http://www.smarterhomes.org.nz/design/glazing/double-glazing-glass-options/
Energywise http://www.energywise.org.nz/yourbuildingdes/windows.html
Window Association of NZ http://www.wanz.org.nz/how_dg_works.htm
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