Double Glazing & 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.


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. 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 a good option for noise, and reduced fading (99%UV blocked) 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.

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.

This table compares the performance of some glass and frame options:

 Single glazingStandard double glazingDouble glazing with low-E glassDouble glazing with low-E glass plus argon gas
Aluminium framesR0.15R0.25R0.31R0.32
Thermally broken aluminium framesR0.17R0.31R0.39R0.41
Timber / uPVCR0.19R0.36R0.47R0.50

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.

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 cold locations.

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).

In a well-insulated home, windows and other glazed areas are the biggest source of heat loss. 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.


Glass Types

Annealed float glass does not resist high stresses from the impact of an object. When broken, it shatters into large sharp pieces

Toughened safety glass is up to five times stronger than annealed float glass and offers the highest resistance to impact. It is ideal for applications in doors, shower screens etc. If broken, the whole panel of glass shatters into small pieces of blunt granules which are relatively safe. Additionally, the shattered glass falls out quite easily.

Laminated Glass has the about the same impact strength resistance as that of annealed float glass (e.g. 6mm = 6.38mm). If broken, glass remains intact on its PVB interlayer and shards do not fly out. Because of its high resistance to penetration, laminated glass is very safe and can be used in a wide variety of applications.


Understanding Low-e Glass

Low-e coatings play an important role in the overall performance of a window and can significantly affect the overall heating, lighting, and cooling costs of a home.

But what exactly is low-e glass? How does it work? To answer these questions, the “e” in low-e glass must be explained.

The “e” stands for emissivity. Emissivity is the ability of a material to radiate energy. When heat or light energy—typically from the sun or HVAC system—is absorbed by glass it is either shifted away by air movement or re-radiated by the glass surface.

In general, highly reflective materials have a low emissivity, and dull darker colored materials have a high emissivity. All materials, including windows, re-radiate heat in the form of long-wave infrared energy depending on the emissivity and temperature of their surfaces. Radiant energy is one of the important ways heat transfer occurs with windows. Reducing the emissivity of one or more of the window glass surfaces improves a window’s insulating properties. Therefore, having low-e glass ultimately can improve the insulation of a home from external temperatures in any climate.

To reduce the emissivity of glass, low-e coatings have been developed to minimize the amount of ultraviolet and infrared light that can pass through glass without compromising the amount of visible light that is transmitted.