Glazing options for temperature control

Thermal performance of windows and glazed doors is determined by the type of glass used, the framing material, and the unit’s construction.

On this page:

• Insulated glazing units (IGUs)
• Solar control glazing
• Framing materials
• Specifying solar heat gain coefficient (CHGC)

All of these factors together determine the thermal – as well as optical – performance of a glazing unit. All, therefore, must be considered together, along with other elements of passive design such as window size, placement and orientation, and insulation in other parts of the building envelope.

Insulated glazing units (IGUs)

Insulated glazing units (IGUs) may be double- or triple-glazed, although double-glazed units are more common.

IGUs can limit heat transfer by:

• conduction – due to the high thermal resistance of the air or gas in the space between glazing panes
• convection – due to the inability of the still air between the panes to move and transfer heat
• radiation – due to two or more panes interrupting the radiation of energy and the low emissivity when using a low-emissivity (low-E) surface.

The minimum spacing between glazing panes should be 12 mm, but the gap can range between 10–16 mm, unless Krypton gas is used, in which case, a 6 mm space is considered ideal.

Argon and krypton gas

Heat transfer through windows can be reduced by using argon or krypton gas between the panes of double glazing.

Argon gas is a low-cost, clear, non-toxic, naturally occurring gas with a lower thermal conductance than air. Use of argon between glazing panes instead of air can reduce the amount of heat conducted across the gap and improve the R-value performance by approximately 15%.

Argon also substantially improves the R-value of IGUs where the gap between the panes is narrower than the recommended 10 mm minimum. Therefore, if the gap between panes must be reduced to 6–8 mm, such as when retrofitting IGUs into existing timber frame windows, argon gas-filled units should be used.

Krypton has a better thermal performance, particularly when small gaps are used, but is more expensive and is not available in New Zealand.

Other benefits of insulated glazing units

In addition to improving the thermal performance of windows, IGUs also provide other benefits such as reducing condensation, reducing noise transmission, and reducing the need for drapes or curtains (although these should still be installed for use on cold nights).

Solar control glazing

Solar control glazing is any glazing that is tinted, has a reflective or a low-E coated surface, or has spectrally selective characteristics. Many of these properties are frequently combined in one product to provide greater solar control.

Tinted glazing

Metal oxides are added during manufacture to produce tinted glazing panes. The solar control properties of tinted glazing operate on the principle that darker colours absorb more heat than lighter colours, so a dark tinted glazing will typically reach a higher temperature than a lightcoloured tinted glazing.

Tinted glass is therefore sometimes referred to as heat-absorbing glass.

Reflective glazing

Reflective glazing, has coatings typically added to the surface of glazing panes that have a higher reflectivity than standard glass. The reflectivity can be tuned to selectively:

• reflect short-wave solar radiation, or
• reflect long-wave heat radiation.

The reflective ability depends on the particular coating and on the orientation of the glazing. High-reflectivity coatings generally have low light transmission properties, but reflective glazing systems with lower reflectivity and higher light transmission properties are also available for domestic use.

Reflective coatings may be added to tinted glazing to further enhance the solar control performance of the glazing.

The reflection is typically towards the side with brighter illumination, so at night, the direction of the reflectivity can change so that the reflection is to the interior.

Low-E glazing

Low-E coatings are applied to the surface of glazing panes with specific solar control properties. A microscopically thin metallic coating is added to the glazing surface, which lets light through but reflects short-wave infrared radiation. This suppresses the radiant heat flow and lowers the U-value of the glazing. Converting one pane of a clear double-glazed IGU to low-E glazing is approximately equivalent to adding a third clear pane.

Low-E coated glazing is generally clear, and different types of coatings are available for high, moderate or low solar gain.

Low-E glazing is typically only used with insulated glazing units, which provide protection to the coating from the possibility of damage. The low-E coating is either:

• on the cavity-face of the inside pane to keep heat in (colder climate)
• on the cavity face of the outside pane to keep heat out (warmer climate).

Other locations are possible for specialist applications. Occasionally, low-E glazings are used as single panes.

Framing materials

The thermal performance of any window frame will depend on its construction, but in general timber and PVC frames provide better thermal insulation than aluminium.

The following table compares the U-values for centre of glazing (U_cog) and glazing system plus frame (U_total) standard windows with clear single and insulated glazing units and different frame types. See measuring glazing performance: key concepts for an explanation of U-values.

The information in the table is taken from NZS 4218:2009 Thermal Insulation – Housing and small building, Appendix C, Tables C1 to C4, based on the Window Energy Rating System rating for a standard window 1800 mm wide x 1500 mm high with a central mullion and one opening light.)

Frame type	Ucog (W/m2 0C)		Utotal (W/m2 0C)
	Single glazing - clear glazing	IGU - clear	Single glazing - clear glass	IGU - clear
Aluminium	5.88	2.73	6.70	3.90
Composite	5.88	2.73	6.58	3.86
Thermally broken aluminium	5.88	2.73	6.04	3.22
PVC or timber	5.88	2.73	5.23	2.75

Specifying solar heat gain coefficient (SHGC)

When specifying glazing units:

• Where passive solar gain is desirable, east-, north- and perhaps west-facing windows should have windows with high SHGC.
• Where late afternoon passive solar gain is undesirable, west-facing windows should have a low SHGC (less than 0.4).
• Where passive solar cooling is required, low SHGC windows should be used for north- and west-facing windows.
• For south-facing windows, the SHGC has less effect, but where passive solar gain is desirable, glazing should have a high R-value and a high SHGC.

The SHGC does not necessarily have a direct correlation to visible light transmittance. For example, some advanced tinted float glazing will allow a high transmission of visible light but also have a low SHGC.

See measuring glazing performance: key concepts for an explanation of SHGC.