Double Glazing

What is Double Glazing?

Double glazing units are commonly referred to as insulated glass units. This is when the glass consists of 2 or more panes that are isolated by a desiccant-filled spacer and sealed with a sealant. While the main function of a double glazing is to improve the comfort of building occupants, it can also reduce heating and cooling costs by reducing the flow of heat from the outside to the inside, or vice-versa – depending on the climate. 

The insulation value, commonly referred to as the U-value, is dependent on the configuration this type of glass includes – thickness and spacer width. Double glazing units incorporating solar control products such as Low-E, Soft-coated Low-E and reflective glass will significantly reduce:

  • Solar heat gain
  • Glare control
  • Air conditioning costs
  • Noise reduction

However, double glazing can also improve security measures as well, since it is harder to penetrate from the outside layers. For this reason, it is widely used in applications such as curtain walls, lower level windows, and industrial products such as freezers and coolers.

When air that is saturated with excess water vapours comes into contact with a colder surface, condensation will naturally form on the glass surface. The temperature at which condensation occurs is known as the ‘dew point. Once the air reaches the dew point, condensation can develop on the hot air side of the windows during colder weather, however, with double glazed glass, the side temperature is significantly lower maintaining a warmer atmosphere on both sides of the glass. Double glazing unit is recommended to provide a thermal barrier between the inside and the outside, thereby minimising the occurrence of condensation.

Shading Coefficient

The ratio of solar heat gain through a window to the solar heat gain through a single light of 3mm clear glass under the same set of conditions. Dimensionless and varying between 0 and 1, the smaller the number, the better the window is at stopping the entry of solar heat.



The heat flow rate through a given construction is expressed in W/m2/°C. The lower the U-Value, the less heat is transmitted through the glazing material. Values given for summer calculated for outside air temperature at 32°C, outside air velocity at 2.8 m/s, and inside air temperature of 24°C, and a solar intensity of 783 W/m2. Winter night time U-Values are calculated for outside air temperature at -18°C, outside air velocity at 5.5 m/s, and a solar intensity of 0 W/m2.

Relative-Heat Gain (RHG)

The total amount of heat gain through a glazing system specified summer conditions, incorporating the U-Value and the Solar Heat Gain Coefficient. The conditions are 783 W/m2 outdoor temperature of 32°C indoor temperature of 24°C and 2.8 m/s wind.

RHG = U SUMMER X (32-24) + SHGC X (783)
Expressed in terms of W/m2

Solar Heat Gain Coefficient (SHGC)

The fraction of incident solar radiation which enters a building as heat. It is based on the sum of the solar energy transmittance plus the inwardly owing fraction of absorbed
solar energy on all lites of the glazing(This measure equates to the Sun’s direct transmittance energy plus the part of this energy absorbed by the glass and re-radiated inside). Dimensionless and varying between 0 and 1, the smaller the number, the better the glazing is at preventing solar gain. 3mm clear oat has a SHGC of 0.86. It is preferred over the shading coefficient since it can be used for solar incidence angles other than normal to the glass surface. SHGC can also be calculated as 86% of the Shading Coefficient.

Double glazing is also useful in regulating temperature. When air with excess water vapour come into contact with a colder surface, condensation will form on the surface (the temperature at which the condensation occurs is the ‘dew point’). Double glazing can provide a thermal barrier between the inside and the outside, minimizing the occurrence of condensation, regulating temperature and preventing water damage to surrounding areas. Double glazing is widely used in applications such as curtain walls, windows and industrial units such as freezers and coolers.

Insulation Comparison U-value W/m2
Single Pane 5.60-6.20
Single Pane Low-E 3.80-4.20
Standard IGU 2.40-2.70
Low-E IGU 1.60-2.10
Low-E/Argon gas IGU 1.50-2.00
Low-E/Argon gas/ Triple Glaze 0.25-0.33

U-Value VS. SHGC

Conduction, convection and radiation are measured by the U-value where direct transmit- tance energy from the Sun is measured by the SHGC. Why use both measures?

In general terms where buildings are artificially cooled or heated in any climate, glass with a lower U-value will reduce energy costs. However, for warm climates when we combine the SHGC and U- value into one total heat gain number RHG on an unshaded glazing, the SHGC which becomes more relevant. The Sun’s direct heat (measured by SHGC) con- trols a much larger percentage of the total heat gain when compared to other heat flows (as measured by U-value). For warm climate unshaded windows, control of the Sun’s direct energy with a glass that has a lower SHGC is the first important step in design.

As previously mentioned, a lower U-value will further assist in heat gain reduction and lower energy costs. Improving the U-value will further improve and lower the total heat gain impact on the building. By using glass with a lower U-value provides both benefit to daytime shaded windows and during warm nightime conditions.It is often the surroundings of the building including objects, the ground, other buildings etc which absorb heat and re- radiate it (long wave radiation) towards the window. These objects may continue to release this energy throughout the day and into the night. Energy use can thus be reduced through selection of glass with lower U-values. Preferred glazing solutions for warm climates are low SHGC glass with an integral low-E coating or low SHGC low-E coated glass combined in IGU’s.


The acoustical performance of windows and doors is affected by:

  • Glass size
  • Glass thickness
  • Airspace gap
  • Presence of laminated products
  • Framing members
  • Gaskets, sealants, weather stripping
  • Window design

Sound transmission class (STC) is the standard method for rating sound attenuation characteristics of glass products and window assemblies. The higher the STC rating, the higher the sound attenuation properties of the window.

Glare Reduction

Reducing annoying glare can be achieved through controlling the amount of daylight that passes through the glass. Though it should also be noted that glare is subject to individual perception. Some situations may require other methods to control glare such as external barriers, blinds, ceramic fritted patterns or matrixes on the glass itself or removing the cause of the glare.

Acoustic Properties
Baseline unit construction: STC Rating 28 3mm Glass/ 6.5mm Airspace/ 3mm Glass
Property Change in STC Ratting
Increase air space thickness
6.5mm to 13.0mm +2
13.0mm to 25.0mm +3
Change glass thickness One pane Two panes
3mm to 6mm +2 +4
6mm to 12mm +2 +5
3mm to 2.2mm -2 -3
Mismatch glass thickness increased from 2:1 to 3:1 +1
PVB laminated addition of 0.76mm +4
Increase PVB Thickness from 0.76 to 1.52mm +2
Replace air with Argon gas no charge