Energy Efficient Terms - Capral

Energy Efficient Terms

Before embarking on the important journey of choosing windows and doors for your home, it may be useful to become familiar with a few commonly used energy efficient terms.

Double Glazed Units or Insulated Glass Units (IGUs)

IGUs provide thermal insulation and improved acoustic performance. Consisting of two panes of glass, separated by an air or gas filled cavity, an IGU is described in terms of the thickness of the outer pane in millimetres, followed by the cavity width and finally the thickness of the internal pane; e.g /1/. Generally, a 1mm to 1mm cavity width provides optimum performance.


Heat loss and heat gain can occur through leaks in the window or door assembly. Good seals and quality workmanship minimise this.

Laminated Glass

Panes are assembled from two sheets of glass with an adhesive interlayer. Laminated glass when broken will not shatter or splinter leading to greater safety and security. Laminated glass eliminates nearly 99% of harmful UV rays, reducing fading to floors and furniture. A specialised interlayer in laminated glass can further reduce Solar Heat Gain Coefficient.

Reflective Coating

A metallic coating is applied to one side of the glass in order to significantly increase the amount of reflected visible and infra red heat.

Low E Glass

A thin coating that is applied to the glass surface to enable it to reflect short wave (direct solar) heat or long wave (re-radiated/reflected) heat.


The direction a home faces can affect how much energy is used heat and cool. Ideally a home should be oriented north-south to prevent the sun’s rays from penetrating in summer, while still allowing for solar warming in winter. It is impractical for many homes to achieve optimal orientation; however, high performance windows can help achieve a high level of control over a home’s thermal performance.

Passive Cooling

Passive cooling encourages cooling breezes and air movement in summer as an inexpensive way of cooling your home. Passive cooling is best achieved through: orientation to cooling breezes, increased natural ventilation, a light coloured roof and wall finishes, and appropriate windows and glazing to minimise heat gain.

Passive Solar Heating

Passive solar heating aims to keep the summer sun out while still allowing the winter sun in. As well as using orientation and shading, passive solar heating is best achieved through effective insulation, draught sealing and high performance glazing. Window frames and the type of glazing used play a big part in passive solar heating.


Shading a home can improve comfort and increase energy efficiency. It is
important to install effective shading devices to block summer sun from the north while still allowing the lower angled winter sun in.

Solar Control Glass

Glass that reduces heat gain derived from direct solar radiation. This may be achieved via interlayers, body tints, reflective coating or Low E coatings.

Solar Heat Gain Coefficient (SHGC)

SHGC is a measure of the amount of solar radiation transmitted through a window and released into the home as heat. The lower a window’s SHGC, the less solar heat it transmits; a low SHGC is vital for warm or hot climates.

Toned / Tinted

Usually green, grey, bronze or blue, toned or tinted glass can shade internal areas and reduce the amount of heat entering through the window. This will keep the building cooler and reduce glare and UV rays.

U Value

U Value measures the rate of heat flow through a window, due to temperature difference between the indoor and outdoor environment, taking into account the effect of the frame, glass, seal and any spacers. Heat is lost and gained through a window by the combined effects of conduction, convection and radiation. The lower the U Value, the higher the energy efficiency of the window or door.

Visible Transmittance (VT)

VT is a numeric measure of how much visible light is transferred through the window. The higher the VT, the more daylight that is openly transmitted.

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