Low Energy House - Low E Glass - Double and Triple Glazing

A double glazed unit consists of two sheets of glass separated by an air gap. A triple glazed unit consists of three sheets of glass separated by two air gaps. The panes of glass are kept apart by spacers and the edges are sealed. The air gaps provide the insulating layer that slows down the heat loss and reduces the opportunity for condensation to form

Low Emissivity Glass - Low E Glass

In order to achieve the U-values required by the building regulations low emissivity glass must be used. Double and triple glazing units with low emissivity glass (low E) will perform better than ordinary sealed units.

Low E Glass will:

• Reduce heat loss, saving energy by maintaining a comfortable environment at lower thermostat settings.
• Reduce cold spots and downdraughts near windows, improving comfort and increasing usable floor space
• Increase inner glass surface temperatures reducing condensation inside the window.
• Reduce capital and running costs of heating systems

Low E Coating

Low emissivity (low E) glass incorporates a very thin layer of metallic coating on one surface. To protect it from wear, the low E coated surface is positioned in the outer face of the inner pane in a double or triple glazing layer. The coating allows heat from the sun to enter the building but significantly reduces heat loss from inside the building by reflecting radiant heat back into the room.

Transparency of Low Emissivity Glass

In most instances, the transparency of the glazing is not significantly affected by the low E coating although a very slight tint is discernable in certain circumstances, particularly from the outside.

Pane Separation in Double and Triple Glazing

Double and triple glazed units for house windows are generally available with air gaps from 6mm to 20mm. In frames with narrow air gaps, the closeness of the panes can cause heat to pass from the inner pane to the outer. Where the air space is wide, air currents can set up within the air gap which also causes warm air to pass from the inner pane to the outer.

Double Glazed Unit Thickness

The thickness of the glass will depend on the size of the window. In house high performance windows, the most popular air gap width is 16mm. With a glass thickness of 4mm, the unit is 4-16-4 giving a total double glazed unit thickness of 24mm (1 inch). This size works well for replacement window frames.

Insulating Spacers

Thermally, the weakest point of a double or triple glazed unit is at the edge where the spacer is incorporated. Spacers are usually made of aluminium, which conducts heat, and can create a cold bridging effect at the edge of the unit. Aluminium spacers should be buried deep inside the rebate in order to minimise a thermal bridge. Sealed units are available that incorporate insulated spacers and reduce the cold bridge effect, improving the overall performance of the unit.

Inert Gas filled Spacers

Air is a good insulator but inert gases are better because they have lower thermal conductivity which reduces the amount of heat that is conducted through the window. The cheapest and most commonly used inert gas is argon, although krypton and xenon can also be used. Gas filled units are generally used in triple and double glazing to reduce the unit depth.

Triple Glazed Windows

Expertly designed assembled and installed triple glazed windows will significantly reduce energy usage in a new dwelling.

Double Glazed Windows

Old and badly fitting window frames should be replaced with energy efficient double glazed windows to save energy and improve comfort.

If you have one or more of the windows replaced in your home then Building Regulations will apply to the work. If the installer you employ is registered you will not have to involve the Building Control Service. On completion of the work the registered installer must give your local authority a certificate confirming that the work complies with Building Regulation