Most standard double glazed units are constructed with a perimeter hollow aluminium spacer which holds the glass spaced apart to provide a cavity or space between the glass.
The hollow aluminium spacer is filled with a drying agent to absorb any moisture within the cavity, which can contain air or inert gas. The perimeter spacer is placed approximately 4mm inwardly of the perimeter of the glass to provide a void which is filled with sealant to provide the perimeter seal.
The depth of the spacer is normally 6mm plus the hermetical seal, provides an overall seal depth of 10mm, requiring the window rebate depth to be at least 12mm. Aluminium spacers are highly conductive and lead to heat loss.
Unfortunately the aluminium spacer between the two panes of glass, causes a cold bridge around the perimeter and therefore perimeter loss of insulation.
This can be evidenced at the onset of condensation which forms initially around the perimeter of the double glazed unit. (see 4 - Warm Edge Technology).
The U-Values of standard double glazed units must comply with Building Regulations of minimum U- Value in England of 2.0 and Scotland 1.8.