Heritage double glazed units have a cavity or space between the two panes of glass which is filled with inert insulating gas as the component to increase insulation values.
Argon is the primary type of inert insulating or low conductive gas used in standard double glazed units. Argon does not have the best insulating properties though, although Argon gas is by far the cheapest gas, hence its wide use in double glazing manufacture.
For compliance with Building Regulations, standard Double glazing units need to use wide cavities of 12mm to 16mm, which is reflected in the bulky window units necessary to accommodate the overall thickness of 20mm to 24mm.
Other inert gases which provide better insulation, although more expensive, do provide the better insulation value in smaller cavities. If these gases are used in wider standard cavities it is most likely that convection or movement will occur within the wider cavity, effectively transferring cold from the outer pane to the inner pane, losing insulation properties.
Convection does not occur in smaller cavities, therefore it is very effective in Heritage Double Glazed Units, enabling them to comply with Building Regulations.
Heritage Double Glazed Units with thin cavities, inert low conductive gases, small perimeter seal depths and U Values which comply with Building Regulations enables window construction with slimmer sections to produce a much more overall visually pleasing window.