Most standard double glazed units are constructed with cavities around 16mm to 20mm and filled with Argon gas. The wide cavities are necessary to achieve the required insulation or U Value. Argon is a light gas and necessary for wider cavities. Argon cannot produce the best insulation in small cavities which require Krypton and Xenon to provide the best results.

Some standard double glazed units still use an aluminium spacer to provide the required cavity. However this causes a cold bridge around the perimeter evidenced as condensation around the perimeter in cold conditions.

Slimlite Double Glazed Units by using warm edge spacer, which is now becoming popular  avoids this problem, as the difference in insulation from centre to perimeter is almost nil.

Example: Slimlite Double Glazed Units
Constructed: 4mm Low E/4mm Cavity, gas/4mm Clear - U Value 1.9 BSEN673
National Physical Laboratory Report on total overall U Value 1.96 BSEN8990.