Double Glazing Works Because Of The Gas Between The Panes

Two sheets of glass insulate far better than one, and the reason is not that there is twice as much glass. Glass is a poor insulator and the gap does the work.

Glass conducts heat readily

Heat passes through glass without much resistance. Doubling the thickness of a single pane changes its insulating performance only marginally.

Still air, by contrast, conducts poorly. Trapping a layer of it between two panes introduces a genuine barrier where previously there was none.

The gap has an optimum width. Too narrow and conduction dominates, too wide and the gas begins circulating, carrying heat across by convection instead.

Heavier gases perform better than air

Sealed units are usually filled with argon rather than air. It conducts less readily and, being denser, is more reluctant to set up convection currents.

The improvement is real but modest, and the gas slowly diffuses out through the seal over many years. Performance therefore declines gradually rather than at a stroke.

Because the loss is invisible, an old unit may still look perfect while insulating measurably worse than it did when installed.

Coatings handle the radiated heat

Conduction and convection are not the only routes. Warm surfaces radiate infrared, and that radiation crosses the gap unimpeded.

A low-emissivity coating on one inner surface reflects infrared back towards its source while remaining transparent to visible light. It returns heat to the room in winter.

This coating typically contributes more to modern window performance than the gas fill does, and it is the main reason recent units outperform older double glazing so clearly.

The spacer is the weak edge

The panes are held apart by a spacer bar around the perimeter, and that bar bridges the gap. A metal spacer conducts heat straight around the insulating layer.

This is why condensation and cold spots appear at the edges of a window first. Warm-edge spacers using less conductive materials exist specifically to reduce that path.

The spacer also contains a desiccant, which absorbs any moisture sealed inside during manufacture and keeps the cavity dry.

Misting means the seal has gone

When condensation appears between the panes, the perimeter seal has failed. Humid air is entering, the desiccant is saturated, and the gas fill has long since escaped.

The unit still keeps weather out, but its insulating value has fallen towards that of a single pane with an air gap.

Nothing inside can be cleaned or refilled in place, which is why the sealed unit rather than the window is the item replaced.