How double-glazed windows are made

The insulation in a double-glazed window is the gap between the panes, often filled with argon.

stations: 6
things upstream: 55
raw sources: 18
steps from the ground: 8

The short version

Two panes of glass are cut, washed and separated by a hollow spacer bar filled with moisture-absorbing desiccant. Sealant bonds the panes to the spacer to make a closed gap. The gap is filled with air or argon, and the edge is sealed again. Some panes carry an invisible low-emissivity coating.

Cut the panes

Sheets of glass are scored and snapped to size. Panes usually range from 3 to 10 mm thick.

Goes in
Window glass
Wash and dry

A washing line scrubs the glass in clean water and dries it, because any dust left inside the unit stays there for good.
Bend the spacer frame

A hollow spacer bar is bent into a frame and filled with desiccant. The desiccant absorbs the moisture trapped inside during manufacture, so the glass won't fog.

Goes in
Aluminum
Seal pane to spacer

A butyl sealant, a rubbery compound that blocks moisture and gas, bonds the spacer to the glass. The second pane is pressed on, forming a closed cavity.

Goes in
Adhesive
Fill with argon

Air in the cavity is swapped for argon, a heavier, poorer conductor of heat, and the cavity is closed.

Goes in
Argon
Seal the outer edge

A secondary seal of silicone or similar sealant is applied around the edge to hold the unit together and keep out water.

Goes in
Silicone

End of the lineDouble-glazed window Next it goes into aluminum window frame.

Worth repeating

Fitting a second pane of glass to improve insulation began in Scotland, Germany and Switzerland in the 1870s.
A single pane of glass has an R-value of only around 1.
Low-emissivity glass has a thin metallic coating, usually on the second or third glass surface, that reflects infrared light.