Double Glazed Units Explained

A double glazed unit — also called a DGU or IGU — is two panes of glass sealed around a spacer bar with a sealed cavity between them. It arrives as one sealed component and is fitted as one.

What the cavity does

The cavity does the work, not the glass. Air, or sometimes an inert gas, is a poor conductor of heat compared with glass. Trapping a layer of it between two panes slows heat transfer in both directions — rooms stay warmer in winter and cooler in summer.

The sealed cavity also disrupts sound transmission, so a DGU is noticeably quieter than single glazing.

What 20mm and 24mm mean

The number is the total thickness of the unit, not the glass. It is glass plus cavity plus glass.

• 20mm — typically 4mm glass, a 12mm cavity, 4mm glass.
• 24mm — a wider cavity or thicker panes, giving better thermal and acoustic performance.

Which you need is usually decided by your window frame, not by preference. The frame has a rebate of a particular depth and the unit has to fit it with the correct clearance.

They cannot be cut, ever

This is the single most important thing to understand. A DGU is sealed at manufacture. Cutting it breaks the seal, and a broken seal means the unit fails — moisture enters the cavity and it fogs.

So unlike every other product we cut, there is no adjusting on site. The measurements have to be right before it is made.

Measuring for a replacement

Two dimensions matter and they are not the same thing.

Sight size is the glass you can see. Rebate size is the full size of the unit including what is hidden inside the frame. You need the rebate size, plus the clearance the frame requires.

You also need the rebate depth, because that determines whether a 20mm or 24mm unit will physically fit.

If you are unsure — and this is a case where being unsure is entirely reasonable — your window supplier or a glazier can confirm the sight size and rebate depth. It is worth the phone call. A DGU that does not fit cannot be modified.

Replacing a failed unit

The usual sign is fogging or moisture between the panes — visible condensation you cannot wipe away because it is inside the sealed cavity.

That means the perimeter seal has failed. There is no repair. The unit is replaced, not fixed. Anything claiming to “de-fog” a unit by drilling and drying it is treating the symptom; the seal is still gone.

The good news is that only the unit needs replacing, not the window or the frame.

When a DGU is worth it

• Rooms that are hard to heat or cool
• Bedrooms and living rooms facing a busy road
• Large glazed areas where single glazing is the weak point in the envelope
• West-facing glass in a Melbourne summer

It is worth being realistic: replacing one window in a house of single glazing gives a local improvement, not a whole-house one. The benefit scales with how much of the glazed area you address.

Lead time

DGUs take longer than anything else we supply. The unit has to be assembled and sealed after the glass is cut, and the seal needs to cure. Order with that in mind rather than expecting a same-week turnaround.

See the double glazed units range for 20mm and 24mm made to your measurements, or email us at info@affordableglassonline.com.au to check your sizes before ordering.