What is an IGU (double glazed unit)?

Most “double glazing” in Australia is an Insulated Glass Unit (IGU): two panes separated by a spacer, sealed around the edge, with dried air or gas inside. The key point is the cavity is dry and sealed — that’s why moisture should not appear between panes.

If moisture gets into the cavity, the unit has lost its sealed integrity — and performance drops (insulation, clarity, sometimes sound control).

3 types of condensation (and the one that matters)

• Inside surface condensation (room side) — usually indoor humidity + ventilation/heating behaviour.
• Outside surface condensation (external face) — can be normal on high-performing glass on cold mornings.
• Between-the-panes fogging — this is the red flag. It typically indicates the IGU seal has failed and moisture has entered the sealed cavity.

What fogging between panes really means

Fogging, misting, or water droplets between panes is strongly associated with seal failure. Once the seal is compromised, humid air can enter the cavity and condense, and any insulating gas (like argon) can leak out over time.

A quick homeowner test (the “ice test”)

In Australia, AGWA published a simple “ice test” concept to help distinguish normal surface condensation from likely IGU failure. Internal condensation/fogging patterns during the test can indicate moisture has entered the sealed unit.

If the fog is between panes and doesn’t wipe off, the practical fix is usually replacing the IGU (not the whole frame) — assuming the frame is still sound and the system is serviceable.

Shelf life: how long double glazing lasts (realistic expectations)

Here’s the part most people don’t cover: IGUs have a finite service life. The glass panes may last decades, but the edge seal system and desiccant are consumable components — they age.

Typical lifespan ranges (why it varies)

• Well-made + well-installed IGUs can last a long time, but seal aging and moisture ingress are common long-term failure modes.
• Harsh conditions (high UV, coastal salt air, high heat cycling, poor drainage) can accelerate seal degradation.
• Installation quality matters: drainage, setting blocks, edge clearances, and compatibility of glazing materials materially affect survival.

A useful way to think about it: the “shelf life” of an IGU is mainly the shelf life of its edge seal integrity, not the glass itself.

Repair options: replace IGU vs replace the entire window

Option A: Replace the IGU only (often the best value)

If the frame is structurally sound, hardware works, and the glazing system is serviceable, you can often replace just the sealed unit. This restores clarity and thermal performance without the cost of new frames.

• Common for aluminium and uPVC systems designed for reglazing
• Best when fogging is isolated to one or two units
• Typically faster than full window replacement

Option B: Replace the whole window (when it makes sense)

• Frame failure — warped timber, corrosion, water damage, poor structural integrity
• Obsolete system — glazing beads/gaskets unavailable or not designed for safe reglazing
• Multiple failures across many IGUs (end-of-life cluster)
• Upgrade goal — acoustics, bushfire requirements, higher energy performance

Why IGUs fail (and how to slow it down)