What causes condensation on double glazed windows and what does it mean?

For Australian homeowners who invested in double glazing to eliminate winter condensation, spotting water droplets or fog on their glass can trigger immediate panic. However, condensation on double glazed windows is not a single issue with a single cause. Depending on where the moisture forms, condensation can be proof that your windows are insulating with world-class efficiency, a warning sign of high indoor household humidity, or definitive evidence of a failed sealed unit.

The simplest diagnostic tool is the Finger Swipe Test: run your finger across the glass. If you can wipe the moisture away from inside your room, the cause is indoor humidity and ventilation. If you can wipe it away from outside your house, your windows are performing brilliantly. But if your finger touches dry, smooth glass on both sides while water droplets, mist, or white mineral streaks remain locked inside, your Insulated Glass Unit (IGU) has suffered a hermetic seal failure.

Condensation on the outside: why it's a sign of superior insulation

Seeing mist, dew, or fine droplets on the exterior face of your double glazed windows on crisp autumn or winter mornings is completely normal—and is actually proof of superior thermal performance.

On clear, still nights, atmospheric water vapour naturally condenses onto any cold outdoor surface (such as grass, car windscreens, and exterior walls). On older single-glazed windows, indoor heat continuously leaks straight through the thin glass into the atmosphere, warming the outer surface and keeping it above the outdoor dew point. You rarely see exterior condensation on single glazing because the window is actively wasting heating energy.

By contrast, a high-performance double-glazed IGU traps your home's warmth indoors. Because virtually no heat conducts through the argon gas cavity to warm the outer pane, the exterior glass cools down to the outdoor ambient temperature. When that outer surface drops below the outdoor dew point, atmospheric moisture condenses on the outside. As soon as the morning sun rises or a breeze picks up, this exterior moisture evaporates harmlessly. No maintenance is required.

Condensation on the inside (room-facing glass): dew points and humidity

If moisture beads on the room-facing surface of your double glazed window, it is not a window defect; it is an indoor humidity and ventilation challenge.

Condensation forms when warm, moisture-laden indoor air contacts a surface at or below the dew point temperature. Everyday domestic activities—including breathing, hot showers, cooking, and indoor laundry drying—release substantial amounts of water vapour into the home daily.

Double glazing keeps the centre of the inner pane substantially warmer than single glass, which drastically reduces room-side condensation. However, if indoor relative humidity exceeds 60% to 65% in an unventilated bedroom overnight, even an insulated inner pane can eventually drop below the dew point on freezing winter mornings.

In addition, if your windows use non-thermally broken aluminium frames or standard aluminium spacer bars, heat conducts outward through the perimeter metal. This creates a 'cold edge effect' where a thin 15mm–25mm band of condensation beads along the bottom glass edge. Practical steps to resolve room-side moisture include:

• Ventilate Moisture at the Source: Always run ducted exhaust fans in bathrooms during and for 10 minutes after showers, and use rangehoods that vent externally (not recirculating) when cooking.

• Avoid Unflued Gas Heaters: Portable unflued gas heaters release significant water vapour into the room air during combustion, rapidly driving indoor humidity above the dew point.

• Promote Convective Airflow: Keep internal heavy block-out curtains open slightly at the bottom, or crack open trickle vents to allow warm room air to circulate across the glass face.

Condensation between the panes: what seal failure actually means

Condensation trapped between the two glass panes—appearing as a permanent cloudy haze, water droplets, or streaky white calcium deposits inside the cavity—is the definitive symptom of a failed Insulated Glass Unit.

Under Australian Standard AS 4666, an IGU is hermetically sealed during factory assembly using a dual-seal barrier: primary polyisobutylene (PIB) to block moisture vapour and retain argon gas, backed by secondary structural silicone or polysulphide. Inside the hollow perimeter spacer bar is a crystalline molecular sieve desiccant engineered to absorb any trace moisture remaining after assembly.

When physical damage, manufacturing defects, or decades of thermal expansion breach this hermetic perimeter seal, ambient outdoor air is drawn into the cavity. At first, the internal desiccant absorbs this incoming moisture. But once the desiccant reaches maximum saturation, it can absorb no more. The moisture vapour then condenses into visible water droplets on the internal glass surfaces (Surfaces #2 and #3).

Debunking DIY 'Fixes': Online videos occasionally suggest drilling tiny holes in the glass or frame, flushing the cavity with alcohol, or using a hairdryer to clear internal fogging. Australian fenestration engineers caution that these DIY hacks are destructive: once the desiccant is waterlogged, it cannot be dried in place, and drilling holes permanently vents the insulating argon gas, leaving an unsightly, stained unit that continues to fog. The only compliant, long-term solution is to deglaze the frame and install a fresh factory-sealed IGU.

The #1 killer of IGU seals: blocked weep holes and water pooling

While quality double glazing has an expected service life of 20 to 30 years, premature seal failure within 5 to 10 years is almost always caused by poor frame drainage.

All architectural window frames are engineered with sub-sill drainage slots and weep holes designed to channel wind-driven rainwater away from the glass rebate. However, over time, airborne dust, tree sap, and spider webs can clog these tiny drainage weep holes. When rain falls, water becomes trapped inside the bottom frame rebate, forcing the lower edge of the double glazed IGU to sit submerged in standing water.

While secondary sealants are moisture-resistant, prolonged continuous water submersion chemically degrades the polysulphide or silicone bond. Once the secondary seal softens, moisture penetrates the primary PIB vapour barrier and enters the cavity. Regularly clearing window frame drainage tracks and inspecting exterior weep holes twice a year is the single most effective maintenance habit to ensure your IGU seals last 30+ years.

The fixed window reglazing advantage

If an Insulated Glass Unit does eventually fail after decades of service, replacing the glass in a fixed aluminium window (picture window) is remarkably simple, clean, and cost-effective.

In a fixed window, there are no moving sashes, balances, or complicated roller tracks to dismantle. A licensed glazier simply unclips the snap-in aluminium glazing beads from the frame, removes the failed IGU using heavy-duty suction lifters, clears and checks the bottom rebate drainage slots, and beds a brand-new AS 4666 certified double-glazed unit into place with fresh santoprene rubber gaskets.

The entire reglazing process takes under 45 minutes per window. The architectural aluminium frame, external weather flashings, internal timber reveals, and surrounding plasterboard remain 100% undisturbed, restoring your fixed view window to pristine clarity and full thermal performance.

Frequently asked questions

Can condensation between double glazing panes be repaired without replacing the glass?

No. Once moisture enters the sealed cavity and saturates the internal desiccant, the hermetic seal cannot be restored in the field. The affected IGU must be deglazed and replaced with a new factory-sealed unit. Reputable Australian manufacturers offer 10-year warranties that cover IGU seal failure.

Why did condensation suddenly appear after installing new double glazed windows?

Older single-glazed windows are notoriously drafty, allowing moist indoor air to leak out through frame gaps. Modern double glazed windows create an exceptionally airtight building envelope. Unless homeowners adjust their ventilation habits (running bathroom fans and opening windows during cooking), moisture generated indoors has fewer escape paths, elevating relative humidity and occasionally causing room-side condensation.

Does condensation on window frames cause black mould?

Yes. Persistent room-side condensation that drips onto timber reveals or porous plasterboard provides the damp organic environment required for black mould (Stachybotrys) spores to colonise. Regularly wiping away winter room moisture and increasing ventilation is essential to maintain healthy indoor air quality.