Key Takeaways
- Condensation on windows forms when warm, moist indoor air meets a cold glass surface, so it is a sign of humidity and surface temperature rather than a faulty window.
- Where the moisture appears tells you the cause: inside the room, outside the glass, or trapped between the panes.
- Better ventilation, lower indoor humidity and warmer glass surfaces are the three levers that reduce condensation.
- uPVC double glazing keeps the inner pane warmer than single glazing, which cuts the chance of moisture forming on the inside.
What Causes Condensation on Windows?
Air holds water vapour, and warm air holds more of it than cold air. When that warm, humid air touches a surface cooler than its dew point, the vapour turns back into liquid droplets.
Windows are usually the coldest surface in a room, so they are where the droplets show up first. On a cold Melbourne morning, the glass can sit well below room temperature.
- Showers and cooking: both release large amounts of vapour in a short time.
- Drying clothes indoors: a full rack can release litres of water into the air.
- Unflued gas heaters: these add moisture as a by-product of burning gas.
- Poor ventilation: closed rooms trap vapour until it finds a cold surface.
- Cold glass: single glazing and aluminium frames drop to the outside temperature quickly.
Where the Condensation Appears Tells You the Cause
Not all window condensation means the same thing. Checking the location is the quickest way to work out what is happening and whether anything needs repair.
| Location | Likely cause | What to do |
|---|---|---|
| Inside the room | High indoor humidity meeting cold glass | Ventilate, reduce moisture, consider better glazing |
| Outside the glass | Cool glass meeting humid outdoor air, common on very efficient windows | Usually no action, it clears as the day warms |
| Between the panes | Failed seal on a double glazed unit | Have the unit assessed and replaced |
Condensation on the outside can actually be a good sign. It happens because the outer pane stays cool, which means less heat is escaping from your home through the glass.
Moisture between the panes is different. It means the sealed unit has failed and outside air has entered the gap, so cleaning will not fix it.
Why It Matters Beyond Wet Glass
Occasional light fogging is normal. Persistent condensation is worth taking seriously, because the moisture does not stay on the glass.
- Mould growth: damp window reveals and curtains are common places for mould to start.
- Damaged finishes: water running onto sills can stain and lift paint or timber.
- Air quality: mould spores and damp air can affect people with asthma and allergies.
- Wasted energy: cold, damp rooms are harder and more expensive to heat.
Condensation management has also become part of Victorian building rules. Under the National Construction Code, condensation management requirements became mandatory in Victoria from 1 May 2024, as set out by Victoria’s building standards guidance.
Practical Ways to Reduce Window Condensation
Start with the moisture sources and the airflow, because these cost little and help immediately. Windows then do the rest of the work by staying warmer.
- Run bathroom and kitchen extractor fans during and after showers and cooking.
- Vent clothes dryers outdoors and avoid drying washing inside where you can.
- Open windows briefly each day to swap damp air for fresh air, even in winter.
- Keep furniture and heavy curtains slightly away from glass so air can circulate.
- Use a dehumidifier in rooms that stay damp, such as laundries and bedrooms.
- Wipe away condensation each morning so moisture does not soak into reveals.
If condensation keeps returning despite these steps, the glass itself is probably too cold. That is when upgrading the window makes the difference.
How uPVC Double Glazing Helps
The inside surface of a window stays warmer when the glazing insulates well. That is the main reason double glazing reduces indoor condensation compared with a single pane.
Ultimate Windows double glazed units are built to keep heat in and the inner pane warm. Our double glazed windows are made from uPVC, which is a natural insulator and does not chill the frame the way bare metal can.
- Insulating gap: a sealed 16mm space between panes limits heat transfer through the glass.
- Argon gas as standard: included at no extra cost to improve insulation across the gap.
- Warm edge spacers: non-metal spacers keep the glass edge warmer, where condensation often starts first.
- Low-E glass options: a coating that reflects heat back into the room.
- Compression seals: tight seals limit draughts that cool the frame and glass.
Our uPVC windows are up to 4.5 times more energy efficient than standard aluminium-framed windows. If you are comparing materials, see our guide to aluminium frames compared with uPVC.
A Note on Fit and Ventilation
Better windows reduce condensation, but they also make homes more airtight. A tighter home holds moisture unless you keep ventilating.
Many people open a tilt sash on a window for a few minutes each day. Fresh air and warm glass together control moisture much better than either alone.
- Choose operable windows: styles that tilt open let you vent without a full opening.
- Use trickle ventilation: small controlled openings keep air moving in cold weather.
- Get installation right: a sealed frame and gap stops cold air bypassing the glazing.
Our in-house team handles every job, and you can see how it works on our professional window installation page.
When to Replace Windows Instead of Managing Moisture
Habits and ventilation solve many cases. Replacement makes sense when the window itself is the cause and the problem keeps coming back every cold season.
- Single glazing: a single pane is cold in winter and fogs easily, whatever you do indoors.
- Failed sealed units: misted glass between panes cannot be repaired by cleaning.
- Damaged frames: rotting timber or warped aluminium lets cold air and moisture in.
- Mould on reveals: repeated mould after cleaning points to a cold surface that needs fixing.
- High heating bills: if rooms stay cold and damp, heat is likely escaping through the glass.
Frequently Asked Questions
Is condensation on the inside of windows bad?
Occasional light condensation in cold weather is normal. Persistent heavy moisture is a warning sign of high humidity or cold glass, and it can lead to mould on reveals, sills and curtains if left unchecked. Improving ventilation and glazing usually solves it.
Why is there condensation on the outside of my double glazed windows?
Outside condensation forms when the outer pane is cooler than the surrounding air, often on clear, still mornings. It shows the glass is insulating well because heat is not escaping from inside. It normally clears as the sun warms the glass.
What does moisture between double glazed panes mean?
Moisture trapped between the panes means the seal on the glass unit has failed, letting humid air into the gap. It cannot be wiped away. The sealed unit generally needs to be replaced, and a glazier can confirm whether the frame can be kept.
Will double glazing stop condensation completely?
Double glazing greatly reduces condensation because the inner pane stays warmer, but it cannot remove moisture from the air. If indoor humidity is very high, droplets can still form. Combining double glazing with ventilation and humidity control gives the best result.
Get Expert Advice on Condensation and Better Windows
If your windows are streaming every morning, our team can assess the problem and recommend a fix, from ventilation tips to new uPVC double glazing. Book a free measure and fixed-price quote, or call 1300 898 782 to talk it through.
