Water droplets on window glass form when warm, humid air meets a cold surface. A little morning fog on the inside of the glass is normal, but persistent condensation is a warning that moisture is building up where it should not be. Left alone, that moisture feeds mould, rots timber frames, and damages paintwork. This article explains why condensation forms, how it differs between winter and summer, and the practical steps to stop it at the source.
Condensation rarely stays on the glass. The same moisture that fogs windows collects in attics, wall cavities, and cathedral ceilings, where it is harder to see and more expensive to fix. Understanding the dynamics in one spot helps protect the whole house.
Why Condensation Forms on Window Glass
Air holds water vapour, and warm air holds more of it than cold air. When air cools to the point where it can no longer hold its moisture, the excess turns into liquid water on the nearest surface. That temperature is the dew point. On a cold night the glass is often the coldest surface in the room, so it is the first place where moisture appears.
Dew Point and Relative Humidity
Relative humidity compares the moisture in the air with the maximum the air could hold at that temperature. At 50 percent relative humidity and 21 degrees Celsius, the dew point is roughly 10 degrees. If the glass surface drops to that temperature, water starts to form. Keep the room warmer, drier, or the glass warmer, and the fog disappears.
A broader look at how to treat condensation in buildings shows the same pattern repeating room by room: the fix always comes back to temperature, humidity, or ventilation.
Reading Humidity Levels
- 30 to 50 percent relative humidity is the comfortable band for most homes
- Below 30 percent, static shocks and dry skin become common
- Above 50 percent, condensation risk climbs quickly in winter
A hygrometer costs a few dollars and takes the guesswork out of the numbers. Measure in the morning, before heating and cooking add moisture to the air. Normal living releases several litres of water vapour into the air every day, and showering, cooking, drying clothes indoors, houseplants, and even breathing all add up quickly in a tightly sealed house.
Winter and Summer Condensation Compared
Condensation appears on different sides of the glass depending on the season, because the temperature gradient flips between winter and summer.
Two Seasons, Two Problems
| Season | Where moisture appears | Main cause | Typical fix |
|---|---|---|---|
| Winter | Inside face of the glass | Warm interior air meets cold glass | Ventilation, lower humidity, warmer glass |
| Summer | Outside face of the glass | Hot humid exterior air meets air-conditioned glass | Shading and air movement; usually clears itself |
| Between panes | Inside the sealed unit | Failed seal lets moisture in | Replace the sealed unit |
In winter the interior air is warm and often humid while the glass is cold. Windows are thinner barriers than insulated walls, so they become the coldest spot in the room and the first surface to condense.
In summer the pattern reverses. Air conditioning cools the glass below the dew point of the humid outdoor air, and water forms on the outside of the pane. Outside condensation usually clears on its own as the sun warms the glass, and it is a sign the window is doing its job.
The same outdoor-to-indoor temperature contrast that fogs glass also drives moisture problems in roofs. A cathedral ceiling with poor ventilation collects condensation in the same way, and the fix follows the same logic of airflow and insulation.
How to Stop Condensation on the Inside of Windows
Interior condensation is the most common problem, and it is also the most fixable. Work through these steps in order, because most homes need more than one.
Ventilation Changes That Work
- Run exhaust fans in the kitchen and bathroom while cooking or showering, and leave them on for 10 to 15 minutes afterwards.
- Open trickle vents or crack a window in rooms that stay damp.
- Move drying racks and clothes airers out of bedrooms and into a ventilated space.
- Keep furniture away from radiators and windows so air can move across the glass.
- Wipe down wet sills and frames each morning before the water soaks in.
High humidity does more than fog glass. It softens paint, swells timber, and can make doors and windows stick, so fixing the moisture problem also protects the joinery.
Whole-House Humidity Control
- A dehumidifier in the coldest or dampest room pulls litres of water out of the air each day
- Keep the heating on low rather than letting rooms cool right down, because cold air reaches its dew point faster
- Insulate or replace the coldest panes so the glass surface stays above the dew point
- Check for hidden moisture sources: leaks, unvented dryers, and damp basements push humidity up all winter
Condensation on the outside of the glass needs no treatment. Condensation inside, between the panes, or running down the frame needs attention. A quick check with the back of a hand tells you which surface is wet, and that determines the right fix.
Condensation Between Panes and Failed Seals
Moisture between the panes of a double-glazed window is different from interior fog. The space between the panes is sealed, and the seal contains a desiccant that keeps the air dry. When the seal fails, the desiccant saturates, humid air gets in, and water condenses on the inside of the outer pane.
Is the Window Unit Still Insulating?
A fogged unit has lost most of its insulation value. The gas that filled the gap, usually argon, has leaked out and been replaced by damp air. The window still looks the same from outside, but the U-value has degraded, so the unit loses heat faster and fogs more easily.
When Repair Beats Replacement
- If the frames and hardware are sound, replace only the sealed unit, not the whole window
- Check whether the unit is under warranty before paying for a replacement
- On old homes with timber frames, restoring old windows can save the character of the house while new sealed units restore performance
The decision depends on the frame. A modern PVC frame with a dead unit is a simple glass replacement. A century-old sash with a dead unit, on the other hand, often deserves restoration rather than demolition, and restoring old windows keeps the original joinery in service.
Dealing with a Blown Double-Glazed Unit
When the seal fails completely, you are dealing with a blown double glazed window unit. The glass looks permanently fogged, and wiping the inside face is impossible because the moisture is trapped between the panes.
Replacement Steps
- Confirm the failure: fog that does not clear with wiping and reappears after sunny days points to a failed seal.
- Measure the unit carefully: width, height, and glass thickness, including the spacer bar.
- Order a replacement sealed unit with the same dimensions and glass specification.
- Remove the beads or frame clips, lift out the old unit, and bed the new one on fresh sealant.
- Refit the beads and check that the drainage holes in the frame are clear.
Tackle this on a mild day. Hot sun makes the sealant runny, and cold weather makes the glass brittle, so a moderate temperature gives the cleanest job.
When to Call a Professional
Large units, upstairs windows, and glass with safety or acoustic ratings are worth handing to a glazier. The replacement unit itself is usually the bulk of the cost, so the labour saving on a DIY install is modest.
Long-Term Prevention and Maintenance
Condensation control is a habit, not a one-off fix. A short routine each season keeps the glass clear and the moisture under control.
A Seasonal Maintenance Routine
- Autumn: check seals and weatherstripping, clear window wells and drainage holes, service the heating system
- Winter: run extractor fans, keep humidity between 30 and 50 percent, wipe sills weekly
- Spring: clean tracks and hinges, check paint and putty around the frames
- Summer: shade south and west windows to reduce the temperature swing on the glass
The physics that fogs windows also makes plumbing pipes sweat in summer, when cold pipe surfaces meet humid air, and the same moisture-control approach protects both. Check the sealant beads around the glass once a year and replace any that have cracked or pulled away from the frame.
Signs You Need a Damp Survey
Persistent mould on frames, water staining inside walls, or condensation that returns within days of fixing the ventilation points to a deeper problem. A damp survey or building inspection traces the moisture to its source before it spreads further.