Waking up to puddles on the window sill is one of the most common household complaints in Scotland between October and March, and it is almost never a fault with the windows. It is a humidity and ventilation problem showing itself on the coldest surface in the room.
This guide explains the mechanism, the practical fixes in order of effectiveness, and the one type of condensation that genuinely does mean something has failed.
How to reduce condensation on windows: step by step
- 1
Work out where the moisture comes from
A family of four puts several litres of water into the air daily through breathing, cooking, showering and drying laundry. Identify the biggest sources in your home.
- 2
Extract at source
Run the kitchen extractor while cooking and for ten minutes after, and the bathroom fan during and for twenty minutes after a shower. Keep the doors to those rooms closed while they run.
- 3
Stop drying clothes indoors
A single load of washing dried on a radiator releases around two litres of water into the house. Use a vented or condenser tumble dryer, or dry in one closed room with the window open.
- 4
Open the trickle vents
Leave trickle vents open all year. They exist to provide background ventilation and closing them is the single most common cause of condensation complaints.
- 5
Keep heating steady
A low, constant background temperature keeps surfaces above dew point. Short intense blasts of heat in an otherwise cold house produce more condensation, not less.
- 6
Improve air movement
Pull furniture a few centimetres off external walls, avoid overfilling wardrobes on outside walls, and leave internal doors ajar in unused rooms.
- 7
Add a dehumidifier if needed
In a persistently damp property a dehumidifier is a cheap and effective backstop while you deal with the underlying causes.
The three types of window condensation
| Where it appears | What it means | Serious? | Action |
|---|---|---|---|
| Inside face of the glass | Room humidity meeting cold glass | No, but manage it | Ventilate and heat |
| Outside face of the glass | Very efficient glass — the outer pane stays cold | No, it is a good sign | None |
| Between the panes | The sealed unit has failed | Yes | Replace the glass unit |
| On the frame or reveal | Cold bridging or poor sealing | Sometimes | Check insulation and sealant |
| Black mould on the seal | Persistent high humidity | Yes, health concern | Clean and fix ventilation |
Why new windows sometimes make it worse
Old draughty windows ventilated the house constantly, badly and expensively. Replace them with well-sealed units and the incidental ventilation disappears, so the moisture that was always being produced now has nowhere to go and shows up on the glass.
This is why trickle vents are required on most replacement windows. They are not a design flaw; they are what keeps the house breathing once you have sealed it up.
The fixes ranked by how much they help
- Stop drying laundry on radiators — by far the biggest single win in most homes.
- Use extractor fans properly, with the door shut and running past the end of the activity.
- Open trickle vents and leave them open.
- Keep a steady low background heat rather than heating in bursts.
- Ventilate bedrooms in the morning — open windows for fifteen minutes after getting up.
- Move furniture off cold external walls to let air circulate.
- Add a dehumidifier in the worst room.
When condensation is a building problem
If condensation and mould keep returning despite good ventilation habits, look beyond the windows. Cold bridging at lintels and reveals, missing or slumped cavity insulation, a failed damp-proof course, or a blocked underfloor vent all produce cold surfaces that will always attract moisture.
Black mould concentrated in one corner of one room usually points at a localised cold spot rather than a whole-house humidity problem.
Misting between the panes is different
If the moisture is between the two panes and you cannot wipe it off from either side, the sealed unit has failed and no amount of ventilation will help. The good news is that the frame is usually fine and only the glass needs replacing — typically £95 to £260 per unit rather than the cost of a whole window.
We do a lot of this work across Central Scotland and will always tell you if the frame is sound enough to keep.
Common questions
No, the opposite. It means the outer pane is staying cold because so little heat is escaping through the glass.
Want a straight answer on your own windows or doors?
DM Installations fits and repairs windows and doors across Central Scotland — Cumbernauld, Glasgow, Falkirk, Stirling and the surrounding towns. Free survey, written quote, and an honest answer on whether you need a repair or a replacement.
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