In this article, you’ll discover:
- Why condensation forms on windows (and why it’s not a fault)
- The difference between interior, exterior, and between-pane condensation
- Four practical solutions ranked by cost and effectiveness
- When upgrading your glazing actually makes sense
Waking up to windows streaming with water is frustrating. And if you’ve got beautiful timber windows, it’s natural to worry about moisture causing damage.
Here’s the thing: condensation on windows isn’t usually a window problem. It’s a ventilation and humidity problem. Your windows are simply the coldest surface in the room — where moisture in the air turns back into water droplets.
This guide covers all the causes and solutions, so you can tackle the problem properly.
What Causes Window Condensation?
Condensation is water vapour turning back into liquid when it meets a cold surface. The scientific term is ‘dew point’ — the temperature at which air can no longer hold its moisture. Three factors determine whether you get condensation:
- Indoor humidity levels — a family of four produces 10-15 litres of water vapour daily through breathing, cooking, and washing
- Surface temperature — how cold the window glass is
- Ventilation — whether moist air is being replaced with drier air
Modern homes suffer more because they’re airtight. Older houses were draughty — not ideal for comfort, but excellent for preventing condensation. When you insulate and draught-proof without adding ventilation, moisture has nowhere to go.
Interior vs Exterior vs Between-Pane Condensation
Condensation on the Inside
Interior condensation — moisture on the room-facing surface — is the most common type. It indicates high indoor humidity combined with cold window surfaces. Particularly common in bedrooms (we breathe out moisture overnight), kitchens, and bathrooms.
Condensation on the Outside
Here’s something that surprises many homeowners: external condensation is actually a sign of good thermal performance. It happens when the outer glass is so well insulated from your home’s heat that it stays colder than the outside air. If you’ve recently upgraded to low-e glazing and see morning condensation outside, don’t worry — it clears as the sun warms the glass.
Condensation Between the Panes
Moisture trapped between the glass panes means the seal has failed. The inert gas (usually argon) has been replaced by humid air. Failed sealed units need replacing — the good news is that with timber windows, you can usually replace just the glazing unit rather than the whole window.
Four Solutions: From Simple to Significant
Solution 1: Improve Ventilation
The single most effective solution. You need moist air out and drier air in.
Trickle vents are small ventilators built into window frames, providing constant background ventilation without significant heat loss. Building Regulations require them in new windows precisely because they’re so effective. Keep them open — closing them defeats the purpose.
Extract fans in kitchens and bathrooms should run during cooking/bathing and for 15-20 minutes afterwards. Humidistat-controlled fans that activate automatically are particularly useful.
Window opening: Even in winter, five minutes of ‘purge ventilation’ (windows wide open) is more effective than leaving them on a crack all day.
Solution 2: Reduce Humidity at Source
Sometimes you’re producing more moisture than ventilation can handle:
- Drying clothes indoors: One load releases ~2 litres of water. Use a vented tumble dryer or dry in a closed room with window open.
- Cooking: Cover pans and always use the extractor.
- Showers: Keep bathroom door closed; run fan during and after.
- Unflued gas heaters: Produce huge amounts of water vapour. Avoid if possible.
Solution 3: Use a Dehumidifier
If ventilation alone isn’t enough — particularly in older properties or basements — a dehumidifier can help. Modern refrigerant models extract several litres daily and cost around 3-5p per hour to run.
Aim for 40-60% relative humidity. Below 40% causes dry skin; above 60% encourages condensation and mould. A simple hygrometer (under £10) lets you monitor levels.
Solution 4: Upgrade Your Glazing
If you’ve got single glazing or failed sealed units, upgrading makes sense. Modern double glazing with a decent U-value (1.4 W/m²K or better) keeps the inner glass warmer, making condensation less likely.
Triple glazing and argon-filled units perform even better. But honestly, for most UK homes, good double glazing combined with adequate ventilation solves the problem.
Quick Diagnosis Guide
Where’s the condensation? Here’s what to do:
- Inside surface → Improve ventilation and reduce humidity sources
- Outside surface → Nothing to fix — your glazing is working well
- Between panes → Seal has failed — replace the glazing unit
- Throughout house → General ventilation problem — consider whole-house approach
Frequently Asked Questions
Why do I get condensation on my new windows?
New windows are more airtight than old ones. Less background ventilation means humidity builds up. It’s not a fault — you just need to adjust habits. Keep trickle vents open and ventilate briefly each day.
Is condensation damaging my timber frames?
Occasional condensation won’t harm properly finished timber. Persistent heavy condensation pooling on sills can eventually penetrate the finish. Wipe away pooled water regularly and maintain frames with microporous paint.
What humidity level should I aim for?
Keep relative humidity between 40-60%. Below 40% causes dry skin and respiratory issues; above 60% encourages condensation and mould. A cheap hygrometer helps you monitor this.
Will secondary glazing help?
Yes — secondary glazing warms the primary window and reduces humidity transfer. A good solution for listed buildings where replacing original windows isn’t permitted.
Getting Condensation Under Control
Window condensation comes down to moisture meeting cold surfaces. Address the moisture, warm up the glass, or improve ventilation — and the problem improves. Start simple: open trickle vents, use extract fans, avoid drying clothes indoors. If that’s not enough, add a dehumidifier or consider upgrading your glazing.
At Timber Windows Direct, all our windows come with integrated trickle vents and high-performance sealed units. Get in touch to discuss replacement windows for your property.
