How to Stop Internal Window Condensation (Causes & Fixes)

Waking up to windows streaming with water inside your bedroom is frustrating, but it is rarely a fault with the glass itself. Learn the lifestyle causes of internal condensation and exactly how to stop it.

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If the water droplets sit on the glass inside your room where you can touch them, it is a ventilation issue, not a broken window.

1. Why Are My Windows Wet on the Inside?

If you can physically wipe the water off the glass from inside your bedroom or kitchen, you are dealing with internal condensation. This is rarely a fault with the actual window itself. Instead, it is a clear visible indicator that the humidity levels inside your home are too high.

The Science of the Dew Point:
To understand why this happens, imagine pouring an ice-cold drink on a warm summer day. The outside of the glass quickly becomes wet with droplets.

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When warm, moist air hits a cold surface (like a glass or a window pane), it releases water droplets.

Air naturally contains invisible water vapour. A typical family of four produces around 14 litres of water vapour a day simply by breathing, cooking, and showering. Warm air from your central heating can hold onto a lot of this moisture. However, when that warm, humid air hits the coldest surface in the room (which is almost always your window glass), it rapidly cools down. As it cools, it can no longer hold the moisture, turning it into physical water droplets on your window frame and glass.

2. The “New Window” Paradox

A very common complaint we hear is: “My old, single-glazed timber windows never had this much condensation. Since I installed these expensive new double-glazed windows, my house is soaking wet!”

This is entirely true, but for a very specific reason. Older windows were usually incredibly draughty. While you slept, the wind simply blew through the gaps in the frame, carrying the moist air away with it. Unfortunately, those draughts also carried away all your expensive central heating.

Modern uPVC and aluminium windows are designed to be completely airtight to comply with UK Building Regulations. They are incredibly good at sealing the house. Because they seal the house so effectively, they trap the heat in—but they also trap the moisture in. If you do not actively let that moisture out, it has nowhere to go except onto your glass.

3. The Health Risk: Black Mould & Cold Walls

Leaving internal condensation unchecked is not just an aesthetic annoyance; it can be a genuine health hazard. Constant moisture sitting on your window sealant, uPVC frames, or adjacent walls creates the perfect breeding ground for toxic black mould (Stachybotrys chartarum).

The Danger of Cold Walls:
It is crucial to understand that it’s not just your windows at risk. If you try to save money by letting your house get too cold during the winter, the actual fabric of the building (your solid or cavity walls) drops in temperature. When the warm, moist air inside your home inevitably touches these freezing external walls, the condensation forms directly on the plaster or wallpaper instead. Over time, this constant dampness inevitably leads to severe black mould growth spreading across your walls and ceilings.

How to Safely Clean Window Mould:
If you already have black spots growing on your window frames or silicone sealant, remove it immediately using this safe method:

1. Protect yourself: Wear rubber gloves and a face mask. Never scrub dry mould, as this releases dangerous spores into the air you breathe.
2. The Solution: Mix 1 part household bleach to 4 parts warm water. (Alternatively, use a 50/50 mix of white vinegar and water).
3. Apply: Gently spray or dab the solution directly onto the black mould spots. Let it sit undisturbed for 15-20 minutes.
4. Wipe & Dispose: Clean the area with a damp cloth, dry it thoroughly with paper towels, and immediately throw the used towels in an outside bin.

4. KJM’s 5-Step Ventilation Checklist to Stop It

To permanently stop condensation on the inside of your windows, you must change the atmospheric conditions inside your home. Follow this daily checklist:

• 1. Leave Trickle Vents Open: If your new windows have trickle vents (the small sliding vents at the top of the frame), keep them open 24/7. They are mandated by Building Regulations specifically to provide secure, low-level background ventilation without causing a massive draught.
• 2. The “Short, Sharp Burst” (Cross-Ventilation): When you wake up, open windows on opposite sides of your property for exactly 10 minutes. This creates a powerful ‘through-draft’ that completely flushes out the 8 hours’ worth of moist, stale air you exhaled overnight. This short, sharp burst of cross-ventilation replaces the damp air with fresh, dry air very quickly, doing so without giving your walls and furniture time to cool down.
• 3. Stop Radiator Drying: Drying wet laundry directly on hot radiators is the number one cause of winter condensation. It instantly pumps litres of water directly into your rooms. Always dry clothes outside, in a tumble dryer, or in a closed room with the window slightly open.
• 4. Use Your Extractor Fans: Always run your kitchen hood when boiling pasta, and run your bathroom extractor fan for at least 15 minutes after you finish showering. Keep the bathroom door closed so the damp air doesn’t escape into your hallways.
• 5. Invest in a Dehumidifier: If you live in a highly airtight new-build property, purchasing a plug-in electric dehumidifier or installing a loft-based PIV (Positive Input Ventilation) system will physically suck the excess moisture out of the air.

5. Frequently Asked Questions