Timber - PVC - Aluminium - Windows, Doors & Conservatories in Hampshire
The Ultimate Guide to Double Glazing Condensation: Inside, Outside & Between the Panes
The Ultimate Guide to Double Glazing Condensation: Inside, Outside & Between the Panes
It remains one of the most common questions we receive at our Andover showroom: “I bought new windows to stop condensation, so why do I still have it?” Discover the practical truth behind wet windows.
Why are your double-glazed windows covered in condensation? Before spending money on repairs, it helps to identify exactly where the moisture is sitting. If the water forms inside the room on the glass you can touch, this is typically a ventilation issue caused by trapped humidity, rather than a window fault. If the condensation appears on the outside glass facing the garden, it is actually a good sign that your windows are highly energy-efficient. However, if the fog or mist is trapped directly between the two panes of glass, the perimeter seal has failed. In this scenario, you will need to replace the sealed glass unit.
🏠 1. Inside the Room
Status: Not a window fault.
Cause: High indoor humidity.
Fix: Improve room ventilation.
🌳 2. Outside the House
Status: Good news!
Cause: Excellent thermal insulation.
Fix: None required.
🌫️ 3. Between Panes
Status: Unit failure.
Cause: Broken perimeter seal.
Fix: Replace the glass unit.
Page Contents
Condensation naturally occurs when warm, moist air meets a cold surface (known as the Dew Point). Because your windows often represent the coldest surface in a room, they act as a clear indicator for the overall humidity levels trapped inside your home.
1. Interactive Tool: The Condensation Diagnoser
Unsure whether you need a simple ventilation fix or a professional glazing repair? Tell us exactly where the water is forming below, and our logic engine will help diagnose the problem.
2. Internal Condensation (Lifestyle & Health)
If the physical water droplets sit directly on the side of the glass you can touch from the inside, this is generally not a window fault. Instead, it indicates a ventilation issue within the property.
The Science: Dew Point
Air naturally contains invisible water vapour, and warm air holds more water than cold air. When warm air trapped in your home (heated by radiators) meets the cold glass surface, it cools down. As it cools, it loses the ability to hold onto that airborne moisture, releasing it as liquid water droplets. This precise moment represents the Dew Point.
A typical family of four produces around 14 litres of water vapour a day simply by breathing, cooking, and showering. If that volume of water has nowhere to escape, it inevitably settles on the glass.
⚠️ The Health Risk: Black Mould
Leaving windows persistently wet can cause issues. Constant moisture sitting on frames can lead to black mould (Stachybotrys chartarum). According to the NHS, inhaling or touching these mould spores can cause:
- Respiratory issues (wheezing and coughing).
- Asthma attacks.
- Allergic reactions (sneezing and red eyes).
✅ KJM’s Ventilation Checklist
- Use Trickle Vents: Keep them open 24/7. They provide secure, continuous background ventilation without causing significant draughts.
- The “Morning Blast”: Open all bedroom windows for ten minutes every morning to help clear stale night air.
- Stop Radiator Drying: Drying wet clothes directly on hot radiators releases a large amount of moisture instantly into your rooms.
- Extractors: Always use extractor fans while showering or boiling pans in the kitchen.
3. How to Safely Clean Black Mould
If you already have mould growing on your window sealant or uPVC frames, you should remove it carefully to avoid spreading the spores.
🧽 The Safe Removal Recipe (Step-by-Step)
Safety First: Always wear rubber gloves, safety goggles, and a mask. Avoid scrubbing dry mould, as this releases spores into the air.
- The Solution: Create a mix of 1 part household bleach to 4 parts warm water. Alternatively, use a 50/50 mix of white vinegar and water (effective for porous surfaces).
- Apply: Gently spray or dab the solution directly onto the mould.
- Wait: Let it sit undisturbed for 15-20 minutes.
- Wipe: Clean the area with a damp cloth, then dry thoroughly with a clean, absorbent cloth.
- Dispose: Throw away all used cloths immediately into an outside bin.
4. The “New Window Paradox” Explained
Customers sometimes ask: “My old timber windows didn’t have this problem!”
This is often true, but for a specific reason. Older timber windows were generally quite draughty. The wind moving through the gaps carried the moist air away. While it helped with the condensation, it also carried away your central heating.
The Modern Reality: Modern uPVC and aluminium windows are designed to be airtight to comply with UK Building Regulations (Part L). Because they seal the house so effectively, they trap the heat and the moisture. This is exactly why active ventilation, such as using trickle vents, is so important.
5. External Condensation (Excellent Efficiency)
If you wake up on a crisp Autumn morning and discover mist on the outside of your windows (facing into the garden), there is no need to worry.
🏆 It Represents a Badge of Honour!
This phenomenon generally only happens on highly energy-efficient windows (utilising Low-E glass), typically during the transitional seasons of Spring and Autumn.
It proves that your inner pane of glass is keeping the heat securely inside your house, meaning the outer pane remains cold. Because the outer glass is cold, morning dew naturally forms upon it—just like it does on the grass or your car windscreen.
6. Failed Units (Between the Panes)
If you see water droplets, persistent mist, or fog trapped between the two sheets of glass (where you cannot wipe it off from either side), the sealed unit has failed.
This means the black rubber perimeter seal has perished, allowing moist external air to enter the cavity while the insulating gas (Argon) escapes. This frequently happens due to age or thermal stress over time.
❌ Can a failed unit be repaired?
Generally, no. We cannot clean the sealed inside of a glass unit. Some companies may offer to drill tiny holes to vent the moisture, but this destroys the thermal efficiency. The only secure fix is replacement.
The Good News: You rarely need an entire new window frame. KJM can usually remove the failed glass unit and fit a new, energy-efficient pane directly into your original frames.
7. Frequently Asked Questions
Yes. Utilising a dehumidifier or installing a permanent PIV (Positive Input Ventilation) system in the loft is an effective way to help cure persistent condensation in modern, airtight homes.
Condensation trapped between the panes (representing seal failure) is covered as it constitutes a product failure. Conversely, surface condensation accumulating inside the room is not covered, as it represents a lifestyle and ventilation issue rather than a broken window.
Overnight, the external temperature drops, cooling your window glass. At the same time, your family is asleep inside the house, breathing out warm, moist air for around eight hours with the bedroom doors closed. This combination creates the perfect atmospheric conditions for morning condensation to form.
📚 Explore Our Performance Glazing Hub
Now you understand the causes of condensation, explore the specific upgrade and repair options relevant to your home:
Need to Replace a Failed, Misted Window?
If you have a blown, foggy window unit that is obscuring your view, we can help. KJM Group manages professional glass replacement services across Andover and Hampshire.
Contact KJM for a Free Repair Quote
Mark Pearce is a highly experienced specialist in the double glazing industry, having built his knowledge from the ground up through hands-on experience in installation, manufacturing, sales, and business management. Beyond his role at KJM, Mark is a keen blogger, writer, and traveller, often sharing insights on the latest innovations in the home improvement industry.
- Certified Fire Doors: The Fire Stop Collection - 15 January 2026
- Premium Hardware for Profile 22 Doors: Ultion Sweet & Fab&Fix - 15 January 2026
- 2026 Design Trends: The 4 Window & Door Styles Defining the Year - 19 December 2025
