Why condensation Occurs in double-glazed windows
Condensation is simply water vapour that turns back into a liquid when warm, moist air meets a surface that’s cold enough to make the air drop below its dew point. Air always contains some level of moisture, so the warmer it is, the more moisture it can hold. When that air brushes against a cooler pane of glass, it loses that heat, causing the mist to turn into tiny droplets that rest on the surface of the windowsill.
Double glazing is made to reduce this effect since the two panes are separated by a sealed cavity that's filled with dry air or an inert gas like argon. Since the cavity reduces the heat flow, the inner pane stays closer to the room's temperature while the outer pane stays in line with the outdoor conditions. A warmer inner pane means that the indoor air is less likely to fall below its dew point at the glass, so condensation is less likely to occur with double glazing as opposed to single glazing.
However, condensation can appear in three distinct places on double-glazed panels; these places are the room-side panel, the outside panel and between the panes. Condensation that's on the room-side panel is usually a sign that there's high indoor humidity from activities like cooking, showering or limited ventilation. Condensation that's on the outside panel is a sign that heat has radiated and cooled down on the pane below the outdoor air temperature. If there's condensation between the panes, it's usually a sign that there's a failed edge seal and moist air has infiltrated the cavity and saturated the spacer’s desiccant, which allows the vapour to condense inside.
Types of Condensation
Broadly speaking, there are three types of condensation: internal, external, and condensation that occurs between the panes. Each type has different causes and requires different solutions.
Internal condensation occurs on the inside surface of the window, facing the room. It forms when warm, moisture-heavy air comes into contact with the colder glass, causing it to drop below the dew point and release water droplets. This type of condensation is extremely common during winter since most rooms are heated but are poorly ventilated. To prevent this, it's crucial to make sure your home has good airflow. By running extractor fans in kitchens and bathrooms, keeping trickle vents open, and using dehumidifiers, you can effectively lower your indoor humidity levels.
External condensation appears on the outer pane of the glass either early in the morning or late at night. This happens when the outer surface cools down due to clear skies and still air, causing the indoor air to drop below the outdoor dew point. While it can look concerning, this type is actually a sign that the double glazing is highly efficient since the heat from inside isn't escaping outside. External condensation isn't typically harmful, though it can be minimised by improving your air circulation outside the window.
Condensation between the panes is the most serious type of condensation since it's a sign that there's an issue with the window’s seal. If the airtight seal breaks down, moisture can infiltrate the insulating cavity, and once it's inside, the desiccant material that normally absorbs the moisture becomes saturated, allowing misting or fogging to appear.
Unfortunately, this type of condensation cannot be simply wiped away or treated with ventilation. The only real preventative measure is investing in high-quality double-glazed units with durable seals. If this type of condensation does occur, the affected sealed unit will need to be replaced.
Short-Term Fixes
Condensation in double-glazed windows forms due to excess indoor moisture meeting the cold glass. As a short-term fix, you should wipe down the wet panes with a microfibre cloth or a window vac every morning, especially when it's cold. It's vital not to let the water pool on the windowsill since it can cause mould to grow.
After wiping the panes, you should give the room a 5 to 10 minute “purge ventilate” by opening the windows to create a cross-breeze. You can run extractor fans during and after cooking or showering for 15 to 20 minutes, and keep the bathroom doors closed while bathing to reduce the amount of moisture that's in the air. A portable dehumidifier that's set to 50-55% relative humidity (RH) can make a huge difference in poorly ventilated or moisture-prone areas.
For preventative maintenance, you can clean and unblock the window frame drainage/weep holes so that any moisture can escape. You should also inspect the silicone and internal seals for gaps and re-seal any cracks to prevent cold draughts and reduce the risk of moisture seeping into the inner pane. The gaskets and weather-stripping should also be checked since any flattened or perished rubber should be replaced in order to keep the sash closing properly.
Long-Term Solutions
One long-term solution to stop condensation forming is to upgrade your glazing package. Modern A-rated sealed units with low-emissivity coatings, such as argon, keep the inner panes closer to your natural room temperature and warm-edge spacers can lift the edge-of-glass temperature and reduce the risk of mist forming around the perimeter of the window frame. If your double-glazing is old or if there's consistent fogging between the panes, it's best to replace the sealed unit or consider installing triple glazing for more protection.
Beyond trickle vents, long-term humidity appliances, such as MEV (centralised extract), continuously pull air out from wet rooms, whereas dehumidifiers remove excess moisture from the existing air. MVHR (heat-recovery ventilation) supplies fresh, filtered air to rooms while reclaiming heat, which is ideal for small homes with limited space. PIV (positive-input ventilation) introduces filtered air into rooms to gently lower the humidity throughout the entire house.
To reduce the rate of moisture permanently, you should duct tumble dryers outside or use a heat-pump dryer, fit tight-sealing lids and use a vented cooker hood. In larger homes, you should consider using a plumbed dehumidifier to tackle different problem zones around the house at once.
It's crucial to keep your indoor humidity level at around 40-60%, and check this level with hygrometers, and annually inspect your seals, gaskets, drainage slots and trickle vents. These steps can reduce the risk of condensation forming and keep your windows warmer, clearer and longer-lasting.
We provide fast, reliable double-glazing repair services to restore clarity, warmth and security to your property. We can fix misted units, damaged seals, panes and more.
