Overview
Condensation on windows forms when warm, moist air meets a cooler surface. There are three distinct types – internal, external, and between the panes – each with a different cause and a different solution. Understanding which type you have is the first step to dealing with it effectively.
Why condensation forms on windows
Condensation is water. More specifically, it is water vapour from the air that has cooled enough to turn back into liquid and settle on a surface.
Every home produces moisture throughout the day. Cooking, showering, breathing, drying clothes indoors – a typical Scottish household generates several litres of vapour every single day. When that moist air makes contact with a surface that is cooler than its dew point, condensation appears.
Windows are often the first place it shows up. Glass have a high thermal conductivity compared to walls and ceilings, so it cools quickly. That makes it the most likely surface in any room to trigger condensation first.
This is a new problem. But it does affect Scottish homes more noticeably than many, given the combination of cooler outdoor temperature and well-insulated modern interiors that limit natural air movement.
The three types of window condensation
Not all condensation is the same. The location of the moisture tells you a great deal about what is happening and whether any action is needed.
| Type | Location | Cause | Action required |
| Internal | Inside surface of glass | High indoor humidity, poor ventilation | Yes – address ventilation and moisture sources |
| external | Outside surface of glass | High-performance glazing, cold nights | No – this shows that your double glazing is performing well and keeping that heat inside your home |
| Between panes | Inside the sealed unit | Failed sealed unit seal | Yes – sealed unit requires replacement |
Internal condensation
Internal condensation appears on the room-facing side of the glass. It is most visible first thing in the morning, particularly in bedrooms, kitchens, and bathrooms.
This type of condensation is not caused by faulty windows. It is caused by an imbalance between indoor humidity levels and ventilation. The more moisture in the air, and the less fresh air circulating, the more likely condensation becomes.
Common contributions include:
- Cooking without an extractor fan running
- Dyring laundry indoors, particularly on radiators
- Long shows in poorly ventilated bathrooms
- Overfilled rooms with restricted airflow
- Blocking or covering trickle ventilators in window frame
The solution is a combination of reducing moisture at source and improving ventilation. Trickle ventilators — the small slots fitted as standard to every CR Smith window — are specifically designed to allow a continuous, low-level exchange of air without draughts. Keeping them open, particularly in kitchens and bathrooms, makes a significant difference.
If you notice heavy, persistent internal condensation across multiple rooms, it is worth reviewing the overall ventilation in your home. Scottish Building Regulations (Standard 3.14) require rooms to have adequate ventilation through window openings and trickle ventilation. CR Smith fits a trickle ventilator in every window as standard to help homeowners meet this requirement.
External condensation
External condensation appears on the outside face of the glass, typically in the early morning. It often disappears within an hour or two as the sun warms the glass.
Many homeowners are surprised — and sometimes concerned — to see this on newly installed windows. In fact, external condensation is a sign that the glazing is doing what it is supposed to do and performing well.
High-performance, energy-efficient glass is designed to retain heat inside the property. This means the outer pane stays relatively cool. On clear nights, when the outdoor temperature drops and humidity rises, moisture from the air settles on that cool outer surface — the same way dew forms on grass.
Older, less efficient windows allowed more heat to escape through the glass. The outer pane stayed warmer as a result, so external condensation was less likely. Modern A-rated double glazing keeps that heat inside where it belongs — which is exactly the point.
External condensation requires no action. It is a natural weather effect and will clear on its own. The CR Smith Lorimer range of windows achieve an A energy rating with a U-value of 1.4W/m²K, so customers occasionally notice this effect, particularly in the first weeks after installation.
Condensation between the panes — when to take action
Condensation appearing between the two panes of a double glazed unit — inside the sealed unit itself — is a different matter entirely. It cannot be wiped away. It sits permanently in the cavity, often leaving a milky or hazy appearance on the glass.
This is caused by a failure in the glass unit’s perimeter seal. Once the seal is compromised, warm moist air from outside can enter the cavity. As temperatures fluctuate, that moisture condenses on the inner glass surfaces.
A sealed unit is made up of six components working together: two panes of glass, a warm edge spacer bar, a primary butyl seal, a secondary hot melt sealant, and a desiccant. The desiccant — similar to silica gel — is housed inside the spacer bar and is designed to absorb any residual moisture within the cavity. When the outer seals fail, the desiccant eventually becomes saturated and condensation becomes visible.
Seal failure can be caused by age, physical damage, or manufacturing defects. CR Smith sealed units are tested to BS EN 1279 Parts 2 and 3, which defines performance requirements for insulated glazed units including moisture penetration and gas retention. Units are tested annually by the British Standards Institution and daily within the CR Smith manufacturing plant to check gas fill, seal quality, and moisture resistance.
If you have condensation between the panes of a CR Smith window, contact the team. The sealed unit will need to be replaced. This is a straightforward replacement – the frames themselves are unaffected.
How CR Smith windows are built to manage moisture
The CR Smith Lorimer range is manufactured in Scotland and engineered with moisture management built in at every level.
Each sealed unit is filled with argon gas rather than air. Argon is a denser gas that improves thermal performance and reduces heat loss across the cavity. The units are flooded with argon gas rather than injected, which provides a more consistent fill — CR Smith maintains a minimum 90%, or greater, argon gas content in accordance with BS EN 1279 Part 3.
The warm edge spacer bar is made from insulating glass-reinforced plastic composite. Unlike aluminium spacers used in lower-specification units, warm edge spacers do not conduct heat or cold to the same degree. This reduces the temperature differential at the edge of the glass, which is one of the zones most vulnerable to condensation.
Every unit includes desiccant within the spacer bar. This silica-based material absorbs moisture from within the cavity, preventing condensation between the panes and helping to maintain the clarity and thermal performance of the glazing over its full lifespan.
Trickle ventilators are fitted as standard to every Lorimer window. These allow continuous background ventilation which helps to manage indoor humidity levels without requiring windows to be left open.
Steps you can take to reduce condensation at home
The most effective approach combines good habits with good ventilation. Here are practical steps that make a real difference in Scottish homes.
Keep trickle vents open. This is the single most important step. Trickle ventilators are designed to allow continuous, low-level airflow without draughts. Blocking them — with paint, tape, or by covering them with curtains — restricts ventilation and contributes directly to condensation.
Use extractor fans in kitchens and bathrooms. Cooking and showering produce large volumes of moisture in a short time. Run the extractor during and for at least 15 minutes after these activities. If your bathroom has no fan, opening the window after a shower makes a significant difference.
Avoid drying clothes indoors on radiators. This is one of the most significant contributors to high indoor humidity. A heated airer with a cover, or a tumble dryer vented externally, is a much better option during wet Scottish weather.
Maintain a steady background heat. Rooms that go from cold to warm quickly experience sharp changes in relative humidity, which accelerates condensation. A consistent low background temperature is more effective than heating intermittently to high levels.
Wipe down surfaces after condensation forms. Persistent moisture on window sills can lead to mould growth over time. Wiping the glass and frame with a dry cloth in the morning removes the moisture before it settles.
Consider a dehumidifier in problem rooms. In particularly humid rooms — utility rooms, bathrooms without external ventilation, or basement conversions — a dehumidifier can reduce ambient moisture levels considerably.
Key Takeaways
- Condensation on windows has three distinct types – internal, external, and between the panes – each with a different cause and a different response.
- Internal condensation is caused by high indoor humidity and insufficient ventilation. Keeping trickle vents open and improving airflow are the most effective solutions.
- External condensation is a positive sign of thermally efficient glazing and requires no action.
- Condensation between the panes indicates a failed sealed unit seal, and the glass unit should be replaced.
- CR Smith Lorimer windows are manufactured with warm edge spacer bars, argon gas fill, silica desiccant, and trickle ventilators fitted as standard – all designed to minimise the conditions that lead to condensation problems.
FAQ’s
New, high-performance double glazing is more thermally efficient than older windows. The inner pane stays warmer because less heat is escaping through the glass, which can initially seem to make condensation worse. What is actually happening is that the warm, moist air is condensing on the next coolest surface — outer pane of glass, causing external condensation. The new windows are performing exactly as intended.
Only if the condensation appears in between the panes of the double glazed unit should it be regarded a fault.
- Internal condensation on the room-facing surface of the glass is a ventilation and humidity issue, not a product fault. To help manage this, keep trickle vents open, use an extractor fan in bathrooms and kitchens, maintain a steady background heat and follow the further steps detailed above.
- External condensation on the outside pane of glass is a sign of good thermal performance and that the hot air inside your home is being retained.
- Only condensation between the two panes of the sealed unit indicates a product issue, specifically a failed seal, and the unit will need to be replaced. At CR Smith, this would be done under the terms of our product guarantee (for ten years).
A trickle vent is a small ventilator fitted to the top of a window frame. It allows a continuous, low-level exchange of air between the home and outside without the need to open the window. CR Smith fits trickle vents as standard to every Lorimer window. They should be left open in all habitable rooms to maintain healthy air circulation and reduce the risk of condensation. Closing them — particularly in bedrooms and kitchens — restricts ventilation and increases humidity.
Persistent internal condensation that is not managed can lead to mould growth on window reveals, sills, and surrounding walls. This is caused by sustained damp conditions rather than the condensation itself. Mould can damage decorative finishes and, in serious cases, affect air quality. Condensation between the panes will not cause structural damage to the window frame, but it does impair the thermal performance of the sealed unit and should be addressed. External condensation causes no damage at all.
External condensation typically clears within one to two hours as sunlight warms the outer glass surface. It is most common during spring and autumn, when overnight temperatures drop sharply but daytime temperatures are mild. It is more likely to occur on north-facing windows that receive less direct sunlight. Once the glass reaches ambient outdoor temperature, the condensation evaporates naturally.
