Why do my windows get wet every morning?
Windows are usually the coldest surface in a room, so they are where moisture in the warm indoor air condenses first. Overnight, humidity rises — from breathing, and from a home that is closed up and less ventilated — while surfaces cool. By morning, the air near the glass has been chilled below its dew point and the moisture appears as water on the window. It is basic building physics, and it points to two things you can change: too much moisture in the air, and surfaces that are too cold.
Window condensation is often the first visible warning of a wider moisture and ventilation problem. Left unmanaged, it leads to mould on the frames and reveals, damage to the finishes, and a persistent musty smell. Diagnosing why it happens is the route to stopping it.
Too much moisture, too little ventilation
A typical household adds several litres of moisture to the indoor air every day — cooking, showering, drying clothes and simply breathing. In a well-ventilated home that moisture is carried away; in a home that is sealed up without adequate ventilation, it builds up and condenses on the coldest surfaces. This is why condensation often appears or worsens after new windows or draught-proofing: the home became more airtight, but the moisture had nowhere to go.
Cold surfaces and thermal bridges
The colder the surface, the more readily condensation forms. Single glazing, poorly performing units and the cold reveals around a window are prime spots. If mould appears on the reveal or in the corner beside the window, a thermal bridge is usually chilling that surface. Warming these surfaces reduces condensation as effectively as reducing humidity — and the best solution usually does both.
How Retrofit IQ investigates window condensation
We measure the conditions causing it. Humidity, temperature and surface readings, plus dew-point analysis, reveal how close your air is to condensing and where. Thermal imaging locates the cold surfaces and thermal bridges around the windows. We review your ventilation — trickle vents, extract fans, background ventilation — to find where it falls short. Where relevant we test airtightness. The result is a diagnosis of the true cause, not a guess.
Part of a full Building Performance Investigation
Window condensation is a symptom of the wider relationship between moisture, cold surfaces and ventilation. A window condensation investigation is most valuable as part of a full Building Performance Investigation, which ties these together into one prioritised plan and stops the problem returning elsewhere in the home.
Secondary glazing and window upgrades
Because the glass is usually the coldest surface, improving it reduces condensation — but the right upgrade depends on the situation. Replacing failed or single-glazed units with modern insulated glazing raises the surface temperature so condensation forms less readily. Where replacement is not possible, for instance in a conservation area or with valued original windows, secondary glazing adds an inner layer that keeps the inner surface warmer. Neither is a complete answer on its own, though: unless ventilation and humidity are also addressed, condensation can simply migrate to the next-coldest surface. Our investigation shows which combination your home needs.
Curtains, blinds and trapped cold air
Heavy curtains and blinds can, perhaps counter-intuitively, make window condensation worse. By trapping a pocket of still, cold air against the glass overnight, they let that surface become even colder, so more moisture condenses on it by morning. This is why bedroom windows with thick curtains often stream the most. It does not mean removing your curtains — but it is a clue that the real issue is a cold surface and moist air, and that warming the surface and improving ventilation will do more than any window dressing.
Seasonal patterns and what they tell us
Window condensation that appears in autumn, peaks in the coldest months and eases in spring is the classic signature of internal condensation driven by indoor humidity and cold glass. Recognising this seasonal pattern helps distinguish it from other moisture problems and confirms where to focus. It also explains why the problem can seem to 'fix itself' in summer only to return each winter — the underlying imbalance never went away, the weather simply stopped exposing it. A measured investigation resolves the cause so the pattern does not repeat.
New-builds and flats
Owners of new-builds and flats are often surprised by condensation, assuming a modern home should be free of it. In fact, airtight modern construction can trap moisture readily if ventilation is inadequate or misused, and flats — with limited external walls and sometimes restricted window opening — can be particularly affected. The high airtightness that saves energy makes deliberate ventilation essential. Our investigation is just as relevant to a modern flat as to a period house, because the physics of moisture, cold surfaces and ventilation applies to both.
What you receive
Trickle vents, extract fans and the quick wins
Some window condensation can be eased with simple measures, and a good investigation will tell you where they apply. Opening trickle vents that have been shut, running bathroom and kitchen extract fans long enough to clear the moisture, drying laundry outside where possible, and keeping a gentle background of heat all reduce the moisture load and the sharpness of the temperature swings that cause condensation. These quick wins are worth doing — but where they are not enough, they point to a deeper imbalance between moisture, ventilation and cold surfaces that measurement will uncover.
Why bedroom windows stream first
The bedroom window is usually the first to run with water each morning, and for good reason. Overnight, the occupants add moisture to the air for hours while the room is closed up and the heating is low, so surfaces are at their coldest. The glass, being the coldest surface, reaches the dew point first and the moisture condenses there. If your morning routine includes wiping down bedroom windows, it is a clear sign of overnight humidity meeting cold glass in an under-ventilated room — the exact conditions our investigation is designed to measure and resolve.
Internal condensation versus condensation between panes
It matters where the condensation forms. Water on the room side of the glass is internal condensation, driven by indoor humidity and cold surfaces — the focus of this investigation. Condensation on the outside of the glass, in contrast, is usually a sign of a well-performing, well-insulated window and is harmless. Misting between the panes of a sealed unit indicates a failed unit that needs replacing. Distinguishing these is part of a proper assessment, so you address the right problem and are not sold a fix you do not need.
When window condensation signals a bigger problem
Window condensation is often the earliest visible warning of a whole-home moisture and ventilation imbalance. If it is accompanied by mould on the frames, a musty smell, damp in corners or condensation on the walls themselves, the windows are simply the first place a wider problem is showing. Treating only the windows in that situation misses the point. This is why we look beyond the glass to the humidity, ventilation and cold surfaces across the home, so the underlying cause is addressed before it surfaces elsewhere.
- A clear diagnosis of why your windows condense
- Humidity, temperature and dew-point findings
- Thermal images of cold surfaces and reveals
- A ventilation assessment and where it falls short
- A prioritised plan — moisture, surfaces and ventilation together
A real-world investigation example
A homeowner wiped water off the bedroom windows every winter morning and had started to see mould creeping onto the frames. Humidity logging and dew-point analysis showed overnight humidity in the closed bedroom climbing high while the single-glazed windows dropped to the coldest surface temperature in the room, so the air beside the glass reliably reached its dew point before dawn. Heavy curtains made it worse by trapping a pocket of cold air against the glass. Improving overnight ventilation, managing moisture at source and upgrading the coldest windows brought the morning condensation under control and stopped the frame mould developing.
Common homeowner mistakes
The most common mistake is simply wiping the windows each day and treating it as normal, while the underlying moisture and cold-surface imbalance quietly feeds frame mould. Another is buying a dehumidifier and considering the problem solved, when it only masks the symptom. Homeowners often assume new double glazing alone will fix it, but without addressing ventilation the condensation simply migrates to the next-coldest surface. And confusing harmless external condensation, or a failed sealed unit misting between the panes, with internal condensation leads people to replace windows that were not the problem.