Condensation forms when moist indoor air meets a surface cold enough for the water vapour to condense. Traditional single glazing can therefore show condensation during cold weather, but the window itself is not always the underlying cause.
Start with indoor moisture
Cooking, showers, drying clothes and normal occupation all add moisture to indoor air. If ventilation is limited, relative humidity rises and condensation becomes more likely. Bedrooms can be particularly affected overnight. Regular ventilation and effective extract fans are important before assuming the window needs replacement.
Why does it appear on the glass?
Single glazing is usually one of the coldest internal surfaces in a room. Water therefore becomes visible there first. Condensation between panes of a sealed double-glazed unit is different and can indicate that the unit has failed.
Will draught proofing make condensation worse?
Draught proofing reduces uncontrolled air leakage and improves comfort, but a building still needs planned ventilation. After reducing draughts, occupants may need to use trickle ventilation, extract systems or regular purge ventilation more deliberately. Our draught proofing, secondary glazing or double glazing guide compares the options.
Can glazing upgrades help?
Secondary glazing or suitable double glazing can raise the temperature of the internal glazed surface and may reduce condensation on the room side. They do not remove moisture from the building, so humidity and ventilation still matter. See secondary glazing and slimline double glazing.
Check the timber as well
Persistent water running onto meeting rails or sills can damage paint coatings and eventually contribute to decay. Keep surfaces dry where practical and repair failed paint, putty and external weathering defects. If the timber has softened, our rotten timber repair page explains the repair approach.
If condensation is accompanied by sticking sashes, decay or failed glazing, contact us for a window survey.