The exceptions matter though. Condensation forming inside the structure rather than on it, or mould appearing in one cold corner, points to a gap in the insulation or a missing vapour control layer, and that is worth chasing down.
Construction moisture comes first
A modest extension can contain several thousand litres of water in its screed, blockwork, plaster and timber. That water has to leave, and it leaves into the room. For the first heating season you will see more condensation on the glass than you will ever see again.
The way through it is gentle background heat kept steady rather than blasted on and off, trickle vents left open, and a dehumidifier if you need to speed things up. Drying too aggressively cracks the plaster and shrinks the joinery, so steady wins.
When it is a design or build problem
Look at where it forms. Condensation spread evenly over the glass is normal. Condensation and mould concentrated in one corner, along the line of a steel beam, or at the junction between the new wall and the old one is a cold bridge, which means insulation is thin, missing or interrupted at that point.
Mould on the ceiling of a flat roofed extension, or damp insulation found when a light fitting is removed, suggests interstitial condensation: warm moist air getting into the roof build up and condensing on the cold side. On a cold deck roof that is a ventilation failure. On a warm deck roof it means the vapour control layer has been punctured or was never continuous.
- Even misting on glass: normal, especially in the first winter
- One cold corner with black spot: cold bridge
- Damp inside the roof build up: vapour control or ventilation failure
- Streaming on the inside of a rooflight kerb: uninsulated kerb
Ventilation, heating and moisture
Three levers control condensation. Reduce moisture at source by using extract fans that actually run for long enough and vent outside rather than into a void. Add ventilation so humid air leaves, which in a sealed modern extension usually means trickle vents and a decent bathroom or kitchen extractor. Raise surface temperatures by keeping background heat on rather than heating in short bursts.
Bifold and large sliding doors deserve a note. Big areas of glass are always the coldest surface in the room, so they collect condensation first even when everything else is right. Thermally broken frames and warm edge spacer bars make a visible difference here.
Fixing a genuine cold bridge
Where a thermal image or the mould pattern confirms a cold bridge, the honest repair is to insulate the area properly. That can mean opening up a reveal, insulating around a steel that was boxed without any, or dry lining a section of the old wall where the new work meets it.
It is disruptive but it is finite, and it removes the problem rather than managing it. Anti mould paint over a cold bridge just moves the mould six inches to the side.
Common questions
How long should condensation last in a new extension?
Expect a noticeable first winter and a much drier second one. If year two is as wet as year one, something structural or behavioural is keeping moisture in the room.
Are trickle vents really necessary in a new extension?
In a well sealed room, yes. Without a route for humid air to leave, moisture builds until it condenses on the coldest surface, which is usually the glass or a cold corner.
Why is there mould only in one corner?
That corner is colder than the rest of the room. It is nearly always a break in the insulation, a poorly insulated lintel or steel, or a junction between new and existing walls.
Does a dehumidifier fix the problem?
It manages symptoms and genuinely helps during the drying out year. It is not a cure for a cold bridge or for a roof with a ventilation fault.
Can you check an extension you did not build?
Yes. We are often asked to look at other people's work, and the visit and written opinion are free.