Why has condensation suddenly got worse in my house?

What this usually means

Condensation is the visible result of a balance between three things: how much moisture is in the indoor air, how cold the surfaces are, and how effectively ventilation removes the moist air. When that balance is stable, the condensation you get is roughly constant; when it suddenly worsens, one of the three has shifted. The instinct is to focus on the wet windows or walls, but those are just where the imbalance shows. The productive approach is to ask what is different now compared with before it worsened, because a sudden change almost always has a specific, traceable trigger rather than appearing spontaneously.

The most common triggers fall into the three categories. Surfaces get colder when the weather turns — a cold snap drops surface temperatures below the dew point that were previously just above it — or when heating is reduced to save money, so rooms and their surfaces run cooler. Moisture rises when the household generates more of it: more people at home, drying washing indoors, a new tumble dryer vented incorrectly, more cooking, or even a new fish tank or houseplants. Ventilation falls when an extractor fan fails or is disconnected, trickle vents are closed, a chimney is capped, or the house is draught-proofed, has new windows fitted, or is otherwise sealed up — all of which reduce the background air change that used to carry moisture away. Frequently a sudden worsening combines two of these, such as colder weather plus less heating, or new windows plus unchanged moisture.

Recent improvements are a particularly common and counter-intuitive trigger. Draught-proofing, new windows or insulation make a home more airtight, which is good for warmth and bills but reduces the accidental ventilation that previously removed moisture — so unless ventilation is deliberately provided, the same daily moisture now has nowhere to go and condenses. This is the 'seal tight, ventilate right' principle: tightening the envelope without addressing ventilation predictably increases condensation. Identifying which change tipped the balance — colder surfaces, more moisture, or less ventilation, or a recent retrofit — is what allows the right response, whether that is restoring ventilation, removing a moisture source, or warming cold surfaces, rather than wiping the windows each morning.

Common causes

Signs and symptoms

Condensation onset is governed by the relationship between the indoor air's dew point and the temperatures of the surfaces it contacts. The dew point rises with the air's moisture content, so increasing the indoor vapour load — through additional occupancy, unflued moisture sources or drying washing — raises the dew point towards the prevailing surface temperatures. Independently, surface temperatures fall with colder external conditions and with reduced heating, lowering them towards the dew point. Condensation appears or intensifies whenever a surface temperature crosses below the dew point, so any change that raises the dew point or lowers the surface temperature can trigger a sudden worsening; commonly two changes act together, such as a cold snap coinciding with economised heating.

Ventilation sets the steady-state moisture content by exchanging humid indoor air for drier outdoor air, so a reduction in ventilation raises the indoor vapour load for the same generation rate and lifts the dew point. Mechanical extract failure, closed trickle vents and capped flues each reduce this exchange directly. Less obviously, fabric airtightness improvements reduce the infiltration that previously provided unintended background ventilation: draught-proofing, replacement windows and air-sealing all lower the air-change rate, so a home that was incidentally ventilated through its leakage becomes under-ventilated unless purpose-provided ventilation compensates. The result is a higher indoor humidity and more condensation immediately after the works, despite the warmer, less draughty interior — the well-known consequence of tightening an envelope without addressing ventilation.

Diagnosing a sudden worsening is therefore a matter of identifying which term in the balance changed and when. Logging the indoor temperature and relative humidity reveals whether the vapour load has risen and whether surfaces are reaching the dew point; correlating the onset with weather, heating patterns, household changes and any recent works isolates the trigger. The remedy follows directly: restore or provide ventilation where air change has fallen, remove or correctly vent a new moisture source, or raise surface temperatures through heating and, where surfaces are persistently cold, insulation. Treating the condensing surface — wiping windows, repainting — addresses none of these and the condensation persists, which is why locating the change that tipped the balance is the efficient route to resolving it.

How to tackle condensation that has suddenly worsened

Work out what changed — colder surfaces, more moisture or less ventilation, including any recent retrofit — then restore the balance rather than wiping the symptom.

1. 01

   Identify what changed

   Pin down what altered around the time the condensation worsened — weather, routine, a fan, or works.

2. 02

   Check the surface temperatures

   Establish whether colder weather or reduced heating has dropped surfaces below the dew point.

3. 03

   Find any new moisture source

   Look for added occupants, drying washing, cooking or a wrongly vented appliance raising humidity.

4. 04

   Restore lost ventilation

   Repair extractors, open trickle vents, and provide ventilation if the house has been sealed up.

5. 05

   Warm persistently cold surfaces

   Where surfaces stay cold, heating and insulation lift them above the dew point.

6. 06

   Rebalance moisture and ventilation

   Adjust the balance of warmth, moisture and ventilation so condensation no longer forms.

How to prevent it coming back

• Provide ventilation whenever you draught-proof, reseal or replace windows.
• Keep extractors working and trickle vents open.
• Vent tumble dryers correctly and avoid drying washing in unventilated rooms.
• Keep surfaces warm enough in cold weather to stay above the dew point.