Mould in a house is almost always a moisture problem: mould spores are already present, but they only grow when there is persistent damp and poor ventilation.

Main causes of mould in a house

  • Condensation from everyday living
    Activities like cooking, showering, drying clothes indoors and even breathing release a lot of water vapour into the air.

When warm, moist air hits cold surfaces (windows, external walls, corners), it cools and turns into water droplets, creating damp patches where mould can grow.

  • Poor ventilation and air circulation
    Closed windows, blocked vents, never using extractor fans, or permanently shut rooms trap humid air inside.

This keeps relative humidity high (often above 75–80%), which is ideal for mould growth.

  • Cold surfaces and temperature imbalance
    Poor insulation, cold external walls, thermal bridges, and under‑heated rooms make surfaces colder than the indoor air.

The bigger the temperature difference, the more condensation forms on those cold spots, especially in corners, around windows and on outside walls.

  • Penetrating damp (water coming through the structure)
    Leaking roofs, damaged brickwork, cracked render, faulty window seals, and clogged or broken gutters let rainwater seep into walls and ceilings.

Those damp areas then stay wet long enough for mould to colonise, often seen as stains, peeling paint or blistering plaster.

  • Rising damp from the ground
    In older or poorly built houses, moisture from the ground can travel upwards through walls if there is no effective damp‑proof course or it has failed.

This typically causes damp and mould along the lower part of ground‑floor or basement walls.

  • Plumbing leaks and internal water sources
    Dripping pipes, overflowing baths or sinks, leaking radiators, or faulty appliances (like washing machines) can soak floors, walls, and hidden cavities.

Even slow, hidden leaks can keep materials like plasterboard and timber damp enough for mould to thrive behind furniture or inside walls.

  • Building defects and poor maintenance
    Inadequate insulation, thermal bridges, poorly fitted windows, and unsealed joints all create cold, damp‑prone areas.

Lack of maintenance—like not cleaning gutters, ignoring damaged roof tiles, or not repairing cracks—allows ongoing moisture ingress and chronic mould issues.

  • Moisture‑absorbing materials and dust
    Materials like drywall, wood, wallpaper, and porous plasters absorb moisture easily and act as a food source for mould.

Dust, skin flakes and other organic debris on surfaces further feed mould once humidity and dampness are high.

Typical “quick scoop” signs mould is forming

  • Musty or earthy smell in certain rooms.
  • Dark spots or fuzzy patches on walls, ceilings, window reveals, or behind furniture.
  • Peeling or bubbling paint or wallpaper, especially on external or lower walls.
  • Persistent condensation on windows, especially in the mornings.

Why it’s such a talking point right now

In recent years, especially with rising energy costs, many people heat fewer rooms and keep windows tightly shut to “save heat,” which increases condensation and mould problems.

Public health agencies now regularly issue guidance on damp and mould because prolonged exposure is linked to breathing issues, allergies, and worsened asthma.

Simple example: a winter day

  • You cook dinner, shower, and dry clothes on radiators with windows closed and no fans on.
  • Warm, moist air fills the house, then hits cold outside walls and window glass.
  • Water droplets form there, keeping the surfaces damp. After a few days or weeks of this cycle, mould starts to appear in the colder corners behind wardrobes and around windows.

Mini table: moisture sources vs mould risk

[7][9] [7][3] [10][1][3] [1][3] [9][3]
Moisture source How it leads to mould
Everyday humidity (cooking, showers, drying clothes) Raises indoor humidity; condenses on cold surfaces, creating damp patches.
Poor ventilation Traps moist air; keeps humidity high so spores can grow easily.
Leaks and penetrating damp Continuously wets building materials; mould feeds on damp porous surfaces.
Rising damp Ground moisture climbs into walls; lower walls stay damp and grow mould.
Poor insulation / cold bridges Makes some surfaces very cold, encouraging condensation in specific spots.
**TL;DR:** Mould in a house is caused by excess moisture plus poor ventilation and cold surfaces, whether that moisture comes from daily life, leaks, or damp rising through the structure.

Information gathered from public forums or data available on the internet and portrayed here.