What causes mould on ceilings?

Question

Is the ceiling mould in a rental down to condensation or a bigger problem? We open windows when we can, but mould keeps coming back, so I’m not sure if it’s just everyday living or something like insulation or ventilation being poor.

Nathan · Answer

Condensation (most common cause)
Warm, moist air from showers, cooking, drying clothes, and even breathing rises and hits a cold ceiling surface. The moisture then condenses, creating perfect conditions for mould growth. This is very common in bathrooms, kitchens, and bedrooms.

Poor ventilation
Lack of extractor fans, blocked vents, or not opening windows allows humidity to build up, leading to persistent damp air.

Cold spots / thermal bridging
Poor insulation or cold bridges in ceilings (especially around loft hatches, steel beams, or external walls) cause cold surfaces where condensation forms.

Leaks from above
Roof leaks, damaged flashing, cracked tiles, leaking pipework, or bathroom leaks from the floor above can introduce constant moisture into the ceiling.

High indoor humidity
Drying clothes indoors, aquariums, tumble dryers not vented externally, and overcrowding can all push humidity levels high enough for mould to grow.

Perry · Answer

Poor extraction from fans, blocked air bricks etc can mean theres not enough air movement in the room.

Cold spots.  
Lack of insulation in roofs / lofts / exterior walls.

Moisture ingress 
Roof leaks / plumbing leaks allowing water ingress to building fabric.

Indoor behaviour 
Tumble dryers / cookers / electric fan heaters.  Closing off vents or turning off extraction fans.    Tropical pet tanks etc..    hot showers with no vent / windows open.

Daniel · Answer

Damp and wet seeping in thru the structure of the property.

What causes mould on ceilings?

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