What causes a musty smell in a room?
A musty smell is produced by mould and mildew growing somewhere, releasing compounds you can smell before you can see any growth. Air freshener masks it; finding the moisture source is the only real fix.
The smell strength gives clues. If it is strongest in one corner or near an external wall, look for cold-bridging or penetrating damp. If it is strongest near the floor, consider the subfloor. If it appears seasonally, it is likely condensation-driven rather than a leak.
The common sources, roughly in order of frequency:
- Condensation on cold surfaces, particularly behind furniture placed tight against an external wall, where air cannot circulate. Moving furniture a few centimetres out is a genuine fix.
- Slow leaks, from plumbing, a roof, or failed sealant around a bath or shower. These often show no visible water because the material absorbs it.
- Poor subfloor ventilation in properties with suspended timber floors. Airbricks blocked by soil, plants or a later extension are a very common cause.
- Rising damp, though this is diagnosed far more often than it occurs, and most cases attributed to it turn out to be condensation or a bridged damp-proof course.
- Contents rather than the building: damp cardboard, stored textiles, or a rug that was wetted and never fully dried.
A cheap humidity meter is worth having. Sustained readings above about sixty percent indicate a moisture problem rather than a smell problem.
Treating visible mould without addressing moisture guarantees its return. Clean it, then find the water. If the source is not apparent, a moisture meter or a survey is more productive than guessing.