Growth beneath a floor covering
A dark, absorbent, slow-drying layer with no airflow. The pile can feel dry to the hand while the padding under it has not dried for weeks.
In detail
Carpet has layers, and the ones that matter are the ones nobody looks at: the backings, the underlay beneath, and the subfloor below that. All three are absorbent, none is ventilated, and a professional body states the consequence — after small water damage the padding stays wet long after the surface fibres feel dry, and that wet dark environment is where growth proceeds undetected. So a musty smell in a room is usually not coming from the carpet surface, and cleaning that surface achieves nothing lasting. The same body notes the smell may be shared with other absorbent things — furniture, skirting boards, plaster, window coverings, bedding, the ventilation system — which is why chasing it around one surface fails. There is no domestic treatment to recommend: the professional guidance is explicit that because of the risks of breathing or ingesting mould this belongs to a specialist trained in microbial remediation. What can be said is diagnostic — if a smell returns with the weather, look UNDER the covering rather than at it, and treat a persistently damp underlay as a replacement question rather than a cleaning one.
Sources for this page
Sources
Consumer Floor and Furnishings Care Information: carpet topics
Institute of Inspection, Cleaning and Restoration Certification (IICRC) · Trade or professional body
Relied on for: That musty odour often originates in the carpet backing or subfloor which surface cleaning cannot reach, that absorbent padding stays wet while surface fibres feel dry, that this wet dark environment allows undetected growth, that the odour returns when the area becomes damp, and that a microbial remediation specialist should be consulted. (states this directly)