Why is there a musty smell coming from under my floorboards?

A suspended timber floor is kept dry by air moving through the void beneath it. A musty smell means that air has stopped moving, and the fix is nearly always outside the house rather than inside.

Difficulty: beginner
Time: 1 hr 30 min
Read: 4 min
Safety: caution

Short answer

Go outside and find every airbrick at low level. Count them, clear them, and remove whatever is blocking them — raised soil, a path, a border, decking, a bin store, render, or an extension built across them. Stagnant void air is what smells, and restoring cross-ventilation is what stops it.

A suspended timber ground floor has no damp proof membrane under it. It is designed to be dried by moving air: airbricks on opposite external walls let air enter one side of the void and leave the other, carrying moisture out with it. That system is quietly disabled by ordinary garden work — a path laid a bit higher, a border built up, decking put against the house — and the first thing you notice is the smell.

Safety

A persistently damp underfloor void is the condition timber rot needs, so the smell is an early warning rather than a nuisance. Do not seal the smell in — blocking airbricks, sealing every gap in the boards or laying an impermeable covering over a damp void makes the underlying problem worse while removing the symptom that told you about it.

What you'll need

a torch
a stiff brush and a screwdriver for clearing vents
a hygrometer (optional)
a mirror or an inspection camera (optional)

Anatomy of a suspended timber ground floor

Step by step

Walk the outside of the house at ground level.Airbricks are small vents low in the wall, usually below the level of the internal floor. On a typical house there are several, on at least two opposite walls. Mark each one you find on a rough plan.
Check each one is genuinely clear.Look for soil banked against it, a raised path or patio covering it, a border or shrub in front, render or paint filling the holes, an old sock of insulation pushed in, or a plastic vent cover fitted the wrong way round. Clear each one with a screwdriver and a brush.
Check the walls that should have them but do not.Extensions, conservatories, porches and garden rooms built against the house very often seal off one whole side of the void, killing the cross-flow even though the remaining airbricks look fine. So do internal walls built off the oversite without a gap left through them.
Look into the void.Lift a board or use an inspection camera through a lifted airbrick. You are looking for standing water, wet oversite, white fungal growth, insulation that has fallen and is lying on damp ground, and builders' rubble heaped so high it blocks the airflow path.
Remove what is in the way inside.Rubble, old bags of insulation, discarded material and packed floor insulation touching the ground all obstruct the flow. Insulation between joists is fine and desirable; insulation filling the void to the ground is not.
Fix the outside ground level if that is the cause.Ground and paving must sit clear of both the airbricks and the damp proof course. Digging a path back down, or forming a small drained channel against the wall, restores the ventilation and the damp proof course at the same time.
Add ventilation if there genuinely is not enough.Where a wall has been extended across, or the void is subdivided, extra airbricks or telescopic vents through a new wall are the answer. Cutting a new airbrick into a solid wall is a mortar and masonry job rather than a specialist one, but it must go below the internal floor level to do anything.
Then let it dry and re-check the smell.A void that has been damp for years does not clear in a week. Give it a season with the ventilation restored before deciding the smell has an additional cause.

Common mistakes

Sealing the floorboards to stop the smell coming up — The smell is the symptom. Sealing it in leaves a damp void doing damage you can no longer detect, and the timber pays for it.
Blocking airbricks in winter to stop draughts — This is the single most common cause of the problem. Deal with the draught at the floor covering and the perimeter instead, and leave the void ventilation alone.
Stuffing insulation into the void without support — Insulation that slumps onto damp ground wicks moisture, blocks airflow and sits against the underside of the joists. It must be held up between the joists with netting or battens, clear of the ground, with the airflow path left open.

If it doesn't work

Musty smell strongest in one corner or one room

Cause: That part of the void has the worst ventilation, often because an extension or an internal sleeper wall blocks the cross-flow — Fix: Trace the airflow path from airbrick to airbrick across that area and open it up, with additional vents or gaps through sleeper walls if needed.

Smell plus visible white growth on joists

Cause: Sustained high humidity in the void with fungal growth established — Fix: Restore ventilation immediately and get the growth identified. Some fungi are a housekeeping problem and some are a serious one, and the difference matters.

Smell only after heavy rain

Cause: Water entering the void from outside — a failed gully, a downpipe discharging at the wall, or ground falling towards the house — Fix: Fix the drainage. The void has to be allowed to dry between wettings or ventilation alone cannot keep up.

Smell in a flat or an upstairs room with no void

Cause: Not a void problem — usually a slow leak into a floor cavity, or a trapped damp area under a fitted unit — Fix: Trace the damp source directly rather than looking for ventilation that was never there.