A house with a suspended timber ground floor depends on air moving through the void beneath it. The joists sit in a space that is naturally damp, and the only thing keeping them sound is a through draught carrying that moisture away. Block the openings and the floor begins to decay quietly, from the underside, where nobody looks.
The openings themselves are unremarkable and easy to lose: a few clay airbricks or plastic grilles low down on the external wall, often behind a shrub or buried by a flowerbed.
Why they get blocked
Almost always by something added later with good intentions. A new path or patio laid at a higher level covers them. A border is topped up with soil or bark year after year until it reaches them. Insulation is pushed into them from inside against draughts. A plant grows across them.
Rendering an elevation and filling them in is the most thorough version of the mistake, and it is not rare.
How many, and where
You want them on opposite walls so air crosses the void rather than entering and stopping, spaced regularly along each elevation, and clear of the ground by enough that splashing and silt do not block them. A single vent on one wall ventilates very little.
Internal sleeper walls can also obstruct the flow, which is why airbricks on one side alone sometimes achieve nothing even when they are clear. That is worth bearing in mind if the symptoms persist with open vents.
The signs that air is not moving
A musty or earthy smell at floor level in a ground floor room is the earliest and most reliable warning, usually noticed on returning to the house after a few days away. Springy boards, a slight give near a wall, and skirtings that feel soft at the bottom follow later.
Lift a corner board or an access hatch if you can. A void that smells damp, has standing water, or shows white strands on the timber needs attention promptly rather than at leisure.
Putting it right
Clearing them is often the entire job: dig the border back, lower the soil, cut back the planting, remove whatever was stuffed in from inside, and brush the grille clear. That is an afternoon with a spade and it frequently resolves the smell within weeks.
Where a path or patio has buried them, a small duct or a periscope vent can carry the opening up above the new level. Where there are too few, more can be cut in by a builder, which is straightforward work on a cavity wall and wants more care on a solid one.
Do not close them for warmth
Underfloor draughts are better addressed above the floor, by insulating between the joists and sealing the gaps around the boards and skirtings, than by closing the vents that keep the structure dry.
Check them each autumn when you clear the gutters, since the two jobs happen at the same time of year and the vents are the one most often forgotten.