Why air bricks and subfloor airflow matter

How a suspended timber floor stays sound

In most properties built before the 1960s, and in many since, the ground floor is timber: joists spanning between walls or sleeper walls, with boards above and an open void below. That void is in contact with the earth, so it is naturally humid, and the timber immediately above it can only stay dry if moist air is continuously replaced with drier air from outside. That is the job of the air bricks, and they are not optional decoration. Guidance for new work looks for an equivalent open area of roughly 1,500mm² for every metre of external wall, with cross-flow from at least two sides of the building so that air moves through the void rather than sitting in it.

How they get blocked

Almost every blockage we find is a by-product of an improvement. A patio, path or drive is laid and buries the air bricks. A conservatory, extension or porch is built across the elevation they served, so the void loses its cross-flow. Someone fills them to stop draughts under the floorboards, or to keep mice out. Insulation is stuffed into the void and packed against the vents. A render or a new plinth is applied over them. Soil and leaves build up in the border in front of them. Sometimes they were never adequate in the first place, particularly in properties where the original open fireplaces, which drew air through the house, have been sealed up.

What happens next

Once the void can no longer dry, the moisture content of the timber rises. Softwood is generally safe below about 18 to 20 per cent; sustained above that, wet rot fungi become active, and joist ends bearing into external walls, which are both the dampest and the most structurally important points, are usually affected first. In an enclosed, unventilated, persistently damp void, dry rot becomes a genuine possibility, and it can spread through masonry to attack timber elsewhere. Beetle activity in older softwood is also more likely to be sustained where moisture content is high. A musty smell from under the floor, springy boards, or a skirting that gives when pressed are all worth investigating.

Effects on the walls as well

A humid sub-floor void does not only affect the floor. The inner faces of the external walls below floor level stay damp, moisture migrates upwards into the plaster, and the resulting low-level damp is regularly diagnosed as rising damp. Where an owner has had a damp-proof course installed but nobody has looked at the void, the damp often persists. Restoring airflow to the void, clearing debris and lowering external ground so the air bricks are clear will in some properties resolve the internal damp entirely, at a fraction of the cost of a treatment.

How to check yours

Walk the outside of the house and find every air brick. Count them, note which elevations they are on, and check whether any are buried, painted over, filled, obstructed by a border or hidden behind a new structure. Look for whether there is a pair of opposite elevations with vents, which is what produces cross-flow. Inside, lift a corner of carpet at the edge of a room and see whether the floor feels or smells damp, and press the skirting and boards near external walls. If there is a hatch or an accessible void, a torch will show standing water, debris, buried sleeper walls and whether the joist ends are stained. Do not disturb suspect material or enter a confined void without proper precautions.

Putting it right

The work is generally straightforward: clear and, where necessary, replace air bricks with larger open-area units; fit periscope or telescopic vents where paving has raised the level, so the vent draws from above the paving; excavate and lower ground where it has buried the vents; add ducts through an extension to maintain cross-flow to the original void; clear debris, rubble and standing water from the void; and remove or reposition insulation that is blocking the path. Where decay has already occurred, the moisture source is corrected first, then the extent of the affected timber is established, and only the members that need replacing are replaced.

When to arrange a timber inspection

Insulating a suspended floor without causing decay

Insulating a suspended timber floor is worthwhile, but it is one of the easier retrofits to get wrong. The insulation must not block the air bricks or the cross-flow path through the void, and it should be supported in the joist depth rather than stuffed into the space below, so that air can still move freely under it. A vapour control layer on the warm side and a breathable membrane on the cold side, installed in that order, prevent moisture accumulating in the insulation. Rigid boards cut tight between joists, or mineral wool supported on netting, are both used; loose material lying on the void floor is not insulation, it is a moisture trap. If you are planning this work, check the void's airflow and moisture condition first, because insulating a void that is already damp accelerates the problem you cannot see.