Air Bricks and Under-Floor Ventilation: Why Does the Floor Smell Damp?

Article

What you are seeing

Many homes built between the 1850s and the 1950s have a suspended timber ground floor: floorboards on joists, with an empty void underneath. The Energy Saving Trust notes these are typical in Victorian and Edwardian properties, and that you probably have one if there are air bricks, sometimes called ventilation bricks, in the outside walls just below floor level.

When that void cannot breathe, damp air sits under the boards. You may notice a stale or earthy smell in the room, a floor that feels cold and clammy, carpets or rugs that smell musty, or skirting boards and the base of the wall that stay damp. The timber itself is the part at risk, because joists and boards that stay wet can decay. Those signs are also shared by condensation, a leak and rising damp, so the aim is to rule things out in order.

Rule these out first, cheapest first

Work down the list. The first four steps cost little or nothing.

1. Step 1, free: find the air bricks and look at them. Walk round the outside of the house at low level. Check each one for soil, leaves, a raised patio or path, a pile of garden material, paint or a plant growing across it. GOV.UK says existing air bricks should be kept clear of debris and obstruction.
2. Step 2, free: clear them and check the ground level. Remove loose debris by hand and keep soil and paving below the level of the bricks. If a path or bed has been raised against the wall over the years, lower it back rather than leaving the vent buried.
3. Step 3, free: check for a water source. Look for a dripping pipe or gutter close to the vent, a leaking waste, or water pooling by the wall after rain. A void that is wet because of a leak will not dry by ventilation alone, and the leak has to be repaired.
4. Step 4, free: use a torch where you can. If there is an access hatch or a cellar, look at the joists and underside of the boards. Visible mould, softness or crumbling timber means stop and get professional advice before doing anything else, as the Energy Saving Trust says for joists showing damp or rot.
5. Step 5, low cost to trades: ask about extra vents. GOV.UK describes air bricks as a cost-effective and easy means of addressing inadequate under-floor ventilation. A builder or damp specialist can advise whether the number and position of vents suits your floor.
6. Step 6, system: lower the overall indoor humidity. Once the void is breathing and no leak is found, any remaining damp feel may be general indoor moisture. That is where a dehumidifier comes in, as the last rung and not the first.

Insulating a floor: keep the ventilation

Warm, draught-free floors are a common reason people look under the boards. The Energy Saving Trust says adding insulation changes the way air and moisture move beneath the floor, which can increase the risk of damp if it is not managed, so good ventilation must be maintained under the floor. It also cautions that plasterboard fixed under the joists can trap moisture if the timber is not dry or airflow is poor. Do not seal the vents to make a room warmer.

The Ashford factor

Ashford Borough Council's flood risk assessment describes outcrops of Gault Clay, Lower Greensand and Weald Clay along the south-west margins of the borough. We do not have figures on how that affects the air under local floors, and we will not invent any. Ashford has a wide mix of housing ages, from older brick terraces to newer estates, so first work out which floor you have. The Energy Saving Trust says many homes built from the 1950s onwards have a solid concrete ground floor, which has no void to ventilate.

When a dehumidifier is not the answer

A dehumidifier does not stop a leak, does not unblock a vent and does not repair rotted timber. GOV.UK describes rising damp as moisture from the ground that is often misdiagnosed, with chemical testing the most appropriate way of confirming it, so a persistent wet band on a wall needs a specialist rather than a machine. Dryzen is also not suitable for rented homes unless the landlord authorises the changes, and it needs a loft to mount in.

Where Dryzen fits

Once the vents are clear and any leak is repaired, a damp feel that remains may be down to humidity in the whole home. A Dryzen system is a loft-mounted dehumidifier with insulated ductwork to the central area or stairway, with a smart app that monitors temperature and humidity. It helps stop the conditions mould and dust mites need; existing mould must still be cleaned off. A free survey measures humidity and checks your loft, and we will say plainly if a system is not the answer.

Quick questions

Should I block up the air bricks to stop draughts?

No. The Energy Saving Trust says not to block air bricks, because they ventilate the space under the floor and stop floorboards rotting. If the floor is draughty, look at sealing gaps at skirting boards and between boards instead, and leave the outside vents clear.

Can I put insulation under my floorboards myself?

The Energy Saving Trust says you should only do so if the joists are in good condition and free from damp or rot, and that ventilation under the floor must be maintained afterwards. If you see signs of damp or rot, get professional advice first.

Will a dehumidifier fix a damp smell from the floor?

It can help stop the conditions mould and dust mites need once the cause is dealt with, but it is not the first fix. A blocked air brick, a leak or ground water in the void needs to be found and fixed first.

Sources: Energy Saving Trust: Floor insulation; GOV.UK: Understanding and addressing the health risks of damp and mould in the home; Ashford Borough Council: Strategic Flood Risk Assessment (2014).

Last reviewed 2026-10-24. General guidance only; this is not a survey of your home.