Leave the Little Holes Alone: A Homeowner’s Case for Winter Ventilation

November has a way of making every gap in the house look like a personal insult. The curtains shift, the hall carpet carries a chill, and the first instinct is to reach for sealant, foam, or a carefully cut square of cardboard. If that square is currently covering an air brick, though, you are not draught-proofing your home. You are quietly changing how it breathes — and damp is usually the bill that arrives later.

What an air brick is actually doing

An air brick is not a design accident. In older UK homes — Victorian terraces, Edwardian semis, 1930s semis and plenty of post-war houses — these small grilled openings allow air to move through spaces that would otherwise sit stagnant. Some ventilate a suspended timber floor, keeping joists and floorboards away from persistent moisture. Some supply air to a chimney or an open fire. Some ventilate a cavity or a roof void. A few do more than one job.

That is why the humble air brick deserves more respect than it usually gets. It is part of the building’s ventilation strategy, not a draught that someone forgot to finish off. Block it with foam, cardboard, silicone or a neatly cut bit of insulation, and the air stops moving. Moisture from cooking, washing, breathing and drying clothes then has fewer places to go.

One hard rule before anything else: never block a vent that supplies air to a gas, oil or solid-fuel appliance. Open fires, wood burners, gas fires and some boilers need permanent ventilation for safe combustion. If you are not sure which vent does what, ask a Gas Safe registered engineer, or a HETAS-registered installer for solid-fuel appliances. Do not guess with this one.

Draughts and damp are not the same problem

Draught-proofing is about stopping unwanted air from moving through gaps you can feel: around window frames, under doors, through letterboxes, along skirting boards. Ventilation is about allowing a controlled amount of air to move through the building so moisture can escape. The two ideas are related, but they are not interchangeable.

A draught is uncomfortable. Damp is expensive. When warm, moist air is trapped inside, it eventually meets a cold surface — a window reveal, an outside wall, a cupboard on a chilly external corner — and condenses. That is when you start seeing misted windows that will not clear, black mould in corners, a musty smell in wardrobes, lifting paint, or dark patches on skirting boards. Timber is particularly unforgiving. Joists and floorboards that stay damp can rot quietly for years before anyone notices.

Modern homes often manage this with trickle vents, extract fans and airtight construction. Older homes usually rely on a looser fabric and a few well-placed vents. Seal those vents and you have removed part of the system without replacing it.

The telltale signs you have closed the door on fresh air

You do not need to panic at the first cloudy window. Condensation happens in most homes at some point, especially on cold mornings. But a pattern matters. Look for water running down glass, mould returning to the same corner, a damp smell under the stairs, rusty fixings in the floor void, or papery paint on a chimney breast.

Also check whether anything has changed recently. Has a new shed, deck, patio or raised flower bed been built against an outside air brick? Has cavity wall insulation been blown in without checking the vents? Has someone painted over the grille because it looked scruffy? Has a piece of furniture been pushed tight against an internal vent? These are common reasons for ventilation to fail without anyone deliberately sealing the house up.

A sensible autumn inspection routine

Set aside twenty minutes on a dry afternoon. Start outside and walk the perimeter of the house. Count the air bricks you can see at low level and any higher up. Take a photo of each one so you have a record for next year. Check that the grille is open, the insect mesh is not clogged with cobwebs, and plants, soil, compost bins, bins or decking are not pressed against it. Keep a clear gap around each vent.

If an air brick has been painted over, carefully clear the slots with a soft brush. If it is full of debris, a vacuum with a brush attachment is usually safer than poking about with a wire coat hanger. A torch and a small mirror help you see into awkward openings. Do not rod a vent blindly; you may damage the mesh or push debris further in.

Inside, check underfloor vents in suspended floors, vents in cupboards, and any grilles near chimneys or appliances. Make sure rugs, boxes and furniture are not blocking them. If you find a vent that has been deliberately sealed and you do not know why, get advice before reopening it — especially near any fuel-burning appliance.

Keeping warm without sealing up

You can still deal with the draughts that genuinely make life uncomfortable. Fit brush strips to sash windows, use a letterbox flap, draught-proof external doors, and hang heavier curtains with a proper overlap. A chimney balloon can help in an unused chimney with no appliance connected, but it must be removed if the chimney is ever used again. Do not stuff insulation into air bricks.

For moisture, use extract fans in the bathroom and kitchen, put lids on pans, dry laundry outside or in a vented tumble dryer where possible, and open windows briefly each day to purge stale air. A dehumidifier can help in a specific damp room, but it is not a substitute for fixing blocked ventilation. If you have trickle vents on modern windows, leave them open — that is what they are for.

Finally, keep the heat steady rather than letting the house swing from icy to tropical. A background temperature helps surfaces stay above the dew point, which makes condensation less likely. It is not about heating the street through an open vent; it is about letting moisture leave before it settles into the fabric.

When to call someone

If damp patches persist after you have cleared vents and improved your daily routine, get independent advice. A qualified surveyor or an independent damp specialist can tell the difference between condensation, penetrating damp and rising damp without simply selling you a treatment. If a gas or solid-fuel appliance is involved, stop and call the appropriate registered engineer.

The little holes in your wall are not glamorous, but they are doing a job. Leave them alone, keep them clear, and save your draught-proofing for the gaps that were never meant to be there.