Shut Doors, Starved Fans: The Small Gap Doing All the Quiet Work in Your Home

There is a moment most of us have had. It is a cold, damp evening in early November, the bathroom fan is rattling away, the window is shut, and yet the mirror is still fogged ten minutes after the shower. You assume the fan is rubbish. Sometimes it is. But quite often the fan is doing its job perfectly well and failing anyway, because the room it is trying to empty has no way of letting new air in.

The culprit is usually a door that seals too well. Not the frame, not the hinges, not even a draught strip. Just a door that sits flush to the floor, with a beautiful carpet meeting it edge to edge, doing a very good impression of a cork in a bottle.

A fan cannot extract air that has nowhere to come from

Extractor fans do not suck air out of a sealed box. They can only move air if replacement air arrives to take its place. In a room with the door shut and no gap underneath, the fan is effectively trying to create a vacuum. It will still spin, still make noise, still feel like it is working, but the actual volume of air shifted drops off sharply. In some cases it simply pulls air back down the very duct it is trying to push it up.

The same principle applies to air conditioning. A unit that is cooling or heating a room relies on air circulating through the space. If a room is sealed tight with a closed door and no return path, the machine ends up working with a small pocket of air near the unit and fighting a losing battle with the rest of the room. You get uneven temperature, longer run times, and a room that never quite feels the way the thermostat promised.

And it is not only mechanical systems. Human beings, cooking, drying laundry and showering all put moisture and stale air into a room. That air needs somewhere to go. If it cannot leave through a gap or a vent, it finds other exits, usually the cold bridge in a corner, which is exactly where you did not want it.

The humble undercut, explained without the lecture

If you have ever looked at the bottom of an internal door and noticed a neat strip of shadow, that is the undercut. Traditionally, doors are hung with a deliberate gap at the base, and in bathrooms and kitchens it is doing real work. A gap of roughly 10mm along the width of the door gives air a genuine route in while still keeping the room reasonably private and warm.

The trouble is that this gap is easy to lose. A new carpet, an added threshold bar, a draught excluder fitted to stop a chill, a door that was trimmed to fit after the flooring went down but never re-hung properly. All of these can close a gap that was never generous to begin with.

It is worth knowing that this is not just a comfort issue. Approved Document F, which covers ventilation in England and Wales, expects a suitable path for air to move into rooms with extract ventilation. Scotland and Northern Ireland have their own equivalents. In practice, a few millimetres of clearance under a bathroom or kitchen door is part of the design, not an accident.

One important caveat: fire doors are a different matter entirely. Never trim the bottom of a fire door to improve airflow. If you live in a flat, a house of multiple occupation or any building with a protected escape route, check with the freeholder or a competent person before touching anything. Airflow can be solved another way.

The five-minute tissue test

You do not need instruments for this. A sheet of toilet tissue and a bit of patience will tell you most of what you need to know.

1. Close the door to the room and turn the extractor fan on (or the air-con unit, if that is what you are checking).
2. Wait a minute for things to settle.
3. Hold the tissue flat against the gap under the door, on the room side, and let go.
4. If the tissue is pulled against the door or sucked towards the gap, air is moving the way it should.
5. If it barely stirs, or drops straight to the floor, the room is starved.

Repeat with the door open. If the fan suddenly sounds different, calmer, or the tissue test changes dramatically, you have found your answer. A second useful trick is to crack the door a couple of centimetres while the fan runs and listen. If the fan note changes, it was working against resistance.

For a slightly more visual version, a smouldering incense stick held near the gap will show you the direction and strength of the airflow. Keep it away from anything flammable and do not do this in a room full of paperwork.

What actually fixes it

If the gap is too tight, you have three realistic options and one of them is free.

Leave the door ajar. Not glamorous, but for a bathroom or utility room it is often the whole solution. A couple of centimetres is plenty. If privacy is a concern, fit a small latch or simply get into the habit of pulling the door to rather than clicking it shut.

Have the door trimmed. A joiner can take a few millimetres off the bottom and re-seal the edge. This is quick, tidy and cheap, and it restores the gap the door was always meant to have. Do not do it yourself with a borrowed circular saw unless you are genuinely comfortable with one.

Fit a transfer grille. Where trimming is not practical (a fire door, a door that sits over a raised threshold, or a room where you want more control), a transfer grille lets air pass through the door or the wall above it while keeping the room visually closed off. These come in simple slatted versions, and better ones include a bit of acoustic baffling so you are not effectively opening a window into the hallway.

Whatever you do, resist the urge to block the gap with a draught excluder in a bathroom or kitchen. In a living room or bedroom, a brush strip is fine. In a wet room, it is working against you.

When it is more than a gap

Some airflow problems are not solvable with a trim. If you have tried the tissue test, opened the door, and the fan still seems feeble, the issue may be ducting that is too long, too bendy or disconnected in the loft. If condensation keeps returning to the same cold corner, the moisture load may be higher than the ventilation was ever designed for. If your air conditioning struggles to cool a room that is not especially large, the return path may be only part of a bigger picture.

That is the point to stop guessing. A short ventilation survey costs far less than redecorating a ceiling with mould wash every spring, and it will tell you whether you need a grille, a new fan, better ducting, or simply a different habit with the doors.

Because the strange thing about a well-sealed house is that it only works when a handful of deliberate gaps are left in place. The one under the door is the quietest and most important of them.