Ventilation and mould prevention

How to check whether the ventilation works

The simplest test takes a minute:

1. Take a thin sheet of paper or a tissue.
2. Open a window in another room — slightly, on trickle vent or on the latch.
3. Hold the sheet against the ventilation grille in the bathroom or kitchen.
4. If the sheet stays against the grille — the extract is working.
5. If it falls — the ventilation is not working.

The second step is the one almost everyone skips, and that is exactly why the test so often gives the wrong answer. If the flat is sealed shut with new windows, air has nowhere to come in from, and even a perfectly sound extract will show nothing. People conclude the duct is blocked and call the property manager, when the problem is at the opposite end.

It is worth running the test twice: first with the windows closed, then with one open. If the sheet holds with a window open but falls with them closed, you do not have a duct problem — you have no incoming air. That is a decisive conclusion, because the solution is completely different and much cheaper.

Ventilation is not a hole in a wall. It is a system with two ends, and without an inlet the outlet does not work.

How natural ventilation works

In most Riga apartment buildings — both Soviet-era panel blocks and pre-war buildings — ventilation is natural, with no fans. It relies on a simple piece of physics: warmer air is lighter than cooler air, so it rises up the vertical duct and leaves above the roof. That movement is called draught.

For it to happen, the system needs three things, and they matter equally:

If any one of the three fails, the whole system stops. And in practice what usually fails is not the third — the duct — but the first and second, which nobody thinks of as part of the ventilation.

Why draught weakens in summer

Another consequence follows from the same physics: natural draught depends on the temperature difference between inside and outside. In winter it is strong, because the difference is large. On a hot summer day, when it is warmer outside than in the flat, the draught can stop completely or even reverse. That is normal and not a fault — in summer, airing through windows does what the duct does in winter.

Why ventilation stops

The first unrecognised reason: sealed windows

Natural ventilation in apartment buildings was designed on the assumption that air enters through the gaps around timber windows — at the time that was taken for granted, because airtight windows simply did not exist. Once the windows are replaced with sealed plastic units, the extract stops, because air has nowhere to come from. The building becomes airtight while the ventilation system stays as it was.

The typical picture looks like this: new windows are fitted, condensation appears on the glass in the first winter, and in the second a patch appears in the external-wall corner of the bedroom. Nobody connects the two events, because a year has passed between them. But the cause is single and logical: the moisture that used to leave through the duct now stays in the room.

The second: renovation

Grilles get painted over along with the wall or papered across, and sometimes a fitted kitchen or bathroom panelling covers them entirely. If problems started after renovation, the first thing worth checking is whether the grilles are still open at all.

The gap under the door — the smallest detail with the biggest consequences

Bathroom and toilet doors should have a gap underneath, usually around 1.5–2 cm above the finished floor. That is not unfinished work — it is part of the system. Through that gap, air from the living rooms enters the sanitary room and leaves from there up the duct.

When doors are replaced, the gap often disappears: the new doors are shorter, fitted tighter, or sealed around the whole perimeter. The result is that bathroom ventilation practically stops, even if the duct is perfectly clean and the flat has a proper air inlet. After a shower the moisture has nowhere to go, and within a few months a patch appears in the ceiling corner.

The same thing happens after a floor covering is replaced: laying new laminate with underlay over the old floor reduces the gap by a couple of centimetres, and it can disappear altogether.

• Check the gap by eye: close the door and see whether light shows underneath it.
• If there is no gap, the door can be trimmed at the bottom — a simple joinery job.
• The alternative is a transfer grille in the lower part of the door, if trimming is not wanted.
• Do not put a thick mat in front of the bathroom door — it seals the gap just as effectively.

Solutions

• Trickle-vent setting on the window — many windows have one, and many owners do not know about it. Turn the handle to roughly 45 degrees and the sash opens by a couple of millimetres. Often that solves the problem at no cost.
• An air inlet valve in the window or wall — if the window has no trickle-vent setting. It is a small duct with a filter and an adjustable opening, built into the window frame or drilled through the wall behind a radiator. A wall valve costs more, but it brings air in directly above the heat source, so the draughty feeling is less.
• A gap under the door — at least for the bathroom and the kitchen.
• Cleaning the grilles — they clog with dust, and sometimes get painted over during renovation. A grille can be taken off, washed in warm water with detergent and put back.
• Checking the duct — if the grille is clean and a window is open but there is still no draught.
• Mechanical extract — a fan in the bathroom, if natural ventilation is not sufficient. It only makes sense together with an air inlet.

The order matters, because it saves money. First check the trickle-vent setting and the gap under the door — that costs nothing. Then clean the grilles. Only after that is it worth thinking about an inlet valve or a fan, and the duct is the last thing to turn to.

When to go to the property manager

The ventilation stack in an apartment building is common property. That means the property manager, not the flat owner, is responsible for keeping it clean and working — your responsibility ends at the grille on the flat side.

Go to the property manager if:

• the grilles are clean and a window is open, but there is no draught;
• smells from other flats come out of the ventilation — cooking, smoke or sewage;
• the ventilation runs in reverse — air comes in rather than leaving;
• the problem affects several flats on the same stack;
• you suspect a neighbour has connected a powerful extractor hood to the shared duct;
• the problem appeared after the building was insulated or after roof works.

It is worth putting the request in writing and attaching specific facts: which room, from what date, what the paper test with an open window showed. If the problem affects several flats, a joint submission works better than separate ones.

In these cases individual measures inside the flat will not help — the problem is at building level. If the ventilation is fine but the humidity is still high, the next place to look is the sources of moisture: how to reduce humidity indoors.