Apartment Ventilation: Why Windows “Cry” and How to Breathe in Winter

The ventilation concept of an apartment building is simple: air enters through window gaps and leaves through kitchen and bathroom extracts. Modern airtight double glazing broke the scheme: air has nowhere to enter. The result is familiar to residents of new buildings: stuffiness, “crying” windows, condensation and mould in corners, especially in frost, when airing means cooling the flat.

The good news: the problem is solvable, and best solved at the project stage, not after renovation. Below we cover how apartment ventilation works, why condensation appears, how a valve differs from a breather and what to include in the drawings.

How apartment ventilation should work

Under norms for residential buildings, fresh air enters through the windows of living rooms and leaves through extracts in the toilet, bathroom and kitchen. This scheme is called natural ventilation: air moves through vertical shafts thanks to differences in temperature and pressure. To calculate air exchange in an apartment, designers generally take no less than 30 m³ of fresh air per hour per person; if each person has less than 20 m² of total area, the calculation is 3 m³/h per 1 m² of living area.

A simple conclusion: the more people live in a flat, the more air is needed. Two people in a bedroom noticeably change the air during the night, and with a closed window and no inflow you wake with a heavy head. All the numbers above are guidelines; for exact requirements see the norms and an engineer’s calculation.

Why windows “cry”: condensation and mould

Condensation appears when warm humid air touches a surface colder than the dew point, the temperature at which water vapour turns into water. Cold glass, reveals and corners of external walls are the first places. In an airtight flat with no inflow, moisture from cooking, showering, laundry and breathing stays inside and settles on cold surfaces.

Condensation creates conditions for mould, though it does not cause it by itself: spores are always in the air, and humidity plays the decisive role. Moisture ruins reveals, swells plaster and rusts fittings. Relative humidity of about 40–60% is usually considered comfortable, and in a bedroom in winter 35–50% is often recommended so that cold windows do not get condensation. Buy an inexpensive hygrometer: it will show what is really going on with the air.

First check the extract

First check the extract. A sheet of paper should stick to the grille in the bathroom and kitchen. If it does not pull, the cause is usually a blocked duct or a general draught failure in the building. Cleaning the duct and repairing shared building systems go through the management company. Changing shared ducts on your own is not allowed.

Note that in tall buildings the draught is stronger near the roof and weaker on lower floors. So on top floors the extract works briskly, while on lower ones it can “reverse”, and smells come from neighbours. No breather will fix an extract that does not pull: there is inflow, but the air has nowhere to go.

Inflow: valve or breather

A window or wall valve is a simple solution with no electrics: air enters in metered amounts and chills little in winter. It neither filters dust nor heats air, and works by pressure difference, so it behaves differently in strong wind and in still air. It is a good option for small rooms and budget renovations.

A breather is a wall unit with a fan, a filter and, as a rule, air heating. It supplies air in the needed quantity regardless of weather. For a city with dusty air and for allergy sufferers it is the best option, and the only full one for a bedroom, where a closed window is a condition for sleep. Many models have a CO2 sensor and operating modes, but a breather does not replace an open window: it keeps air acceptable but does not refresh it as quickly as airing.

Which to choose: comparison

Valve: cheap, needs no socket, silent, but depends on weather, does not filter or heat. Fits small rooms without allergy sufferers and a limited budget. Breather: more expensive, needs a socket and a hole in the wall, filters change periodically, but air is filtered, heated and supplied steadily.

A third option is a supply-and-extract unit with heat recovery. It returns part of the heat of the extracted air and saves heating, but needs ducts and space under the ceiling, so it is less common in ordinary flats and is planned at the project stage. If you are building a house or renovating 100+ m² from scratch, discuss it with an engineer.

What to include in the project

At the rough stage: an outlet for the breather (the hole in the external wall is made before finishing), a socket nearby, and a kitchen extract with a duct and a non-return valve so that neighbours’ smells do not come to you. A hole in the external wall and a grille on the facade may require approval from the management company or homeowners’ association, so clarify it in advance. Choose a place not above a radiator and not behind a wardrobe, where the breather cannot breathe.

Connecting a powerful kitchen hood to the shared shaft must be agreed: a strong motor can steal draught from neighbours. For the kitchen people often choose a hood in extraction mode with a non-return valve or a recirculating hood that does not need an outlet to the shaft. The decision depends on the building.

For air to flow from inflow to extract, doors need a gap at the bottom or transfer grilles: without transfer, the inflow does not work. These items are in our working drawings because “breathing easily” is also designed.

Habits that help

Even with a good system simple habits help. Switch on the kitchen extract while cooking and for a few minutes after. After a shower leave the bathroom door open until it dries. Do not dry laundry in a living room. Air briefly and strongly: 5–10 minutes with windows wide open is more effective than a whole day on tilt.

Do not cover ventilation grilles with decor or tape them over. Do not put furniture tight against an external wall: air needs a gap of a few centimetres, otherwise condensation appears behind the wardrobe. Monitor humidity with a hygrometer.

Typical mistakes

Installing a breather with a non-working extract. Covering a ventilation opening with a stretch ceiling or box without access. Laying the floor under a door without leaving a gap for transfer. Choosing the breather location after finishing, when a hole has to be made in a finished wall. Counting on windows to “solve” ventilation.

If you are only starting renovation, discuss ventilation at the layout stage: then the hole, socket and duct will be on the drawings. Send the apartment plan on WhatsApp or fill in the brief on the site, and we will suggest what to include in your building.