Ventilation in Apartments: Natural and Mechanical

How air is exchanged in a flat, why condensation and mould appear after new windows, and how natural and mechanical ventilation differ in a renovation.

After a renovation some flats develop an unexpected problem: windows mist up in winter, dark patches appear in corners and kitchen smells drift through the whole home. The cause is often that ventilation was ignored during the works. Old timber windows and draughty doors used to exchange air, if unintentionally; airtight new windows close that route.

Why air exchange matters

Every day a flat produces moist and stale air: breathing, cooking, showers, drying laundry, houseplants. Without air exchange, this moisture condenses on walls and cold surfaces. Mould, unpleasant odours and damaged finishes follow. Accumulating carbon dioxide also affects sleep quality and concentration.

The early signs are easy to recognise: droplets at the bottom of the glass in the morning, a heavy feeling on waking, a bathroom mirror that stays fogged long after a shower, a mould spot on the wall behind a wardrobe. If any of these appear, check the ventilation before starting on finishes.

Good ventilation does two jobs at once: it brings fresh air into living rooms and extracts air from the "wet" areas, meaning the kitchen, bathroom and toilet. Air should flow in that direction, from living spaces towards wet rooms.

How natural ventilation works

Most apartment buildings are designed with natural ventilation. The kitchen, bathroom and toilet have a wall grille connected to a vertical duct rising to the roof. Warm air is lighter, so it rises through the duct and draws air out of the flat. For this to work, air must be able to enter the rooms.

Weak points of a natural system:

• it depends on season and wind: in summer, when indoor and outdoor temperatures are similar, the duct draws poorly;
• once windows and doors are airtight, the duct stops working;
• draught may be somewhat stronger on lower floors and weaker on upper ones.

Common renovation mistakes

Covering a vent grille with a suspended ceiling or tiles, taking over the whole duct for a cooker hood, or making the bathroom door fully airtight all disrupt natural ventilation. Dismantling or narrowing the building's shared ventilation duct is not acceptable, since it affects neighbouring flats too.

If a cooker hood is connected to the duct, remember that the same duct also provides general ventilation. In practice the hood gets its own outlet, while a separate grille with a non-return flap is kept for background ventilation.

Bathroom and toilet doors need a small undercut or a door grille so air can flow into these rooms and out through the duct.

Providing air supply

Once windows are airtight, there are several ways to restore air intake:

• trickle vents built into the window frame;
• adjustable through-wall air inlets;
• a micro-ventilation setting in the window hardware.

These solutions soften the flow of cold air and allow exchange without opening the window fully. If the inlet is placed near a radiator, incoming air is warmed as it enters the room.

Mechanical ventilation

In a mechanical system, fans move the air. The simplest version is a duct fan in the bathroom and toilet with a humidity sensor or timer. More complex versions use a central unit that supplies fresh air and extracts stale air throughout the flat. Some of these units include heat recovery: warmth from outgoing air is transferred to incoming air, reducing heat loss in winter.

A central mechanical system needs ducts run in a suspended ceiling, so the decision must be made at the very start of the renovation. The unit's location, noise level, access for filter changes and the duct routes should be shown in the design. In an apartment building, new openings in the facade may not be possible without the building management's consent.

Which solution suits whom

• A standard flat with replaced windows but no condensation: clean existing ducts and add trickle vents; that is often enough.
• A windowless bathroom where damp lingers: a humidity-controlled fan is a sensible addition.
• A large flat on a noisy street where you would rather not open windows: consider mechanical ventilation with heat recovery.

Zaferoğlu İnşaat plans building services, including ventilation, together with the design project in apartment renovations. Hold a sheet of paper to the vent grille: if it does not cling, the duct is not drawing, and that is worth sorting out with us before the works.