Ventilation against mould: what really helps and what does not

Mould does not simply occur because a room receives too little fresh air. It develops when surfaces remain too damp for too long. Ventilation can remove excess indoor moisture and reduce the risk, but it cannot repair leaks, severe thermal bridges, rising damp or already saturated masonry.

Why mould develops

Showering, cooking, drying laundry and people themselves add water vapour to indoor air. When warm humid air reaches a cold wall, window reveal or thermal bridge, relative humidity rises at the surface. Furniture placed tightly against cold external walls can make the situation worse by restricting air circulation.

How ventilation helps

Ventilation replaces humid indoor air with outdoor air. During the heating season, cold outdoor air is often drier in absolute terms even when its relative humidity is high. Once heated indoors, it can absorb moisture and carry it outside during the next air change.

For many living spaces, 40–60% relative humidity is a useful orientation. Poorly insulated homes may need lower winter values. Surface temperature remains decisive, so one reading in the centre of the room cannot guarantee that every wall area is safe.

Typical moisture situations

Bedroom

Two people release moisture overnight. A low continuous airflow is generally more effective and comfortable than short operation at maximum speed.

Bathroom

After showering, moisture should be removed quickly rather than distributed to cooler rooms. Internal bathrooms or special extract-air requirements need an individual system check.

Cooking and laundry

Use available extraction and monitor how quickly humidity falls after the moisture peak. Indoor laundry drying can require longer or higher ventilation.

Basement in summer

Warm humid outdoor air can condense on cold basement surfaces. Continuous maximum summer ventilation may therefore make the situation worse. Control based on indoor and outdoor conditions is preferable.

Window ventilation or automatic ventilation?

Short, wide-open window ventilation can be effective but depends on regular user action. Permanently tilted windows cool surfaces and provide poorly controlled airflow. A decentralized system works continuously, but still needs correct sizing, sensible operating levels and clean filters.

What ventilation cannot solve

No fan repairs a leaking roof, pipe leak, rising damp or a pronounced thermal bridge. Recurring mould, wet patches, crumbling plaster or a musty smell despite plausible humidity levels require a structural investigation. Existing mould also needs appropriate remediation.

Common mistakes

• Switching the unit off because it was undersized and has to run loudly.
• Looking only at room humidity and ignoring cold surfaces.
• Forgetting filter maintenance.
• Distributing bathroom moisture through open doors.
• Trying to ventilate away existing structural damage.
• Keeping rooms too cold.

This article provides general guidance and does not replace project-specific planning, building diagnosis or mould remediation.