What are moulds and fungi in buildings?

Moulds and fungi (Dutch: schimmels en zwammen) belong to the fungal kingdom. They spread through spores that are present in the air almost everywhere. Whether they grow depends mainly on moisture: without a surface that stays damp for long periods, there is no growth.

In buildings there are roughly two groups:

• Surface moulds: black, green or grey patches on walls, ceilings, sealant joints and behind furniture. They feed on dust, paint, wallpaper and plaster. They rarely damage the structure but harm indoor air quality and health.
• Wood-decaying fungi: fungi that break down timber and cause wood rot. Well-known are the wet rot fungi and the destructive dry rot fungus (Serpula lacrymans), which can spread across masonry with its strands and also attack drier timber. These fungi can seriously weaken joists, floors and window frames.

Causes

Mould grows where a surface stays damp for a long time. In building physics, a relative humidity at the surface of around 80% is commonly used as the threshold above which mould growth becomes likely. Typical causes are poor ventilation, cold bridges where condensation forms, leaks, rising damp and construction moisture in new or renovated homes.

Application

• Ventilate: keep trickle vents open, run mechanical ventilation continuously and ventilate extra when showering and cooking. Aim for an indoor relative humidity of around 40-60%.
• Heat evenly: cold rooms and outside walls are more prone to condensation.
• Find the source: mould in one spot often points to a cold bridge or leak. Cleaning without tackling the cause only helps for a while.
• Small patches: remove surface mould with water and detergent; wear gloves and ventilate. Only repaint once the surface is dry.
• Large areas or rot: call in a specialist. If you suspect dry rot, always get an expert survey; all affected timber must be removed well beyond the visible damage.
• Sub-floor voids: a damp crawl space is a common source. Make sure it is ventilated and consider a ground cover membrane.

Related Terms

• Wood rot
• Fungicide
• Condensation
• Relative humidity
• Mechanical ventilation
• Crawl space

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