It started with a smell. Not dramatic, but persistent — a damp, musty odour you cannot quite place. Then you noticed the black specks in the window sill in the bedroom. And behind the bed, on the wall facing the gable end, a greyish discolouration you had not seen before.
Mould is one of the most common moisture problems in Danish homes. And it is not just a cosmetic issue.
What is mould?
Mould is a collective term for hundreds of different fungal species that live off organic material and thrive in damp environments. They spread via spores in the air — spores you inhale.
To grow, mould requires three things: moisture (above 70% relative humidity at the surface), food (timber, plasterboard, wallpaper paste, dust), and time. Remove one of the three, and the mould stops. In practice, it is almost always the moisture you need to control.
Why does it occur?
The most common causes in Danish homes:
Condensation on cold surfaces. Warm, moist air meets a cold wall or window and releases moisture. This happens especially on exterior walls with poor insulation, behind furniture placed against exterior walls, and in window reveals. See also condensation on windows.
Insufficient ventilation. Many older homes have been made tighter with new windows and insulation, but ventilation has not kept pace. Moisture from cooking, bathing and breathing has no way out.
Moisture from the structure. Moisture from the basement, leaking pipes, roof leaks, or rising damp. Here the mould is a symptom of a deeper problem.
High moisture content in new materials. New builds and recently renovated homes can have elevated moisture in concrete, render or plasterboard that has not yet dried fully. This can cause mould in the first year, but typically disappears once the material has dried out.
Health risks
Mould is not just unsightly — it is a health problem. According to the Statens Serum Institut and SBi, mould spores in indoor air can cause:
- Irritation of eyes, nose and throat
- Headaches and difficulty concentrating
- Cough, asthma-like symptoms and shortness of breath
- Allergic reactions — especially in children
- Fatigue and general malaise
Children growing up in homes with mould have up to twice the risk of developing asthma, according to Danish research. This is not something to live with.
How to find the mould
Mould likes to hide. The most common places:
- Window reveals and silicone joints — especially in bedroom and bathroom
- Behind furniture placed against exterior walls — move the sofa 5 cm from the wall
- In corners between exterior wall and ceiling — the coldest surface in the room
- Under floor coverings in basements — often hidden for years
- Behind bathroom furniture and in silicone joints
- In the loft, especially where there are leaks in the roof structure
If you can smell mould but cannot see it, it may be inside the structure — behind plasterboard, under floors or in insulation. A professional needs to investigate with a moisture meter and possibly air sampling.
What you can do
1. Find and resolve the cause
The most important step. Mould is a symptom of moisture — remove only the mould without solving the moisture problem, and it will return. Is it condensation? Lack of ventilation? A leak? A structural defect?
2. Ventilate properly
Keep relative humidity below 50–55% during the heating season. Ventilate for 5–10 minutes morning and evening with cross-ventilation. Use an extractor hood when cooking and an extract fan in the bathroom. Avoid drying clothes indoors without extract ventilation.
3. Remove visible mould
On hard surfaces (tiles, glass, painted concrete): wash with a chlorine-based cleaner, leave to work, rinse off. Wear gloves and a mask.
On porous materials (plasterboard, timber, wallpaper, insulation): the material must be replaced. Mould is embedded deep in the material’s structure and cannot be washed away.
4. Insulate and avoid cold bridges
Cold exterior walls cause condensation. Insulation — either external or internal with the correct vapour barrier — raises the surface temperature and significantly reduces the risk of condensation.
5. Get a professional assessment
For larger attacks (over 1 m²), mould within the structure, or if you have symptoms, you should consult a professional. A mould survey with air sampling and moisture measurements typically costs 3,000–8,000 DKK.
Who is most at risk?
- Flats with poor ventilation — especially bathrooms without a window or mechanical extract
- Basement rooms and semi-basements — high ground moisture and cold surfaces
- Homes from 1960–80 — made tighter with insulation without upgrading ventilation
- Homes where occupants rarely ventilate — especially during the heating season
Rental property: who is responsible?
In rental properties, the landlord has a maintenance obligation. If the mould is caused by structural defects, inadequate ventilation or maintenance failures, it is the landlord’s responsibility to remedy it. Document the problem with photos and dates, and report it in writing to the landlord.
If however it is caused by the tenant’s behaviour — for example never ventilating, drying clothes in the living room without extract ventilation, or blocking ventilation openings — responsibility may fall on the tenant.
If in doubt: contact the tenants’ organisation Lejernes Landsorganisation or the municipality’s housing inspection.
Mould is almost always caused by moisture — read also about damp in walls and poor air quality in the home. If you have children who are frequently ill, mould may be a contributing cause — see our guide to indoor air quality and children’s health.
Sources: SBi “Mould in water-damaged buildings”, Statens Serum Institut, Byggeskadefonden, Bolius.