In this article
Why it happens
Everyday life produces a lot of moisture: showers, cooking, drying laundry, breathing. Warm air holds that moisture; when it meets a cold surface, such as a window, an exterior wall or a corner with poor insulation, it condenses into water. Where that water stays, mould grows.
Three factors make it worse:
- Insufficient ventilation, so moist air stays inside.
- Cold surfaces, typical of older buildings with limited insulation or thermal bridges around windows and balconies.
- Uneven heating, with some rooms kept very cool while others are warm.
Modern, well-insulated buildings with mechanical ventilation are much less prone to it. Older buildings, which make up a large share of the rental stock in quarters such as Limpertsberg, Bonnevoie and the Gare, need more care.
How to prevent it
Ventilate, even in winter
Open windows wide for five to ten minutes, two or three times a day, rather than leaving one slightly open for hours. Short, full airing replaces moist air without cooling the walls. Always ventilate after showers and cooking.
Use the extractor fans
If the bathroom or kitchen has an extractor, use it, and keep it clean. If it does not work, report it.
Heat evenly
Keep all rooms at a moderate temperature rather than switching some radiators off completely. A cold bedroom next to a warm living room is a classic source of condensation.
Mind the laundry
Drying clothes indoors adds litres of water to the air. If you have to, do it in a room with the window open or the extractor on, and never in a bedroom.
Leave space behind furniture
Keep wardrobes and beds a few centimetres from exterior walls so that air can circulate.
Wipe windows in the morning
If windows are wet in the morning, wipe them. It is a two-minute habit that prevents mould around frames.
A hygrometer helps. A small, inexpensive humidity meter tells you when the air is too damp. According to the Housing Ministry's guidance on mould, ambient humidity should not constantly exceed 65 to 70%; if a room sits above that level for long periods, mould becomes far more likely.
How much moisture a household actually produces
It is worth knowing where the water comes from. A load of laundry drying indoors releases moisture into the air continuously, up to around 500 grams of water per hour if it has not been spun, and still roughly 200 grams per hour even after a normal spin cycle. Showers, cooking and simply breathing overnight add further litres. This is why airing a bedroom after a night's sleep, and a bathroom or kitchen immediately after a shower or cooking, makes such a difference: the moisture has to go somewhere, and an open window sends it outside rather than into a cold wall.
The Ministry's own guidance is specific about timing: air bedrooms intensively for five to ten minutes several times during the morning, or for thirty to forty minutes just after getting up, and air bathrooms and kitchens immediately after moisture peaks, with the door to the rest of the home closed so the damp air does not spread. Avoid leaving a window tilted open for hours, since this mostly cools the walls around it without properly renewing the air, which can make condensation on that wall worse rather than better.
What to do if mould appears
- Act early. Small patches can be cleaned with a suitable product. Large areas, or mould that comes back quickly, point to a problem beyond daily habits.
- Photograph it, with the date, before cleaning.
- Tell the landlord in writing if it is more than a small surface patch, or if it keeps returning despite good ventilation.
Who is responsible?
Responsibility follows the cause.
When it points to the tenant
Mould caused by lack of ventilation or heating, or by drying laundry indoors without airing, is generally the tenant's responsibility. Tenants must use the home carefully, and that includes airing it.
When it points to the landlord
Mould caused by the building is generally the landlord's responsibility:
- water infiltration through the roof, facade or windows;
- a leak in a pipe or from a neighbour;
- rising damp in ground floors and cellars;
- a defective or missing ventilation system;
- poor insulation or thermal bridges that make condensation almost unavoidable.
The landlord must provide a home fit to live in, and persistent damp caused by the structure is part of that. Our guide to repairs in a rental explains the general rule.
When it is unclear
Often both play a role: an older building with cold corners, and a household that ventilates too little. A report by a building expert or an energy adviser can identify the cause. Humidity measurements and a thermal camera are standard tools.
Mould at the end of the lease
Mould is one of the most common deductions from deposits. Two things protect you:
- The entry inventory. If there were stains, marks or damp patches when you moved in, they must be recorded. Our guide to the état des lieux explains how.
- Your written reports. Emails to the landlord about recurring damp, with photos and dates, show that the problem was structural and reported, not neglected.
See our guide to security deposits for the rules on deductions.
When choosing a home
At a viewing, look for tell-tale signs: fresh paint in a single corner, dark marks around window frames, a musty smell, a dehumidifier left in a room. Ask about the ventilation and look at the energy certificate. Our viewing checklist and our guide to the energy passport explain what to check.
Our take
Most condensation problems in Luxembourg homes are prevented by habits that take a few minutes a day: airing properly, heating evenly and keeping furniture off cold walls. When mould persists despite that, it usually has a structural cause, and it is the landlord's to fix. Reporting early and in writing is what keeps the question fair for both sides.
In the homes we manage at Aura Properties, we explain ventilation and heating on the first day and act on damp reports quickly. See what is available now.
Sources
This article explains the general framework of Luxembourg residential lease law and is not a substitute for legal advice on your individual contract. For official procedures and current forms, consult guichet.lu or your commune.
