High indoor humidity is a common problem in both older and modern homes. Elevated humidity levels can lead to condensation, mould growth and damage to building materials - but also make the indoor environment heavy and stuffy.
To avoid moisture problems, it is important to understand what levels are acceptable, why problems occur and what measures actually make a difference.
Humidity is measured in relative humidity (RH) and is expressed as a percentage. It shows how much moisture the air contains in relation to the maximum amount of moisture it can carry at a given temperature.
As warm air can carry more moisture than cold air, the relative humidity changes when the temperature changes - even if the amount of moisture in the air remains the same.
In dwellings, it is normally recommended that a RF between 30-50% (where 40-50% is often considered optimal). When the level regularly exceeds about 60-65%, the risk of condensation and microbial growth.
A stable and moderate humidity level improves comfort and reduces the risk of damage to the building.
| Relative humidity (RH) | Assessment | Risk level |
|---|---|---|
| Under 30% | Dry air | May cause dry mucous membranes and static electricity |
| 30-50% | Optimal level | Low risk of moisture damage and good comfort |
| 50-60% | Increased level | Acceptable for short periods |
| 60-75% | High humidity | Increased risk of condensation and mould growth |
| About 75% | Very high level | Significant risk of mould and material damage |
It all starts with measuring humidity to get a clear picture of how the indoor climate actually looks like. A simple first step is to use a hygrometer, which shows the relative humidity in per cent.
Place the hygrometer in the occupied zone - approximately at chest height in the room where you spend most time. Avoid placing it directly next to windows, exterior walls, radiators or in direct sunlight, as this can give misleading readings.
To get a fairer picture, you should measure at different times, for example:
A simple hygrometer will give a good indication, but be aware that cheaper models may have some margin of error. For more accurate measurements, it may be wise to use calibrated measuring equipment or data loggers that record values over time.
In the case of recurring or more extensive moisture problems, professional moisture measurement and ventilation control can provide a more reliable assessment of the cause and the need for action.
The problems are particularly common in summer when hot and humid outside air is ventilated into cooler parts of the building such as basements or crawl spaces.
Lack of air exchange means that moisture from showering, cooking, washing and breathing stays in the home.
When warm, moist air meets cold surfaces, condensation forms. It happens often:
A normal family can add around 5-10 litres of moisture per day through showering, washing, cooking, plants and breathing.
Crawl spaces, cold attics and basements are particularly vulnerable if insulation, drainage or ventilation is poor.
Modern or additionally insulated houses have little natural air leakage. Without balanced ventilation, moisture levels can rise rapidly.
When the relative humidity exceeds about 70-75% for extended periods of time, mould can start to grow on organic material such as wood, plaster and dust.
Possible consequences:
People who are sensitive to indoor environments may find that problems with damp and mould negatively affect their comfort.
Effective ventilation removes moist air and replaces it with drier outdoor air. In dwellings with self-draught or undersized exhaust air, a mechanical or balanced ventilation system can provide better control over air exchange.
I villas, for example, an FTX unit can contribute to:
After showering and cooking, leave the bathroom fan and kitchen hood running for a while to ventilate away excess moisture.
Avoid drying laundry indoors if ventilation is poor and ensure lids are used when cooking to reduce steam build-up.
In areas with recurring problems, a dehumidifier be a good and effective solution.
The choice of dehumidifier depends on temperature, construction and the extent of the moisture problem.
