Moisture Control

Moisture Control

Reduce moisture to improve indoor air quality

When we talk about high or low humidity, we describe the amount of water vapor in the indoor air. Don’t confuse water vapor with steam or mist, which are very fine droplets that can be seen by us when many of them are bunched together, as above a boiling water kettle. Water vapor is an invisible and odorless gas.

The air can’t accommodate unlimited amounts of water vapor. Let’s picture your kitchen filled with nitrogen, oxygen and some tiny amounts of other gases. Water evaporates from the kitchen sink, the boiling soup on the stove, the daisy on the window sill, and from your lungs and skin. But only about 4% of all the air in the kitchen can be replaced by water vapor. Once this amount is reached, the air is saturated and the water vapor has filled all available space (100% relative humidity). Because the air simply can’t take anymore, it will shed some of the vapor as tiny water droplets. In the sky you can see clouds or fog when this happens, and indoors you find yourself in a steamy kitchen or bathroom.

One peculiarity about this whole process is that the 100% level depends on the temperature. Warm air can accommodate much more water vapor than cold air. This explains why you will see condensation droplets on cold windows, walls, and cold water pipes, even so there is no visible steam in the room at all. In other words, the warm room air may have only 50% of the allowable vapor allocation, but this amount is equal to 100% relative humidity when the air near the cold window or wall cools down and the water vapor condenses.

The humidity level affects our comfort and health.

For example, high humidity levels affect our body’s ability to rid itself of excess heat and disorders may develop. On the other hand, low levels dry the mucous membrane in nose and throat and allow bacteria and viruses to by-pass this defense mechanism. My book How the weather affects your health describes these negative health effects in detail. The most comfortable and healthy humidity level is in between 40 and 60% relative humidity.

Water, of course, can come from many other sources. Leaking pipes and taps, overflowing drip pans, pot plants; and human activities, such as taking a shower or cooking; are just a few examples. Moisture may also seep into the building from the ground or the building may have a leaking roof. In addition, new brick or concrete walls and foundations harbor moisture for a long time. Another source is easily forgotten, that is the person itself. A person performing light activities evaporates between 30 and 40 gram of water per hour at room temperature. Add to this all the other sources and the air may have to cope with the vapor that represents several liters of water in a 24 hour period.

How to reduce moisture

As described in previous sections, biological pollution and insect pests thrive and rapidly multiply in moist conditions. High humidity and the resultant moisture are a major reason for excessive allergens, bacteria, viruses and fungal spores in our indoor air. Besides the potential health problems, excessive moisture can also result in damage to the building structure. It is, therefore, very important that we practice some form of moisture control. In most cases this isn’t an elaborate task, as many simple measures, such as improved ventilation, are usually sufficient. The task is even easier if the building in which you live has been designed to prevent moisture entry and has strategies that allows humid air to escape.

You can, of course, invest in a home dehumidifier. They offer a proficient way of reducing humidity at a reasonable cost.

I have divided the many tips and methods into two lists. The first list are the measures you can easily accomplish yourself, whereas the second list contains methods that should be in the building design, require alterations to the building, or need maintenance.


  • Replace indoor air with outdoor air and improve circulation by regularly opening windows, using fans where appropriate, and opening doors between rooms.
  • Avoid moisture build up during household activities such as washing, cooking or having a shower by using fans or opening windows.
  • Check all rooms for condensation, wet spots, and mould growth. Pay special attention to carpets on concrete floors and the space behind furniture items along outside walls. Replace carpets with alternative floorcoverings and move the furniture, if necessary.
  • Clean and dry any wet or damp spots within 48 hours to avoid mould growth.
  • Raise the room temperature if necessary to avoid high humidity and condensation.
  • Use dehumidifiers and air conditioners, especially in hot and humid conditions.
  • Turn off humidifiers if you notice moisture on room surfaces.
  • Check and clean drip pans of appliances such as refrigerators, air conditioning units and humidifiers/dehumidifiers to avoid overflowing.
  • Wipe off condensation droplets that form on window frames.
  • Don’t store firewood indoors as its moisture content may contribute to indoor humidity.
  • An unheated cellar and unventilated attic or crawl space is likely to have a moisture problem.
  • Purchase a hygrometer to monitor humidity levels. A level of between 40 and 60% is ideal. Simple and inexpensive instruments will generally do the job.

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