Humidity in the House
Too Much or Too Little?


Humidity in the house has to be within a narrow range to keep you comfortable and protect your home and everything in it. It is important to be able to monitor and control the level of humidity in the home but that raises some questions:

  1. What is humidity?
  2. How do I measure it?
  3. What is the ideal range for humidity?
  4. Why is too much humidity a bad thing?
  5. Why is too little humidity in the house a bad thing
  6. How do I control humidity?

In this article we shall be concentrating on these six questions. We shall touch briefly on possible solutions in question 6 to offer some pointers for resolving your humidity control problem if you have one.




What is humidity?


Humidity is the amount of moisture in the air. There are three ways to measure humidity.


Absolute humidity is the amount of water in a given volume of air expressed by weight. So, if a room holds 1000 cu ft of air and that air contains 10lb of water the absolute humidity is 10lb. If a room of 2000 cu ft contains 10lb of water the absolute humidity is still 10lb.


Specific humidity is the amount of water in the air expressed as a ratio of the quantities of water to air by weight. So if our 1000 cu ft room has a specific humidity of, say, 1:250 our 2,000 cu ft room, with the same absolute humidity has a specific humidity of 1:500.


Relative humidity is the amount of water in the air expressed as a percentage of the amount of water that air could contain at ambient temperature and atmospheric pressure. So a room at 60°F may have a relative humidity of 50%. If the temperature rises by 20°F that same air could contain twice as much water so the relative humidity falls to 25% while both the absolute humidity and the specific humidity remain constant.


Fortunately we do not have to concern ourselves with absolute or specific humidity in the house. They are only important in order to understand what we mean by relative humidity. To make things simpler still we can ignore the effect of atmospheric pressure on relative humidity in the home as we have no control over it.


How do I measure relative humidity in the house


In the same way that we can measure temperature with a thermometer we can measure relative humidity with a hygrometer. Please note that this is spelt hygrometer, the g is not a typo.


Hygrometers may be mechanical or electronic. They are relatively small devices and are often combined with a thermometer. They range in price from under ten dollars to about thirty depending on quality and the number of functions combined with them.


To obtain accurate readings the hygrometer should be placed away from any direct heat source. It should be left in the place where the reading is to be taken for a period of two hours or so to settle to a reliable reading. Relative humidity varies with temperature so may differ from one part of the room to another.


What is the ideal range for relative humidity in the house?


This is a difficult question to answer. The human comfort zone extends from 30% to 70% RH with variations between individuals. To protect the home against mold and dust mites a level no higher than 50% is required so 45% to 50% is a good target to aim at.


Levels up to 50% will provide protection against condensation in many conditions but if you experience very cold winters and the temperature falls as low as around 20°F (-7°C) or less, relative humidity as low as 30% is required to prevent condensation. This is a very low level of humidity in the house and may be uncomfortable for some.


Why is too much humidity in the house a bad thing?


Very high levels of relative humidity in the house can be uncomfortable. We feel sticky and find breathing an effort.


High humidity is even more uncomfortable in warm conditions as it increases the heat our bodies experience. The body lowers our temperature using an evaporative cooling system. We sweat, the sweat evaporates and takes heat with it. In very humid conditions this evaporation slows and we feel hotter.


High relative humidity also leads to condensation. As temperature falls the relative humidity increases to saturation, we reach the "dew point". This can give rise to damp and consequent damage. Both mold and dust mites need moisture and damp places suit both. Even without condensation high humidity will allow some varieties of mold to flourish.


Mold is unpleasant for many reasons but is particularly unwelcome to allergy sufferers. Dust mites are an unpleasant thought but for those sensitive to the allergens they produce a real menace to health and comfort. Certain types of mold, such as Stachybotrys chartarum (toxic black mold) produce toxins which can affect health. There is much debate about the degree of risk posed by black mold but in general it is a visitor most of us would be pleased to evict.


In the longer term excessive humidity in the home can damage the fabric and the contents of our homes and create health problems.


Why is too little humidity in the home a bad thing


The most immediate consequences of low humidity in the house are physical discomforts for the occupants. These can include chapped skin and lips, itching nose and sore throat, and breathing difficulties. Other short term effects can include shocks from static electricity and problems with electronic equipment.


In the longer term low humidity in the home can be damaging to the fabric of the house, and to its contents, especially those made with wood or leather. In many respects low humidity in the house can be as costly, as unpleasant and as damaging to health as excessive moisture.


How do I control humidity in the home?


Low humidity in the home frequently occurs in areas subject to low temperatures, for all or part of the year. Cold air can hold very little water so if the cold dry air is finding its way into the home this is the most likely culprit. The remedy is to seal all vents through which the cold air can enter. This action will not only improve the level of humidity in the home but will reduce energy costs for heating.

Low humidity in the house also afflicts those who live in warm, arid areas. There is little alternative but to increase relative humidity artificially and the most effective way is to use one or more humidifiers. For those with air conditioning a single, whole house humidifier, integrated with the air-con, is the best solution.


For those with high humidity in the home the first step is to ensure that all sources of water penetration from the outside have been dealt with if possible. Leaking roofs, defects in interior walls and ground water penetration of basements are among many potential causes; deal with these first.


Proper ventilation will help but this can be difficult in very cold or very warm weather. Keeping the house warm will help too, by lowering relative humidity and raising the dew point.


Ventilation is not an easy option in cooler conditions and raising the temperature of the home is impractical in warm climates.


Ensure that all very wet air is vented to the outside by using exhaust fans in bathrooms and kitchens and ducting heated clothes driers to the outside.


Air conditioning reduces relative humidity and is often sufficient for the task. In some cases, however, this is not enough. Air conditioning is cycled on and off according to temperatures, not relative humidity. This may result in the air conditioning shutting down before relative humidity in the house has been sufficiently reduced. This will be particularly probable if your air conditioning unit is "over-sized" and operates for relatively shorter periods.


If you have a persistent problem with excessive humidity in the house and all other measures have failed to reduce it sufficiently you made need to use a dehumidifier. As with humidifiers the most effective method is often to integrate a whole house dehumidifier into the existing HVAC system. If this is not possible, or if the problem is localised, in a basement for example, a free standing unit may be used.


Conclusion


Controlling humidity in the house is both necessary and possible. There are actions you may take to regulate humidity in the house without resorting to additional appliances. If you have tried to do so and the results are inadequate then either a humidifier or a dehumidifier, or both, may be the answer.



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