Relative Humidity Information
What you should know about Relative Humidity, and its effect on your Health, Home / Building, and mold infestation growth.

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Why Monitor/Control the Relative Humidity Levels  Inside My Home or Building?

1.To prevent the growth of mold, dust mites, bacteria, fungi, viruses and lower the rate at which chemical off-gassing occurs from materials in the home. The optimum relative humidity range for the well being of the home and for the health of the occupants is between 30% to 50% RH.

In this day and age, people are spending more of their time indoors (approximately 90%), so the indoor air quality is a vital factor in maintaining healthy lives. A home that is too humid or damp can promote the growth of mold and dust mites. If present at high levels indoors, these microorganisms may adversely affect the health of individuals. The most common symptoms of mold or dust mite exposure are allergic reactions


Surround Air Dehumidifier Product Information

2.Damp structures are more susceptible to a variety of problems such as mold infestation that will work to damage the integrity of the home.

Homes that have high humidity levels are more likely to become infested with termites and carpenter-ants since these insects prefer and migrate towards moist wood, and the steel components of a home may begin to rust if the indoor relative humidity is too high.

Excessive moisture levels in homes can significantly increase the maintenance and operating costs through damage to furnishings, wall coverings, carpets, ceilings, etc.

3. Moisture Problems and Wood.

Wood is a hygroscopic material, meaning that it readily takes up and retains water. It always contains water and exchanges water vapor with the air. For example, wood will adsorb water when the relative humidity is high, and release it when the relative humidity is low. Wood swells as it adsorbs water and shrinks as it releases water, so both its moisture content and dimensions are controlled by the relative humidity of the air.
Inside homes the relative humidity of outdoor air drawn inside is drastically altered by heating and cooling and seasonal changes. It is these extreme fluctuations in relative humidity that may disrupt the wood moisture content and cause structural damage of the wood.

Humidity Control Tips

About Relative Humidity

What is Relative Humidity (RH)?

Relative humidity is defined as the ratio of the water vapor pressure or water vapor content to the saturation vapor pressure or the maximum vapor content at the temperature of the air or gas. The saturation vapor pressure in the air varies with air temperature: the higher the temperature, the more water vapor it can hold. When saturated the relative humidity in the air is 100% RH.

According to the EPA, the relative humidity recommended for houses is between 30% and 50%. This level of humidity is not always easy to maintain, especially in certain “problem” areas of the home. A dehumidifier is often needed in basements and crawlspaces where humidity can be very high. However, in the winter months, the use of the furnace may contribute to most parts of your home suffering from the opposite problems: a lack of humidity. Therefore, monitoring RH levels in your home will allow you to maintain all areas it at the recommended levels and deal properly with seasonal variations.

   Examples of How Relative Humidity Changes Occur

• On a 30 degree F day with an outdoor RH of 60%, the relative humidity indoors can drop to levels as low as 15% when the air is heated to 70 degrees F (as the air is heated it expands giving it the capacity to hold more moisture. If moisture is not added to the air as it is heated the RH level may drop considerably). If the outdoor temperature is 10 degrees F, the indoor relative humidity may fall to 8% or less. These are extreme levels of dryness. As a comparison, the average relative humidity in the Sahara Desert is 25%.
   

• Since warm air can hold more water vapor than cold air, the relative humidity of the air can be changed by simply changing its temperature. If in the winter, outside air at 20 degrees F and 65% RH is drawn inside and heated to 70F without humidification, its RH drops to about 10%. In summer, outside air at 70F and 60% RH flows into a basement at 60F causing an increase in RH up to 82%.

You can see from the above examples why it is important to measure and control relative humidity levels throughout the entire home or building, since the humidity levels in the air may change drastically through the heating and cooling processes. These processes can then lead to extreme highs or lows in the relative humidity levels indoors.

If you suspect unhealthy conditions in your home due to humidity being too high or too low, it is prudent to utilize a tool known as a digital thermo-hygrometer (a.k.a. moisture meter or humidity sensor).  This handy instrument digitally calculates temperature and measures indoor relative humidity levels.  It can store the minimum and maximum temperature and relative humidity as well as provide up-to-date readings of the present conditions.  Using a thermo-hygrometer is the best way to learn and understand your indoor environment.  This unit allows you to evaluated the RH levels in your home and understand the RH patterns, such as highs and lows, that exist.

If you do have excessive humidity, and see evidence of mold problem, then you may need to do a mold inspection and mold testing of your property. Visit Certified Mold Inspector and Do-It-BEST-Yourself Mold Test Kits.

Moisture and Wood

Wood is a hygroscopic material. It always contains water and constantly exchanges water vapor with the air. Wood picks it up when atmospheric relative humidity is high, and gives it off when relative humidity is low. Since wood swells as it adsorbs water, and shrinks as it releases water, both its moisture content and its dimensions are controlled by the relative humidity of the surrounding air. Wood moisture content is equal to the weight of water contained in the wood divided by the oven dry weight of the wood, expressed as a percent.

Despite wide day-to-day fluctuations, average outdoor relative humidity actually changes little from season to season.  However, inside homes the relative humidity of outdoor air drawn inside is drastically altered by heating it and cooling it without humidification or dehumidification.  It is these extreme seasonal swings in relative humidity that may disrupt the wood moisture content and cause structural damage of the wood through changes in dimensions.

Humidity Control Tips

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