Relative Humidity Information
What you should know about
Relative Humidity,
and its effect on your health and home / building.
Why
Measure/Control Relative Humidity?
About Relative Humidity
Moisture and Wood
Why
Monitor/Control
the Relative Humidity Levels Inside My Home
or Building?
-
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 such as runny nose,
itchy eyes, throat irritation, headaches, etc.
Humidity
Sensor Product Information
Surround
Air Dehumidifier Product Information
|
U.S. E.P.A. (Environmental Protection
Agency) on controlling relative humidity indoors:
"By controlling the relative
humidity in a home, the growth of some sources of biologicals
can be minimized. A relative humidity of
30-50% is generally recommended for homes. Standing water,
water-damaged materials, or wet surfaces also serve as a
breeding ground for molds, mildews, bacteria, and insects.
House dust mites, the source of one of the most powerful
biological allergens, grow in damp warm
environments."
Use the affordable
800
Series Relative Humidity Sensor to monitor relative humidity
in your home or building. |
-
Damp structures are more susceptible to a
variety of problems 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.
-
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.
Preventing
Mold Growth in the Home or Building (Includes many
humidity/moisture 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 about 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 a mold problem (Signs
of a Mold Problem), then you may need an air purifier to remove
mold spores from the air.
Ionizers
are typically better than air filters, since they can remove
smaller particles from the air, and do not rely on particulate
passing through them in order to remove them from the air.
Air
Ionizer Buying Guide
Air
Purifier Buying Guide
If
you use a humidifier, make sure that it provides humidity
control, such as the SuperAire model from HumidifiersDirect.com.
Preventing
Mold Growth in the Home or Building (Includes many
humidity/moisture control tips)
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.
Preventing
Mold Growth in the Home or Building (Includes many
humidity/moisture control tips)
