What is the healthy room temperature and relative humidity?
Relative Humidity (RH) 50% is the healthy room RH.
50% RH does not exist at any known place on the surface of the
earth where natural temperature swings eliminate the possibility.
Allergies, colds, flue, germs, molds, mildews, viruses sustained at
50% RH went the way of the dinosaur to become extinct where humans
Why not keep the room at 50% RH?
Conventional HVAC systems can not control RH within 10% accuracy.
Linda can control the RH better with a pan of water on the range
than an expensive HVAC system. Why?
High RH At 70%-100% RH germs, molds, mildew, and viruses thrive
and multiply fast at room temperature.
Black mold thrives inside a dark wall that has 100% RH sustained by
a roof leak. HVAC duct drains become clogged with
living, thriving, growing green goo.
Low RH At 0%-30% RH the "desert" germs thrive and multiply.
Moisture loving molds and mildew may dry and die but
static electricity energizing dust, pollen, allergies, flue,
sinuses, viruses, colds, dry skin, and chapped lips.
Why does RH swing?
RH swings with temperature.
What temperature is healthy and feels good at 50% RH?
"A stable temperature is required to maintain a healthy RH".
I set the thermostat and didn't change it. Why does temperature
The typical HVAC system will swing room temperature 5 to 15 degrees
F. Check the span between the high and low "on to off" heat shut
down and the "off to on" heat turn on. For the total temperature
error add the thermostat error of 2-7 deg F to the controllable span
of 5-15 degrees F.
A stable temperature maintains a constant RH.
A room at 60 deg F and 100% RH may feel 70 deg F.
Increase the temperature to 70 deg F and the RH will automatically
be lowered and more healthy.
A corn stove maintains a constant room temperature. The Relative
Humidity is stable.
Now you know why corn stove owners love the "feel" of corn stove
Water Pressure is the force that drives RH. The partial pressure of
water vapor is maximum at 100% RH.
Total pressure, atmospheric or barometric pressure averages 14.696
psia at sea level or less at high elevations.
Total pressure is the sum of all the partial pressures of each
The partial pressure of water vapor in the air is constantly
changing as the RH changes.
The RH is constantly changing because of the HVAC temperature
Available oxygen supply to the skin and lungs is inversely
proportional to the RH induced water vapor pressure in the air.
Add a few allergies, germs, viruses, dry skin conditions and colds
that multiply rapidly at room temperature
how do we ever survive?
Relative Humidity Measurement
Relative humidity (RH) 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.
Dewpoint is the temperature at which air becomes saturated with
water and begins to condense - forming dew. Therefore at 100 %
relative humidity the ambient or process temperature equals the
dewpoint temperature. The more negative the dewpoint temperature is
from the ambient temperature, the smaller is the risk of
condensation and the drier the gas or air stream.
Barometric Pressure Measurement
Barometric pressure is defined as atmospheric pressure i.e. the
force exerted on a surface of unit area caused by the weight of the
air column above, normally between 950 - 1050 hPa at sea level. It
indicates the presence and movement of weather patterns and affects
many physical measurements.
Why Measure Relative Humidity?
Humidity has a significant effect on our environment. Humidity
measurement gives us an opportunity to control these effects.
The significance of indoor air quality to our health has become
evident. Humans are best suited to and feel most comfortable at
certain humidity and temperature; excess high or low humidity or
temperature cause discomfort.
As most materials are hygroscopic, their water content always tries
to reach equilibrium with the surrounding relative humidity. Thus
each material has its own ideal storage humidity which should be
maintained. Too dry or too humid conditions can destroy the material.
In many production processes the correct measurement and adjustment
of humidity is extremely important for sustaining the high quality
of products and the correct level of energy consumption. The right
humidity makes it possible to optimize energy consumption and
improve end product quality as well as product yield.
At low humidity, static electricity increases. This can be crucial
in the chemical industry where dry powdery material is handled. In
extreme cases static electricity can cause explosions.
Moisture measurement in construction materials. Excess moisture in
structures can cause quality and health problems and economical
losses. Concrete is usually dryer on the surface than deeper down in
the structure. Therefore, a structural humidity profile including
measurement data at different depths is the most reliable approach.
Vaisala's product variety includes humidity transmitters especially
designed for the measurement of structural humidity.
Carbon Dioxide Measurement
Carbon dioxide (CO2) is one of the most common gases in our
atmosphere. It is formed during breathing of humans and animals, in
fermentation and decomposition processes and during the burning of
fossil fuels. Plants need carbon dioxide in the assimilation
process. It can therefore be used as a fertilizer to enhance plant
growth. Carbon dioxide is a good indicator for indoor air quality.
In large concentrations carbon dioxide is a potential safety hazard
and it inhibits bacterial growth.
Diverse CO2 applications
There is a growing need for reliable measurement of carbon dioxide
in environments as diverse as bottling plants and greenhouses, and
classrooms.The lack of small, reliable and inexpensive instruments
has made CO2 measurement difficult and costly. Furthermore, carbon
dioxide is used in many industrial processes, from refining sugar to
hardening molds in foundries. Carbon dioxide is often used to
displace oxygen e.g. in process and packing industries. Fires and
explosions can be prevented by carbon dioxide enveloping.
Potential safety hazards
When the concentration of carbon dioxide rises, people start to feel
tired and listless and have trouble concentrating.With further
increases, CO2 begins to act as an asphyxiant.Carbon dioxide which
is an odorless, colorless gas, displaces air and oxygen in it. CO2
is denser than air, and high concentrations can occur in open pits
and other areas below ground level. Exposure to very high
concentrations of carbondioxide results in unconsciousness or even
death. Occupations where carbon dioxide can rise to dangerous levels
include the brewing and carbonated drink industries,the freezing of
food with dryice, cold storage, cargo ships,and of course plants
where CO2 or dry ice is produced or handled.
Beneficial effects in greenhouses
On the positive side, carbondioxide can enhance plantgrowth. In
greenhouses, the growth rate and development of plants, ranging from
tomatoes to roses, can be improved by controlling the concentration
of carbon dioxide. This raises the productivity and quality of the
crops. To reduce the carbon dioxide consumption and to maximize the
productivity, the CO2 level is typically monitored and measured. If
the carbon dioxide level rises too high, the plants can be damaged
and their growth stunted
Corn Stoves and Air Quality Control
Recirculation of filtered room air through the extreme internal heat
of the corn stove kills germs, mold and mildew improving the quality
of the air. The technical reason to keep the corn stove running
while gone to work is shown below. The technical article on Air
Quality Control demonstrates the importance of the corn stove
maintaining 40-60% relative humidity. The partial pressure of water
vapor equalizes from the outside to the inside. Corn stoves
effectively drop the % RH by maintaining an increased inside
temperature at all times preventing the outside water vapor pressure
from forcing excess water vapor into the insulation of the walls and
Without the affordable warmth of a corn stove in the winter months,
insulation in the walls and ceiling of a home become saturated with
moisture inside the home. In contrast, moisture from outside the
home infiltrates the insulation of a home during the summer months
and will condense inside the walls and between double panes windows
unless it is removed prior to the cool winter weather. The vapor
barrier is installed on the interior side of the wall to prevent
inside moisture from condensing inside the walls during winter
months. The vapor barrier is actually counterproductive in the
summer months trapping outside moisture inside the walls. Moisture
condenses in contact with a cold surface such as a glass window pane
or the cold surface inside the wall. Moisture trapped inside the
walls during summer months, if allowed to remain, will condense
during cold weather and result in molds, mildews, and eventually
structural damage. The warmth from the corn stove takes
approximately two weeks to penetrate the walls and reduce the 70% RH
to 50% RH inside the insulation of the walls and ceiling. A casual
observation notes a distinct lighter wall color at 50% RH as
compared to 70% RH.
Keep the corn stove running to keep the walls warm and dry. Dry
walls are insulators from the outside cold and reduce the energy
bill. Wet walls are conductors, promote heat transfer to the
outside, and increase the energy bill. The "R" value of the wall,
ceiling, and floor was conducted under ideal conditions with no
consideration for moisture infiltration. For these reasons, the
corn stove is even more important for super insulated homes than for
the average air infiltrated home. The insulation must be kept dry
in order to be effective.
Caution the article is technical and not well written but the sketch
Air Quality Control
In 2003, hundreds of North Carolina Central University students were
forced to evacuate campus housing as the school spent millions of
dollars to eradicate toxic mold from new dormitories. Meanwhile,
Duke University had to shut down its engineering library to tackle
mold growth in its book collections. Where was the mold coming from?
In a word, pressure. The key to mold and fungal growth is moisture,
and in both cases improper building pressurization resulted in high
internal humidity levels greater than 70% relative humidity that
encouraged mold production and replication within the buildings.
Building pressure has a significant role in water transport across
the building envelope. Control of a building's pressurization flow
is essential for the reduction of mold and fungal growth (see figure
1), but how can it be done cheaply and efficiently?
Airflow/temperature measuring devices are used to maintain dilution
(outside air) flow rates and building pressure essential to
acceptable indoor air quality (IAQ). They are also routinely used in
laboratory and healthcare institutions for contaminant and infection
IAQ can be substantially improved through the proper control of
outdoor air intake flow. Dilution airflow control is essential to
maintaining acceptable contaminant levels. Indoor pollutants are
generated from components within the building as well as its
occupants. Minimum outdoor air ventilation levels must be maintained
to provide adequate dilution. Therefore, direct measurement of
outdoor airflow rates is essential for acceptable IAQ. Verification
is a requirement of both ASHRAE Standard 62, Ventilation for
Acceptable Indoor Air Quality and the International Mechanical Code.
Proper control requires accurate and reliable airflow measurement.