Fundamentals of Psychrometry (part II)

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Here are some other terms you should get accustomed to...

Dew Point

   The temperature at which water vapor begins to condense is known as the dew piont.  When air is saturated with water vapor, a drop in temperature forces the air mass to release some of its moisture in the form of condensation.  For example, on a warm humid morning the windows of a car are covered with dew or water droplets.  The glass is cold enough to cool the air contacting the glass to the dew point, resulting in condensation.  When drying a structure, you must never allow the inside temperature to reach the dew point or else condensation will occur on inside surfaces.  Maintain inside temperatures using the HVAC system or portable heaters.

Vapor Pressure

   Vapor pressure is the pressure that all substances in a gaseous state exert on surrounding surfaces.  The amount of pressure is determined by the concentration of gases, such as water vapor, in proportion to the area in which it is contained.  A structure along with its contents and air space will dry faster when vapor pressure is lower.  Moisture moves from areas of high water vapor pressure to areas of low water vapor pressure, seeking an equilibrium within the surrounding environment.  As the vapor pressure of the air inside a building decreases, moisture will move from building surfaces into the air space.  Vapor pressure increases as specific humidity increases.


   Permeance is the measure of water vapor flow through a material of a specific thickness or through layers of several materials.  The unit of permeance (ferferred to as "perms") states the amount of vapor flow in grains per hour, through one square foot of surface, when vapor pressure is at one HG.  (1" of Mercury).  Highly porous materials, such as carpets and upholsttery, have high permeance factors, meaning they absorb and release moisture easily.  Nonporous materials, such as plastics, glass, and vinyl, have low permeance factors and so absorb little moisture,

Vapor Barriers

   Vapor barriers are materials that are either impermeable or have a permeance of 1 perm or less.  (A material having a permeance of 1 perm means that during one hour only 1 grain of water vapor could pass through 1 square foot of that materials's surface.)  In other words, a vapor barrier is a material that moisture either cannot pass through at all or cannot pass through quicklyu when it does.  Vinyl wallpaper is one example of a vapor barrier.

Hygroscopic Materials

   Hygroscopic refers to materials that readily absorb and retain water vapor from the air.  Materials that readily absorb water vapor also release it readily under approbpiate drying conditions.  Such materials include carpet, insulation and upholstery.  If the relative humidity rises above 60% in a water damage event, hygroscopic materials can be damaged quickly, since they absorb the mositure so readily and swell.

Equilibrium Moisture Content (EMC)

   Equilibrium moisture content (EMC) is the condition of a material whose moisture content has stabilized in relation to the relative humidity and temperature of the surrounding air.  At EMC a material no longer absorbs or releases moisture at the current relative humidity and temperature.

   The equilibrium moisture content (EMC) of wood materials is that moisture content at which the wood neither gaining nor losing moisture.  The wood is at equilibrium with the condition of the air.  Both the relative humidity and the temperature of surrounding air affect the amount of moisture wood materials can hold and still remain at a condition of EMC.  For example, wood materials within a structure should be between 7.7% and 11% moisture content when indoor conditions are between 40% and 60% RH and above 70 degrees F.  On the other hand, when indoor condtions are between 30% and 50% RH and about 80 degrees F, the EMC of wood materials should be between 6.1% and 9.1%.  The average moisture content of structural wood in North America is 10%

   The length of time that moisture remains in an environment inluences the euqilibrium moisture content of wood.  When the water damage lasts a short duration, the wood's equilibrium moisture conten could be as high as 20% to 25%.  When water stands in a structure, however, wood materials can become waterlogged, with equilbrium moisture content readings increasing as high as 38% to 40%.

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