SebSta: hat „FAQ General:RelativeHumidityinaBuildingComponent“ nach „FAQ:General:RelativeHumidityinaBuildingComponent“ verschoben

2013-06-13T12:25:36Z

hat „FAQ General:RelativeHumidityinaBuildingComponent“ nach „FAQ:General:RelativeHumidityinaBuildingComponent“ verschoben

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Len: Die Seite wurde neu angelegt: = (4): Relative Humidity in a Building Component = I'm trying to make sense of the WUFI results, but I'm confused. What exactly is 'relative humidity' and what is t...

2008-10-02T07:59:26Z

Die Seite wurde neu angelegt: = (4): Relative Humidity in a Building Component = I'm trying to make sense of the WUFI results, but I'm confused. What exactly is 'relative humidity' and what is t...

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= (4): Relative Humidity in a Building Component =

I'm trying to make sense of the WUFI results, but I'm confused. What exactly is
'relative humidity' and what is the relative humidity in a building component
referred to? 


In air the relative humidity is the ratio of the actual water vapor partial pressure
p and the water vapor saturation pressure ps. Example: If the air
temperature is 20°C (and therefore ps = 2340 Pa) and the actual
vapor pressure is 1872 Pa, then the relative humidity is 1872 Pa / 2340 Pa = 0.8 = 80%.


The condition in a porous building material corresponds to a RH of x % if it has been
exposed to air with a RH of x % until equilibrium was reached and no moisture was taken
up or given off any more. 
The moisture in the material is then in equilibrium with the RH of the air in the
pore spaces. At RHs less than ca. 50% this means that a molecular layer with a
thickness of one or a few molecules has been adsorbed at the surfaces of the pores;
at higher RHs capillary condensation occurs.


Here is what happens in detail: the usual formulas for the saturation vapor pressure
(such as in German standard DIN 4108) are only valid for plane water surfaces. At
concavely curved surfaces, where the water molecules are bound stronger, the saturation
vapor pressure is reduced; the more so the stronger the curvature of the surface is.


In a partly filled capillary the interface surface between air and water forms
a curved meniscus whose curvature is determined by the surface energies involved
and in particular by the radius of the capillary. If the air space in such a
capillary is filled with air whose partial water vapor pressure is greater than the
saturation vapor pressure at the meniscus (whereas the RH in the air is still less than
100%), then the air in the immediate neighborhood of the meniscus is supersaturated
and water condenses from the air onto the meniscus, i.e. the capillary fills up.


In a porous material there exists a wide range of pore sizes. In the smallest pores,
any menisci may be curved so strongly that in these pores moisture condenses onto
the menisci from 50% RH in the pore air upwards. The smallest pores get filled
with water, and subsequently larger and larger pores (with smaller curvatures of
the menisci) get filled until a pore size is reached where - because of the larger
pore size and the smaller curvature of the meniscus - the saturation vapor pressure
at the meniscus is equal to the vapor pressure in the pore air. In this way capillary
condensation results in an equilibrium between the moisture content and the relative
humidity in the pore air, even if this RH is less than 100%. The amount of water needed
to fill the pores up to this point depends on the pore structure and the pore
size distribution.


The [[Details:MoistureStorageFunction | moisture storage function]] describes the
amount of moisture taken up in this manner by the building material if it is exposed
to air with a specific RH. Since this relationship between RH and moisture content
is largely temperature-independent, the RH is an important and unique parameter
describing the moisture content of a material.

FAQ:General:RelativeHumidityinaBuildingComponent - Versionsgeschichte

SebSta: hat „FAQ General:RelativeHumidityinaBuildingComponent“ nach „FAQ:General:RelativeHumidityinaBuildingComponent“ verschoben

Len: Die Seite wurde neu angelegt: = (4): Relative Humidity in a Building Component = I'm trying to make sense of the WUFI results, but I'm confused. What exactly is 'relative humidity' and what is t...