Thermal Bridge Heat Transfer & Vapour Diffusion Simulation Program AnTherm Version 6.115 - 8.133 

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On condensing humidity (dew point)

A practical measurement indicator for the amount of vapour contained within the wet air of some specific temperature is given by the relative humidity. It can be measured directly and very precisely with simple instruments - e.g. the hair hygrometer. When wet air wipes some surface of which the temperature is lower then that of the air, then it will show the vapour condensation on that surface when its temperature falls below the dew point temperature of the air. The dew point temperature is set by the value of temperature at which wet air of some specific vapour content will be saturated (by holding other parameters constant), i.e. the relative humidity is 100%.

The dew point temperature is simultaneously the degree of air moisture. Unfortunately it is not easy to measure (e.g. with a dew point thermometer) and by that it has not been adopted in practical use. De facto the dew point provides an ideal indicator to assess condensation risk at building component surfaces. If the dew point temperature is below the actual surface temperature there will be no condensation. To reliably preclude condensation the temperature at any place of the interior surface must be higher than the current dew point temperature of the air.

In practice, however, one will rarely have an idea about the dew point, rather then, on the other hand, about the temperature and relative air humidity. For that reason the term of "condensing humidity" is introduced when assessing surface temperatures in the context of condensation risk.

The "condensing humidity" is the value of relative air humidity at which, when it is exceeded, the vapour condensation will occur on the surface having some specific temperature. It can be calculated by a quotient of saturated vapour pressure at surface temperature and the pressure of saturation at specific air temperature.

The introduction of condensing humidity provides direct normative assessment of the temperature field calculated for some construction. The ÖNorm B8110-2:2003 defines the standard interior climate for residential spaces as follows: 20 °C air temperature and 65% relative air humidity when exterior temperature is 0° C. For negative temperatures of exterior air the maximum allowed humidity for the interior space air has to be reduced by 1% by each degree of exterior air temperature decrease. The condensation risk must be concluded only when the "condensing humidity" calculated for the temperature of the coldest point at the surface goes below the one set within the standard.

Example: When the standardized temperature of exterior air is set for the calculation to -12°C, then the relative humidity of the interior air (at 20°C temperature) shell never go above 65 – 12 = 53%.
If the "condensing humidity" calculated for the coldest surface point is above 53%, then, concluding the ÖNorm B8110-2:2003, there is no condensation risk.
If this is not the case, the construction assessed must be considered non conformant with standards. The areas affected by the condensation risk can be easily made visible by drawing the isolinie of 53% of condensing humidity at that surface.
  Isoline of condensing humidity at 53% (condensation risk assessment)

See also: Condensing Humidity Calculator (Tool), Results report, Selected evaluation function

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2017-03-22 10:19 +0100