Thursday, 12 January 2012

Moist air thermodynamic

Enviromental comfort is one of the most important objectives in buildings, threatened by thermal bridges, defined as the preferential way for the heat loss. There are many blogs that already talk about it, so it isn't an issue I would discuss. However I think it's better to spend few words for the topic in the title, because it's very important in any decision planning .

There are two different types of thermal bridges, geometric and structural ones. The geometric one doesn't depend on the type of material or isolation, but only on the geometry (for example the room corners). The structural bridge depends on the presence of different materials in the structural elements  (for example a concrete pillar in a brick wall). In both cases, however, there is an area at a lower temperature. The problem is not just a matter of energy loss, but also increase the possibility of condensation that can generate an unhealthy mold.

How can form the surface condensation?

We must mention some elements of moist air thermodynamic. First of all the air may contain an amount of water vapor, and this quantity depends on both temperature and pressure (therefore it has two variables).

At constant pressure, if the air temperature increases also increases the maximum amount of water content ("saturation"). In those conditions if the humidity goes up, there is condensation (or fog). For condensation we have a temperature called "dew point", derived using a diagram knowing the air temperature and relative humidity (Mollier diagram).
Normally the air is not saturated and we consider relative humidity (Rh), expressed as a percentage compared to saturation humidity. This depends on human activities and on the amount of water vapor that the building is able to evacuate by walls or windows.
We have an analogous case with the constant temperature; the pressure of water vapor contents is called "partial pressure" (Pv), and the maximum is called "saturation pressure" (Ps), whose values are tabulated. Their ratio (Pv / Ps)  expresses the percentage relative humidity

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