Hi WEI LI,
WUFI has been developed to simulate hygroscopic porous materials. The pore space of these materials contains water both in liquid and in vapour form. The two forms of water content are not independent, their relationship is given by the moisture storage function which describes the amount of liquid water which is in equilibrium with a given relative humidity of the pore air. The moisture storage functions of the various materials are slightly temperature-dependent; in WUFI this temperature-dependence is ignored and moisture storage functions are assumed to be temperature-independent.
The water contents reported by WUFI as the simulation results are only the liquid
water contents.The water vapour in the unfilled pore space is not part of the output since it is usually of no interest to the user, and given the low porosities of most building materials, the vapour mass is negligible when compared to the mass of the liquid water content.
However, even though the vapour portion of the total water content is usually small, the interaction between liquid and vapor must be taken into account if any change of the water content is to be modelled. If relative humidity and temperature in a numerical grid element change, WUFI takes into account
- the change of liquid water content which corresponds to the change of relative humidity (via the moisture storage function),
- the change of vapour content in the air-filled part of the pore volume which is caused by the change of relative humidity,
- the change of vapour content in the air-filled part of the pore volume which is caused by the change in the air-filled portion of the volume when the liquid-filled portion of the volume changes,
- the change of vapour content in the air-filled part of the pore volume which is caused by the temperature-depencende of the saturation vapour pressure.
So even if the component is in a vapour-tight enclosure, because of the last item a temperature change in the material will cause a change of the vapour content in the pore air, which in turn will cause some evaporation or condensation of liquid water. In other words, although in this situation the total
water content in the component must remain constant, the relative amounts of liquid and vapour will change because of the temperature change. In materials with low porosity (= small amount of vapour) the effect will be small, in materials with higher porosity, such as the air layers, the effect will be larger.
Since WUFI's output only reports the liquid
water content, you will see a change in the reported water content, caused by temperature changes, even though the total mass of water (liquid and vapour) remains the same.
Since the moisture storage function describes the amount of liquid water and air layers usually contain no liquid water, their moisture storage function should be zero. However, then WUFI would report zero as the water content and users would be confused because they expect some moisture in the air. Also, since WUFI treats air layers in the same way as porous hygroscopic materials, a (liquid) water content zero would also imply relative humidity zero and WUFI couldn't even compute a non-zero vapour
content for the air layers.
As a makeshift solution, we have given the air layers a small moisture storage function which is close to what's expected as a typical vapour content (a few grams per m³) so that the water contents reported by WUFI are not completely unrealistic. However, this means that an air layer now has both a "reservoir" of liquid water (described by the moisture storage function) and the normal vapour content (implicitly taken into account by WUFI during the simulation). If the relative humidity or the temperature changes, part of that change will be compensated by evaporation from or condensation on that fictitious "reservoir" of liquid water, so that you will see some change in the reported amount of (liquid) water, but that change will not really be the change expected in a real air layer (which has no "reservoir"). That's why we warn users not to rely on the water contents reported for the air layers while the reported relative humidities should be more realistic. Future WUFI versions will have a dedicated material type "air" which implements all the relevant moisture transport and storage processes as appropriate for air without needing a fictitious moisture storage function.