Air Layer Basics

[bek15212]

I am a little confused about air layers so I hope someone can help me out.

There are two types of air layers listed - One with no unique conditions and one "without additional moisture capacity".

When should one choose one over the other?

For example would I choose the simple 70mm (I'm assuming with additional moisture capacity) for the cavity at a brick veneer wall
and the 70mm without additional moisture capacity for the air space in a stud cavity?

What is the difference? Can you give examples?

I find I get the same error message as Majid if I use the plain 70mm, because the porosity is so high the calculation probably returns an untenable result.

Also, the air layers are given in specific thickness - can we modify these thicknesses without throwing a bug in the calculations?

Thanks

[Amandine Piot]

Hi Bek,

the difference between the two types of air layer is described in the end of the help menu "edit hygrothermal sources":

Important note:
Air change sources are often placed into air layers. The moisture released or absorbed by a moisture or air change source will result in the correct change of the relative humidity in the air layer only if the air has an appropriate moisture storage function.

Some of the air layers in the material database are only meant to act as a resistance for heat or moisture flows. They have correct heat conductivities and vapor permeabilities, but they use WUFI's default moisture storage function, which is automatically used whenever no specific function has been entered for a material. This default function corresponds to the moisture contents of mineral wool and is thus too high for air. The free saturation of an air layer with this default function is about 47 kg/m³, while in reality saturated air only contains a few g/m³ (e.g. about 17 g/m³ at 20°C). These air layers may be used when their moisture content is of no further interest, or when droplets condensing on the surfaces confining the air layer shall be simulated.

However, these air layers behave much too sluggish when exposed to an air change source: a source which removes, say, 2 g/m³ of moisture from a saturated air layer will reduce the relative humidity of realistic air to zero within a few hours; this will take more than 2700 times longer when the saturated air layer is assumed to contain initially 47 kg/m³ of moisture.
If you wish to investigate the hygric behavior of an air layer responding to a moisture or to an air change source, you must therefore use a more realistic moisture storage function. To this end, you may enter a simple linear function, starting with a moisture content of 0 kg/m³ and reaching saturation at e.g. 17 g/m³ (or at another saturation corresponding to whichever temperature is representative for the case to be investigated):

? [-] w [kg/m³]
0 0
1 0.017

This still does not allow for the temperature dependence of the moisture storage function of a realistic air layer, but at least the moisture level is at the correct order of magnitude. If such a low moisture storage function creates numerical problems, the adaptive time step control may be able to eliminate them.

This means that, previously, the air was just intended as a resistive layer, without its own sorption curve. WUFI uses automatically its "internal sorption curve", which is much too high for air. The "new" air layers, "without additional moisture capacity" have their own sorption curve, much lower.
You should however be carefull with the porosity: if you have convergence problems, you may need to increase it to 0,99.

Concerning the thickness of the layer, you can change it, but you should take the material that is the closest to your actual air layer: the heat conductivity and vapour resistance factor have been adjusted in order to take into account the effect of convection in no ventilated air layers.

[bek15212]

Thank you