Hi Alberto,
the science behind those numbers is this: Air at rest has a thermal conductivity of about 0.025 W/(mK) and a vapor diffusion resistance factor of µ=1 (by definition). However, the air in an air gap of a building component is usually not completely at rest. Temperature differences across the air layer will cause some convection in the air, and both heat and moisture transport are increased by this additional convective component. The values for thermal conductivity and diffusion resistance have been adapted to include this additional heat and moisture transport. There is also some radiative heat transport across the air layer which is included as well.
The data for the air layers in WUFI's material database have been taken from a table in a (then) standard text on building physics which unfortunately only represents air layers up to 150 mm. As you have seen yourself, extrapolating these data to larger thicknesses is not straightforward. There are formulas, based on similarity theory, which can be used to calculate these values for arbitrary air layer thicknesses. Standard EN 673 "Glass in Building" provides an example for such a set of formulas, and future versions of WUFI are planned to have these formulas implemented.
For example, for a horizontal 250 mm air layer with mean air temperature 23°C and mean temperature difference across the layer of 4 °C, the effective heat conductivity and diffusion resistance factor based on EN 673 are
for heat flow upwards (unstable layering): lambda_eff = 1.5 W/(m K), mu_eff = 0.078
for heat flow downwards (stable layering): lambda_eff = 1.2 W/(m K), mu_eff = 1.0
If in your case the heat flow can be expected to be predominantly upwards or downwards, you may choose the respective options. Otherwise, the next best option is probably to use a mean value.
As to the moisture storage function: We usually recommend using the air layers "without additional moisture capacity" as the hygric inertia of these layers is much more realistic. If there is the possibility of condensation on the surfaces of the air gap, a thin layer (1 mm) of the 'normal' air layers (with higher moisture capacity) should be included there which can take up the condensate. For your 250 mm layer this would mean
1 mm air layer
248 mm air layer "without additional moisture storage"
1 mm air layer
Important: ALL these layers should have the SAME lambda_eff and mu_eff as appropriate for a single layer of 250 mm, since they are an integral part of such a layer and the convection is that of a layer of 250 mm. Using the "1 mm" layers from the database would model very thin isolated air gaps with strongly suppressed convection.
For an example, see pages 13 and 14 of the respective
Guideline.
Kind regards,
Thomas
