Air Layers problem

[WEI LI]

Hi WUFI team,

• The Help document says if dew droplets condensing at the surfaces of the neighbouring materials shall be modelled, then the conventional air layer (the one with additional moisture capacity) should be used.
  I am not quite sure about the reason. In my opinion, we can also use the air layer without additional moisture capacity, and judge if there is condensation by examining the RH of the air layer. Please correct me if I am wrong.
  Currently, I wanna investigate the condensation issue on the roof cavity surface, which is the surface between the metal deck and the air layer. The structure is like "metal deck - 30 mm air layer - PIR board".
  In WUFI, the geometry should be like

  "metal deck - 30 mm air layer (without additional moisture content) - PIR board"

  or

  "metal deck - 1mm conventional air layer - 30 mm air layer - PIR board"

  ?

• I know that the only difference between the metallic and non-metallic air layer is the thermal conductivity. The thermal conductivity of the air layer in WUFI should be able to take care of not only heat conduction but also convection and radiation. If radiation be the root of this difference? If so, does it mean in metallic structure, the radiation is not affecting the air layer, but in non-metallic structure, the radiation is playing a role in the air layer?

Thanks a lot in advance.

[Thomas]

does it mean in metallic structure, the radiation is not affecting the air layer, but in non-metallic structure, the radiation is playing a role in the air layer?

Hi WEI LI,

the metallic or non-metallic nature of the surfaces does not affect the properties of the air itself. But there will be radiative heat exchange between the surfaces and if the two surfaces have different temperatures, there will be a net heat flow. The current versions of WUFI cannot take long-wave radiation inside the component into account, so this radiative heat flow - which occurs in addition to the normal conductive and convective heat flow through the air layer - is taken into account by an appropriate increase of the thermal conductivity which is assigned to the air.

Non-metallic surfaces cause a higher radiative heat flow, metallic surfaces cause a lower heat flow, so depending on the radiative properties of the surfaces the effective thermal conductivity assigned to the air will be different.

Kind regards,
Thomas

[WEI LI]

That is crystal clear. Big thanks, Thomas.
Could you please also check my latest post in WUFI 2D. Cheers.