FAQ:General:VentilatedCurtainWalls

(19): Ventilated Curtain Walls

I want to simulate a ventilated curtain wall; how can I do this? I can model the air gap as an air layer in WUFI but it seems these air layers are assumed to be stagnant, which is certainly not the case in my ventilation gap.

If you model the ventilation gap as an air layer in WUFI, it is indeed treated as a closed air layer without connection to the exterior air. The effect of inner convection on heat and moisture transport across the air layer is allowed for (as a first approximation) by use of effective heat conductivities and vapor diffusion resistance factors.

The air flow and air exchange phenomena in a ventilated air layer cannot be simulated with a one-dimensional program like WUFI-1D; WUFI-2D currently does not take air flows into account.
If the air exchange is large enough, it may be justified to assume exterior air conditions in the air gap. That is, you do not model the curtain facade and the air gap, and you consider the surface of the insulation or the wall itself (as the case may be) as the exterior surface in WUFI's component assembly. Rain must be set to zero (simply by setting the rain absorption factor = 0).
It will be advisable to choose appropriate effective values for the exterior heat transfer coefficient and the short-wave solar absorptivity, but this requires calibration by experimental data.

The same problem is encountered in simulations of roofs, either because of a ventilation cavity in the roof or because of the question how to model the covering and the batten space.

The investigations described in [1] used a simplified treatment of a roof. WUFI-1D simulations were carried out to examine the moisture balance in a fully insulated west-facing pitched roof (50° inclination). The covering and the batten space could be omitted from the simulated assembly because measured temperatures in a similar roof on IBP's testing area were available and could be used to determine appropriate effective surface transfer coefficients. The measurements were taken on the waterproofing foil (i.e. directly on the insulation layer) and were compared with the computed temperatures at the outer surface of the modeled insulation layer which sufficed to represent the whole roof for the purpose of a thermal adjustment.
The thermal surface transfer coefficients were adjusted in WUFI until good agreement between measurement and calculation was reached. This was the case with an effective short-wave absorptivity of as=0.6 and an effective heat transfer coefficient of a=19 W/m²K. The effective absorptivity is roughly identical with the real absorptivity (for red roof tiles), while the effective a is slightly higher than the usual standard value of 17 W/m²K. Obviously the covering and the air in the batten space have no major effect on the thermal behavior of the roof, at least in this case. In particular, the amount of heat removed by convection through the ventilated air cavity seems negligible and the entire heat created in the covering by solar radiation is passed on into the underlay.
The question to which extent this isolated result can be generalised could only be answered by more extensive comparisons with measurements.

[1] H.M. Künzel: Außen dampfdicht, vollgedämmt? - Die rechnerische Simulation gibt Hinweise zu dem Feuchteverhalten außen dampfdichter Steildächer. bauen mit holz 8/98, S. 36-41.