Air exchange rate

[Houri]

I am working on an exterior air gap of a wall system and when I want to add
a new air exchange source, it requires a constant rate or a transient
source file. Any comments that what could be used for each of these
sources? I am modelling the wall for the US climate.

[Philipp]

The air change rate in ventilated claddings strongly depends on the opening on top and bottom. Most of the time no information about these openings and the flow resistances are available. In this case we simulate two different air change rates. One very small (e.g. 0.5 1/h) and one strong (e.g. 30-100 1/h). When both cases leading to a functional construction, the air chance rate is not a strong influencing parameter and the construction is likely robust enough to resist the real climate conditions.
 
Another possibility is to calculate your own air change rate, based on the hourly mean wind speed and use a transient file. After the following equation you can estimate the ACH. A is the Area of the air gap behind the cladding 1m x the width of the air gap and b is the height of your wall. But keep in mind, here are no flow resistances included and the wind direction also plays a role in reality. So treat this with caution, it seems more appropriate to simulate two constant ACH.