Attempting to model an exterior vented cavity.

Unable to use the vented cavity since I have both exterior and interior insulation. Assembly is: -SBS Bitumen -DensDeck -Polyiso (Rigid Insulation) -Plywood -2" Vented cavity -Roof Insulation -Gypsum

If I model the vented cavity as an airgap, I am unable to see the effects of air movement, If I model as an exterior natural vented cavity, I am unable to model the polyiso (rigid insulation exterior of cavity). I attempted to model it as a small 2" zone, and used AFN to model airflow through the walls, however because the zone is so small, it leads to a malformed zone error.

I have tried changing the zone size, but its still an issue. I have modeled it as 10" in height, and that works (sometimes). Results wont be as accurate because my cavity is 2" not 10".

I have tried object SurfaceProperty:HeatBalanceSourceTerm to heat the interior surface, since the error I am getting is that the surface is too cold. also, not working.

Please advise.