If you are using air-permeable insulation like cellulose I think you'd want to comply with the IBC's 1188.8.131.52 in that case, right?:
Insulation shall be located in accordance with the following:
5.1: Item 5.1.1, 5.1.2, 5.1.3 or 5.1.4 shall be met, depending on the air permeability of the insulation directly under the structural roof sheathing.
5.1.1: Where only
is provided, it shall be applied in direct contact with the underside of the structural roof sheathing.
5.1.2: Where air-permeable insulation is provided inside the building thermal envelope, it shall be installed in accordance with Item 5.1.1. In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing in accordance with the R-values in
for condensation control.
5.1.4: Alternatively, sufficient rigid board or sheet insulation shall be installed directly above the structural roof sheathing to maintain the monthly average temperature of the underside of the structural roof sheathing above 45°F (7°C). For calculation purposes, an interior air temperature of 68°F (20°C) is assumed and the exterior air temperature is assumed to be the monthly average outside air temperature of the three coldest months.
I don't think (?) that the green roof changes the vapor-permeability of the roof does it? Any roof membrane would be impermeable, and so would the green roof once all the control layers are in place. So you need to be sure you a) ensure against condensate formation as per 5.1.4 and then b) ensure it can dry to the inside. Are you doing a smart-vapor retarder on the underside of the joists?
When we do that 5.1.4 calc for NYC, I think we use 0C (average of the coldest 3 months) if I remember right.