When I create a 3 foot by 3 foot box in OpenStudio and in the EnergyPlus Idf editor create a say, 10 cm thick wall for each surface of the box. Where is the wall created? As in,
a) Is the air volume inside 3 X 3 X3 = 9 Ft cubed
OR
b) The inside volume is 2.9 X 2.9 X 2.9 = 24.39 Ft cubed?
OR
c) The inside air volume is 2.95 X 2.95 X 2.95 = 25.67 ft cubed? (Assuming the walls are build right in the middle)