From GridLAB-D Wiki
air_volume – Volume of air.
Synopsis
module residential; class house { double air_volume[cf]; } object house { air_volume value cf; }
Default
If no value is input for air_volume, it is calculated by:
air_volume = ceiling_height * floor_area
Using default values produces:
<math>8\text{ft} \cdot 2500\text{ft}^2 = 20,000\text{ft}^3 </math>
Therefore the default air_volume is 20,000 ft<math>^3</math>