The air density is used for calculating the air volumetric flow rate from air mass flows as part of the loads calculation procedure.
The method used in DesignBuilder is the same as that in EnergyPlus for consistency. Air is assumed to be dry and at 20°C. The site elevation is used to calculate standard atmospheric pressure using the equation for 'standard atmospheric' pressure on p 6.1 of the ASHRAE 1997 HOF.
Density = P / (R * T)
R is the Gas constant = 287.05 J/kg-K
T is the temperature in K. Air is assumed to be at 20°C so, T = (20 + 273.15)
P is standard pressure:
P = 101325 x (1.0 − Z x 0.0000225577)5.2559
Z = Elevation above sea level (m)