Density of Dry Air at 20 °C

Air density follows the ideal gas law closely enough that you can compute it rather than look it up: ρ = p/(R T) with R = 287.05 J/(kg·K) for dry air. At 20 °C and one atmosphere that gives 1.204 kg/m³, or 0.0752 lb/ft³: the "standard air" of North American HVAC, usually quoted as 0.075 lb/ft³ at 70 °F. Every familiar duct constant descends from it: 1.08 for sensible heat, 4.5 for total heat, 0.68 for latent.

The trap is altitude and temperature. Air at 1500 m is about 16 % less dense than at sea level, and a furnace flue at 200 °C is 40 % of standard density. A fan is a constant-volume machine, so it still moves the same cfm, but it moves proportionally less mass, delivers proportionally less heat, and draws proportionally less brake horsepower. Denver-based CFM ratings and sea-level heat calculations are the classic mismatch. Humidity moves it the other way, slightly: moist air is lighter than dry air at the same temperature and pressure because water vapour is lighter than the nitrogen and oxygen it displaces.