Greenhouse Cooling Calculator

Required Airflow follows the National Greenhouse Manufacturers Association's own published "Ventilation & Cooling" standard, which sets the summer design rate at 8 cubic feet per minute of exhaust air for every square foot of floor area -- NGMA's design equation is literally Total CFM = 8 x L x W. That base rate is written for a house at least 100 feet long, an 8-foot eave, near sea level, and up to 5,000 foot-candles of interior light, so this calculator corrects it three ways -- short houses get more air per square foot (the air has less run in which to pick up heat), taller eaves scale the rate up proportionally, and Light Intensity scales it against the 5,000 foot-candle reference. Length, Width, and Sidewall Height therefore all raise Required Airflow, as does Light Intensity. Outside Temperature and Desired Inside Temp deliberately do NOT change the airflow rate: exhaust fans can only pull outdoor air through the house, so they can never bring it below outdoor temperature no matter how many you install.

Those two inputs instead set the Cooling Gap to Close -- the temperature drop your evaporative pads have to supply. Fans Needed rounds Required Airflow up to the next whole fan, so it moves in steps. Cooling Pad Area is Required Airflow divided by a 250 ft/min face velocity (the standard for 4-inch cellulose pads), and Water Usage is 0.5 GPM per linear foot of that pad -- the recirculating-pump flow, not the evaporative makeup rate. This calculator does not account for glazing type, shade cloth, elevation above about 1,000 feet, or the wet-bulb depression that determines how much of the cooling gap the pads can actually close.