A common rule for greenhouse fan and pad cooling is one air change per minute in summer: the fans should move the volume of the greenhouse every minute. Pad area then follows from the fan airflow and the speed of air through the pad, about 1.25 m/s for 100 mm pads and 1.8 m/s for 150 mm pads.
How fan and pad cooling works
Exhaust fans on one end wall pull air out. Outside air enters through wet cellulose pads on the opposite wall and cools as the water evaporates. The cooled, more humid air crosses the crop and leaves through the fans.
Step 1: greenhouse volume
Length × width × average height. A polyhouse 40 m × 20 m with an average height of 4 m holds 3,200 m³.
Step 2: fan airflow
One air change a minute means 3,200 m³ per minute, or 192,000 m³/h.
- 54 inch fans at 44,000 m³/h: 192,000 ÷ 44,000 = 4.4, so 5 fans
- 40 inch fans at 23,000 m³/h: 8.3, so 9 fans
Step 3: pad area
Five 54 inch fans move 220,000 m³/h, which is 61.1 m³/s.
- 100 mm pads at 1.25 m/s: 61.1 ÷ 1.25 = about 49 m²
- 150 mm pads at 1.8 m/s: 61.1 ÷ 1.8 = about 34 m²
On a 20 m wide end wall, 34 m² means a pad band about 1.7 m high.
Our fan and pad calculator covers the pad step; enter your own figures.
Layout rules
- Pads on the wall facing the prevailing summer wind where possible, fans on the opposite wall
- Keep the distance from pads to fans moderate; very long houses warm up towards the fan end
- Seal doors, vents and gaps so air enters only through the pads
- Space fans evenly across the wall
Water system
A pad wall needs a distribution pipe along the top, a gutter, a tank, a pump and a filter. We supply PVC and aluminium framing with the full kit.
Adjusting the rule
Shade nets, crop type, altitude and local climate change the design. Crops that need lower temperatures, or very hot regions, may need more airflow; shaded houses may need less. Treat this as a first estimate and ask us to check it.
See greenhouse cooling and cooling pads.
Frequently asked questions
How many exhaust fans does a greenhouse need?
A common rule is one air change per minute: greenhouse volume in m³ × 60 gives m³/h, divided by the airflow of one fan.
How much cooling pad does a greenhouse need?
Total fan airflow in m³/s divided by about 1.25 m/s for 100 mm pads or 1.8 m/s for 150 mm pads.
Which wall should the pads go on?
The wall facing the prevailing summer wind, with fans on the opposite wall.