Propagation/Greenhouse Management
Part 1 – 116 | Unit 1.3
- Physical methods for heating and cooling
Additional heating efficiency and cooling can be achieved through physical/mechanical
means such as the use of shade cloth, white washing, and energy curtains
a) Shade cloth can be purchased in a range of shade densities and can be installed on the
interior or exterior of the greenhouse. Shade cloth reduces light intensity and thus heat
from solar radiation. Relatively inexpensive materials can provide years of service and
reduced cooling costs. However, excess shade for sun-loving crops can lead to weak,
leggy growth that will be more vulnerable to pests and to damage by winds and frosts
when transplanted.
b) White washing, a traditional method of reducing light intensity and temperatures,
reflects solar energy away from the greenhouse, thus reducing interior heat and the
need for cooling. Very inexpensive, but must be removed in the winter months to
improve solar heating potential and reapplied the following season. As with shade
cloth, reduced light levels can lead to weak, leggy growth in some crops.
c) Energy curtains, are the most expensive but most versatile of these physical/mechanical
tools. Energy curtains are retractable coverings, made of either plastics or aluminized
polyesters. When deployed, they trap an insulating layer of air between the crops and
the greenhouse roofing; they reduce the total volume of air that must be heated to
satisfy crop requirements and the metallic fabrics heat energy back into the crop zone.
Additionally, on hot, sunny days, they can be deployed to act as a shade barrier, thus
reducing greenhouse temperatures and the need for additional cooling. Energy curtains
can cost several dollars per square foot to purchase and install, but with every rising
energy costs, the improved energy efficiency they provide can be recouped in as little as
two to three years.
C. Air Circulation
- Active air circulation moves cooler exterior air into the greenhouse to keep temperatures
down. Simultaneously, air movement induces evaporative cooling when plants respire and
when humidity in the air evaporates and absorbs local heat energy.
a) Exhaust fans should be sized according to your climate, crop needs, and the size of the
structure requiring ventilation
i. Propeller-type exhaust fans should be large enough to exchange the entire interior
air volume in just one minute. While this might sound extreme, this is the standard
for active ventilation and can normally prevent interior air from being more than 10ºF
above exterior temperatures.
ii. Inlet vents and pad and fan type cooling systems are normally positioned on the
windward side of the greenhouse to maximize the potential for the movement of
exterior air into the greenhouse
iii. Exhaust fans are normally positioned on the leeward side of the greenhouse to
maximize their potential to move heated air out of the structure
b) Horizontal Air Flow (HAF) Fans, are usually 1-3” in diameter and are attached to the
greenhouse structure at the height of the top of the side walls
i. HAF fans serve to provide consistent air circulation even when the greenhouse is
tightly closed to retain internal heat
ii. HAF fans help reduce excess humidity in the greenhouse, especially when
condensation builds up over night, helping reduce the incidence of fungal issues
iii. HAF fans are normally suspended no more than 50-80” apart and work best when
positioned so that fans on opposite sides of the greenhouse are blowing air in
opposite directions, thus creating a circular movement pattern
iv. HAF fans add little extra air movement beyond what passive and active air circulation
systems provide and are not normally powered on when venting and fans are in use
Lecture 3: Greenhouse Climate Control Systems