Ask many people “how does a greenhouse work” and they’ll probably launch themselves into an explanation about Carbon Dioxide in the air and how it traps heat from the sun; and prevents any heat escaping by radiation etc. Such an explanation is, of course, about the ‘Greenhouse effect’ and global warming; rather than actually explaining how does a green house work?
Greenhouses can be built from various materials with the most traditional being a skeleton metal frame, with glass windows fitted into that frame. Modern greenhouses, especially large scale industrial ones, increasingly have plastic windows in either a metal or plastic framework.
The important point about any greenhouse is its large surface area of clear glass or plastic. This large surface area of clear material allows the maximum amount of sunlight possible to enter the greenhouse, resulting in the ground inside the greenhouse becoming thoroughly warmed. The warmed up ground acts like a giant domestic radiator inside the greenhouse, releasing its heat which then heats up the air inside the greenhouse.
From our school science lessons we know that heat can be transferred in three ways; conduction, convection and radiation. So, it would seem quite reasonable at this point to presume that a convection current starts up inside the greenhouse. However, as the greenhouse is in effect a sealed unit and because the suns heat continues to warm the ground and then subsequently the air inside the greenhouse - whilst the warm air may well rise; it won’t cool! So, the answer to the question - how does a greenhouse work - is quite simply by preventing convection.
Controlling Temperature in a Greenhouse
Whilst the whole purpose of a greenhouse is to keep things warm, the same as getting too cold, if plants get too warm they can die off. Subsequently a well designed greenhouse will have ventilators on either side of its roof and one on each of its walls for approximately every 10 foot of length. Having ventilators at a variety of locations is important so that in windy weather you can open the ventilators on the leeward side of the greenhouse, preventing cold draughts from entering the greenhouse.
Of course you won’t want to be having to constantly going to and from your greenhouse altering the vents to maintain a certain level of temperature and humidity. Fitting automatic ventilation will add to the cost of your greenhouse, but will also save you a lot of time later on. Automatic ventilation doesn’t have to require a power line being run up to your greenhouse.
Greenhouse ventilation systems can operate by oil in a cylinder expanding and contracting as the temperature changes, which in turn operates a piston opening or closing the vents. However, if you do run a power line up to your greenhouse you’ll then be able to install electric ventilation that will fully controlled the heat and humidity; as well as operating fans, a heating system for the colder months and automated watering.
Helping your Greenhouse Work
Unless you live in an extremely cold area you will want to try and avoid having to artificially heat your greenhouse, or at least keep the artificial heating to a minimum. The simplest way to do this is to site your greenhouse where it is guaranteed to get the maximum amount of sunlight possible throughout the year.
Whilst not positioning it in the shade of your home or trees will seem obvious; neither should you position it so that it is over-exposed, particularly in a northern aspect as cold winds will have a rapid cooling effect in the winter.
Also, if you don’t already know, aligning the ridge of the greenhouse roof north-south will maximize the sunlight hitting it across a year, having a solid concrete floor will be a better ‘natural heat radiator’ than an earth floor and a 5 foot high hedge 15 feet from the greenhouse will help to protect your greenhouse from wind-chill; even if it was over 20 feet high.
Photo: The Palm House (built 1844-1848) at Kew Gardens, London, England, Creative Commons License