Greenhouses trap in heat in the same way that the Earth’s atmosphere does. Sunlight passes through the glass of the greenhouse, gets absorbed by the plants and the greenhouse floor and converts to heat. The heat is unable to escape the glass, stays inside the greenhouse and keeps it warm. A process called convection also assists in keeping greenhouses warm. Since warm air rises, the air on the bottom of the greenhouse that has been heated by sunlight absorption rises to the top and pushes the cooler air there to the bottom. This cooler air is then heated by continuing absorption of sunlight, and the process repeats itself. This is why greenhouses are warmer than the outside during the day – even in wintertime – and also why they retain heat during the night.
Transparent greenhouse glass as opposed to colored glass works best because it lets in all three of the basic components of sunlight: infrared light, visible light and ultraviolet, or UV, light. Plants prefer to receive all three of these different types of light, which combined are known as full spectrum sunlight.
While the sunlight gets in, the heat, cannot get out. This is a process known as the greenhouse effect. Greenhouse glass is true glass, but a special type of plastic called polycarbonate is also used in greenhouse construction. Like glass, polycarbonate does exhibit the greenhouse effect but it does not interfere with convection.
Some greenhouses use plastic and some use glass. Plastic greenhouses are lighter and thus easier to transport and they are also less expensive. Glass greenhouses tend to last longer because they are more durable when it comes to age and weathering, making them more permanent. They are also more expensive investments than plastic greenhouses that are harder to move but can end up paying off over time as they generally do not need to be replaced as often. Glass used in greenhouses is also often tempered for safety reasons.
Tempering is laminate that causes glass to crumble into smaller pieces with smoother edges if it breaks as opposed to large, jagged shards with untempered glass. You may be familiar with tempered glass from its use in other applications such as car windshields. Greenhouse glass should always be tempered because it is exposed to breakage risks like high winds, hail and heavy snow.
Sometimes greenhouse glass is also coated in a material known as low-E glazing. This is a thin layer of metal that is semi-transparent so it will still allow light into the greenhouse. It reflects most of the UV light and also some of the infrared light that hits the greenhouse without affecting visible light input.
The purpose of low-E glazing is to prevent excessive amounts of potentially damaging UV radiation from hitting tender seedlings while still providing enough UV for plants to properly photosynthesize and for the greenhouse effect to still work.
The blocking of some infrared radiation also helps in maintaining a more constant internal temperature in the greenhouse, especially when combined with proper ventilation. Additionally, low-E glazing reduces some of the UV radiation that can damage your skin when you are in the greenhouse. Even with low-E glazing, however, it is still a good idea to wear sun protection like sunscreen, sunglasses and protective clothing.
Background information for this article was provided by DIYGreenhousePlans.info.