Greenhouses trap in heat in the same way that the Earth’s atmosphere does. Sunlight passes through the glass of the greenhouse and gets absorbed by the plants therein and the greenhouse’s floor, where it gets converted to heat.
The heat is unable to escape the glass, so instead it stays inside the greenhouse, thus keeping 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 then rises to the top, where it 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 then the outside during the day – even in wintertime – and also why then retain their heat throughout the night.
The Greenhouse Effect and Convection in Action
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.
These types of sunlight can get in, but heat, which is a type of energy somewhat different from light, can not get out.
This is a process known as the greenhouse effect, and this is also what keeps the Earth as warm as it is as compared to the coldness of space.
Greenhouse glass is true glass, but a special type of plastic called polycarbonate plastic is also used in greenhouse construction for the same purpose because it also exhibits the greenhouse effect and it does not interfere with convection. Again, this plastic must be transparent for this process to work properly.
Glass Greenhouses vs. Plastic Greenhouses
Some greenhouses use plastic and some use glass. Plastic greenhouses are lighter and thus easier to transport and they are also less expensive, but glass greenhouses tend to last longer because they are more durable when it comes to age and weathering.
Glass greenhouses are thus more permanent and 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 such as potential moving but also weathering such as high winds, hail and heavy snow.
Greenhouses with Low-E Glazing
Sometimes greenhouse glass is also coated in a material known as low-E glazing.
This is a thin layer of metal that is thin and semi-transparent so as to still let light in and to not be too heavy or expensive.
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.
Gardening in a Greenhouse
Additionally, low-E glazing reduces some of the UV radiation that can damage your skin when you are gardening in your greenhouse, especially when you are doing so for long hours or during the midday in summertime.
Even with low-E glazing, however, it is still a good idea to wear sun protection such as sunscreen, sunglasses and protective clothing if you are gardening under these conditions.
Think of gardening in a greenhouse as being just like gardening outdoors in terms of UV radiation, and often in terms of temperature as well: be sure to stay hydrated during intensive greenhouse gardening work.
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