Greenhouse glass traps heat energy through the transformation of light waves and convection of air. The sun’s light waves are absorbed by plants and soil, converted into heat energy, and trapped inside. Greenhouse glass can be made of tempered glass or polycarbonate plastics, with low-e glazing to protect seedlings and maintain a constant temperature.
The glass in the greenhouse traps heat energy in the same way that the earth’s atmosphere keeps the planet warm: through the transformation of light waves and through the convection of the air inside the greenhouse. Solar radiation reaches the greenhouse, passes through the glass and is absorbed by the soil and plants. They convert it into heat energy, which cannot escape through the glass. The air inside the greenhouse is kept warm by convection, which simply means that the warmer air near the ground rises and the cooler air near the ceiling of the greenhouse falls. Because glass keeps air trapped inside, the convection process allows the air to absorb more and more heat each time it goes up and down.
The sun provides three basic types of light waves: ultraviolet (UV) waves, visible light waves, and infrared light waves. Each of these light waves has a relatively short wavelength and will easily pass through most types of glass, including greenhouse glass. Once inside the greenhouse, the waves are absorbed by the plants and the soil, where they are used for photosynthesis and transformed into thermal energy. Because heat energy has a much longer wavelength than light waves, it cannot pass through glass and becomes trapped inside. This is referred to as the greenhouse effect; it’s the same process that keeps the Earth warm and makes the inside of a car hot on a sunny day.
Along with the transformation of the light wave, another process called convection helps keep the heat inside a greenhouse. The air closest to the ground and plants is heated by radiant heat energy; because warm air is lighter than cold air, it rises naturally. In the atmosphere, warm air will eventually reach a height where it will be cooled again. However, greenhouse glass traps air before it can reach this height and cool completely. Since the air rises and falls continuously inside the greenhouse, it retains its heat and becomes slightly warmer each time.
The term “greenhouse glass” commonly refers to both real glass and the polycarbonate plastics also used to build greenhouses. Glass and clear plastic work basically the same way, allowing light through, but trapping heat inside. The choice of whether to use glass or plastic will depend on the type of structure being built, the amount of money available for the project, and the anticipated life span of the structure. Plastics are generally lighter and less expensive, but not as resistant to weather and age. Glass greenhouses tend to be semi-permanent structures that will cost more to build initially, but will hold up better over time.
The main difference between the greenhouse and ordinary glass is the type of coating, or glazing, applied to it. Greenhouse glass is typically tempered for safety, meaning it has been laminated, similar to a car windshield; if it breaks, it crumbles into small pieces with relatively smooth edges that are less likely to be cut than large, jagged fragments. Tempered glass is highly recommended for greenhouse applications, as the glass will be installed overhead and subjected to wind, hail and snow weight stresses in colder climates.
In addition, low-e glazing can be applied to this type of glass. Low-E glazing is a thin sheet of semi-transparent metal that reflects most of the UV and some infrared light into the atmosphere, while still allowing visible light to enter the greenhouse. By blocking the most intense UV waves, low-E glass can protect the tender seedlings from getting too much solar radiation. It also helps maintain a more constant temperature inside the greenhouse by blocking some incoming infrared or heat waves. With proper ventilation, low-E glass can help prevent your greenhouse from overheating.
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