If you've ever gotten into a car on a sunny day and noticed that it was hotter inside the car than it was outside of the car, you've experienced the greenhouse effect. The greenhouse effect is named so because greenhouses use glass or plastic to trap solar radiation inside of the structures, making them warmer than the air outside and protecting tender plants that could not grow in the outside air. Greenhouse glass allows solar radiation in, but traps it inside of the greenhouse.
Not all greenhouses are covered in glass. Some use plastic sheeting, corrugated plastic or even two-liter bottles instead of glass. The type of glazing or greenhouse covering determines the type of growing effect it will have within the greenhouse. All greenhouse glazing is typically coated with an ultraviolet coating to filter out the sun's ultraviolet rays and protect plants and the people who work in the greenhouse from harm. Glazings for greenhouse glass may be single, double or triple pained. Although a single-pained glazing may be less expensive, it may let in too much solar radiation in the summer and may not insulate the greenhouse enough in the winter to be of practical use in the greenhouse. Double or triple-pained glass will filter out more solar radiation in the summer and keep the greenhouse warmer in winter.
Plants rely on photosynthesis to convert sunlight to energy. The type of glazing that you select for your greenhouse can greatly affect the photosynthesis process. If a greenhouse gets too cold or too warm, photosynthesis will not take place. Additionally, plants thrive more under a glazing that is milky or smoky instead of clear. The smoky glazing scatters the light inside the greenhouse more, which gives more even light distribution to plants and allows them to better photosynthesize. When light goes through a clear glazing, it strikes plants that are closer to the glazing more than plants that are placed further away. The plants closer to the glazing may block light from the plants that are not placed close to the glazing, and the plants that are placed further away may not thrive as well.
New Glass Technology
Although glass was not traditionally considered as versatile as plastic for window glazing, new advances in glass-making technology have changed this. Glass once became brittle with age, couldn't be placed in a greenhouse in large panels, could not be frosted for even light distribution and contributed to heat loss more than plastic. Today, glass can be created in large panels which require fewer greenhouse supports. Newer, energy-efficient coatings may be frosted, and they reduce ultraviolent rays and heat loss by one third while still admitting sufficient solar radiation.