One way to lengthen the growing season for plants is to create a special space, or greenhouse, where a gardener can make optimal use of sunlight, temperature, humidity and other growing conditions. The term "greenhouse window" can be used to refer to a window in a freestanding or lean-to greenhouse, or a type of window installed in a house that projects past the exterior, similar to a bow window. When trying to design the best growing space, gardeners should choose greenhouse windows that provide the light, temperature, humidity, and ventilation required by the plants they want to grow. In addition, greenhouse windows should fit the gardener's budget, and be appropriate for the design and location.
In-house greenhouse windows provide a space in the house with shelves for growing herbs or other small plants year-round. These greenhouse windows may use special low-e glass to prevent condensation build up. Brackets running from the outer edge of the shelf to the exterior of the house may be installed to help support the weight of the shelves and plants. These greenhouse windows are commonly available with vinyl or aluminum frames; aluminum frames tend to be lighter and stronger, but also allow for more heat loss and condensation build up than vinyl frames. Greenhouse windows should fit tightly into the window opening to prevent cold air from harming the growing plants.
For gardeners who want a larger greenhouse space, there are many options. Greenhouses or sun rooms can be installed as additions, or as lean-to or freestanding structures. Ready-to-assemble greenhouse kits are available in a wide range of sizes and materials. Gardeners can also build a greenhouse themselves using a plan. Which greenhouse windows are best for these types of structures may be determined by the choice of design.
Not all greenhouse windows are made of glass; in fact, many popular greenhouse models do not use windows at all. One of the more common materials used in greenhouse design is polyethylene film, a kind of plastic sheeting that covers the greenhouse frame. Polyethylene film comes in various thicknesses and degrees of translucence, and more opaque film can prevent excess sunlight from reaching cool-season or shade-loving plants. Polyethylene film can easily conform to the shape of the greenhouse frame and is relatively inexpensive, but may not be as durable as a more rigid material. Ventilating a greenhouse covered with polyethylene film can also be more difficult.
Polycarbonate, a clear, rigid plastic, is another common material used for greenhouse coverings and windows. It often has multiple layers with a space between them to trap air to help control temperature and prevent condensation inside the greenhouse. If designed to be operable, polycarbonate windows can use automatic openers to provide ventilation. Automatic openers have a fluid-filled cylinder that expands and contracts to open and close the window in response to higher and lower inside temperatures. In contrast to other materials, polycarbonate is generally more expensive than polyethylene film, and also does not allow as much light or heat to pass through as glass.
Glass windows are heavier and can be more or less expensive than other options depending on whether the windows are new or used. Many times, a gardener can find used windows for free, or purchase them fairly cheaply. This is a good option for gardeners who want to build an inexpensive greenhouse from a plan.
Whether new or used, certain features may determine which window is suitable to the greenhouse design. The number of panes is one consideration; single-paned windows allow better transfer of sunlight and heat, but make it more difficult to control temperature than double-paned windows. Windows with UV-protection or low-solar-gain low-e glass may be a better choice when the gardener wants to limit the amount of sunlight plants receive, such as in summer months or in warmer, sunnier climates. As with polycarbonate, glass windows can make use of an automatic opener to provide ventilation.
If the gardener has decided to purchase a greenhouse kit, or has chosen a professional installation, then he may be limited in the choice of window or covering material. Kits may come with either polyethylene film, polycarbonate, or glass windows. Professionals may or may not be able to alter the design of the greenhouse to incorporate the gardener's choice of materials.