A well designed glasshouse can serve many functions and be an extremely productive element in a permaculture system. It creates a self-contained environment in which the designer can modify conditions of heat, light, water and air for a variety of benefits. High value crops can be grown out of their normal climatic range or produced out of season, growing seasons can be extended by starting seedlings of annual plants early, and plants can be propagated and grown in an ideal environment free of many pests, predators and adverse weather conditions. As part of or attached to another structure, such as a dwelling, workplace or animal housing, a glasshouse can provide a temperature buffer and insulation between indoors and outdoors as well as provide heating and induce airflow to assist cooling. A glasshouse is also a warm, sunny, peaceful and beautiful environment to sit in, observe and enjoy.
The Glasshouse Effect: How it Works
Short wave radiation (light) passes through glass freely and is converted to long wave radiation (heat) when absorbed by a solid object. Glass reflects long wave radiation, and the heat is trapped. Rising air warmed by convection from the heated surfaces is also trapped by the glass, or allowed to escape to adjoining structures for heating or to create a draught for cooling. Also, for the protected interior of the glasshouse, wind chill is no longer a major cooling factor.
Efficient Design: Making the Most of Solar Energy
The primary function of a glasshouse is the conversion of light to heat; good design will maximize light exposure and penetration as well as heat absorption and retention. A north* or northeasterly aspect is vital, free as much as possible from shadow cast by nearby buildings or trees. Although many commercial glasshouses favor north/south orientation to achieve even light exposure throughout, east/west orientation ensures higher overall illumination and thermal efficiency. Another way to maximize light transmission is a system of glazing known as ‘ridge and furrow’. Popular in the nineteenth century, it is seldom seen today as it involves extra construction cost and complexity. The walls and roofs were folded in short sections like a concertina, so half the glass was perpendicular to the morning sun and half to the afternoon sun. Besides reducing light lost to reflection, it also reduced the intensity of the midday sun. Taking this concept a little further, one would expect a curved surface to be even more efficient.
Other Considerations
In many climates, cooling in summer will be just as important as heating in winter, and vents should run the whole length of the highest point of the roof to allow the escape of hot air. A corresponding area of vents should also be placed at the lowest point to allow the intake of cool air. A solar chimney or exhaust fans may also be needed and supplementary shading from shade-cloth or whitewash. Most plants’ ability to photosynthesis efficiently decreases at temperatures above 26°C, and temperatures in the mid to high 30°C may be life threatening. Even at moderate temperatures, air movement is beneficial; conditions of high humidity and still air interfere with a plant’s ability to cool itself and function effectively. A glasshouse needs to be protected from strong winds or built strong enough to withstand them. “Suntrap” plantings or earth berms are ideal if they don’t restrict light.
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