Efficient Glasshouse Design

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 maximise 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 favour north/south orientation to achieve even light exposure throughout, east/west orientation ensures higher overall illumination and thermal efficiency. The difference is most significant in winter, when a north/south orientation presents the least area possible directly to the low winter sun; most of the light striking the sides is reflected.

Maximum transmission of light through glass occurs when the glass is perpendicular to the light source; the greater the angle, the more light is reflected. With this in mind we can design to maximise solar gain in winter when light is scarce and minimise it in summer when it is overabundant. The further from the equator the greater this effect becomes. The angle which will place the glass perpendicular to the sun for the winter solstice can be found by taking the latitude and adding 15° (American Orchid Review, Nov 1982); thus for Sydney at latitude 33°, northerly facing glass angled at 48° will allow greatest solar gain in winter. In greater latitudes, using this formula may give an angle so steep as to be impractical for a small glasshouse so that compromise between solar efficiency in winter and practical design may be necessary.

Another way to maximise 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.

Having ensured maximum light transmission, the next goal of good glasshouse design is to convert as much as possible of it to heat, to be stored and reradiated at night. Light converts to heat when it is absorbed by an object. Dark coloured objects absorb more light; a white surface can reflect up to 96 percent of light. The interior of the glasshouse and especially the heat storage and collection surfaces (thermal mass) need to be coloured dark for maximum efficiency (although light coloured surfaces will make more reflected light available for the plants). The southern wall and floor of the glasshouse should be thermal mass, and in some configurations, the southern roof can be also. The floor should be well insulated from the surrounding soil around its perimeter to a depth of one metre, and the southern wall, if not required to transmit heat to an adjoining structure, also well insulated. Heat is also lost through the glass, because although it mostly reflects radiant heat, it readily transmits heat by conduction to the outer surface which is quickly lost by convection to the air. To minimise this loss, insulating shutters or blinds are sometimes lowered at night, or better still, double glazing is used. A more economical alternative is to use an internal liner or horticultural film or plastic to create an insulating air pocket next to the glass.

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 shadecloth or whitewash. Most plants’ ability to photosynthesise 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.

In areas prone to extreme hailstorms, a hailguard of light birdwire or shadecloth may be necessary, though a steeply pitched roof will deflect most hailstones.

Mosts pests and predators can be excluded from a glasshouse, but once introduced, absence of natural enemies can create ideal conditions for the build up of pests. The enclosed environment of the glasshouse is also ideal conditions for integrated pest management, especially biological control. Predatory insects can be released and confined to the target population. Small insectivorous birds may also do well, and provide ongoing pest control, as will frogs and small lizards.

Animals in glasshouses can provide mutual benefits, though in most cases will need to be kept separate from the plants. Chickens, for example, provide CO2 and methane, which stimulate plant growth. Dust from their feathers is also beneficial, as is their body heat at night, and their manure for fertiliser. The chickens benefit from the shelter and extra warmth of the glasshouse. Many other combinations are also possible.

*all directions for southern hemisphere