Wood or aluminium?
Aluminium is usually the material of choice for a glasshouse, whether in natural metal or with a painted finish. A coloured frame may fit into the garden better, especially if the structure can be seen from the home. Aluminium needs no upkeep, and the glazing bars are thin, casting little shade.
Wood is an attractive, traditional building material and better-suited to some garden styles. However, it needs periodic upkeep unless you specify more expensive and lower maintenance cedar-wood timber. Wooden frames tend to be bulkier than aluminium and can cast excessive shade inside the greenhouse.
Shape and size
The taller a glasshouse is at the eaves, the better the light transmission and therefore the wider the range of plants that may be grown. The eaves should be at least 1.5m (5ft) tall, and ideally 1.8m (6ft) or more.
The ridge should be at least 60cm (2ft) above the eaves to allow a door that gives easy access, sufficient slope to shed rain and to encourage loss of excess heat.
Glazing to ground level gives the greatest flexibility, but half-walling, using masonry or timber will reduce heat loss.
Domes and other odd-shaped glasshouses can prove more difficult to manage, especially when trying to ventilate efficiently, and they tend to be more expensive than the traditional shape.
It is important to ensure a greenhouse has sufficient ventilation. Roof vents are the most useful, and ideally should be on both sides of the ridge and equivalent to 15-20 percent of the floor area. Side vents are no substitute for roof ventilation, and while louvred vents allow regulation of air flow, they are hard to draught-proof in winter.
Automatic openers that open or close in response to greenhouse temperatures are useful, but slow to respond and need supplementing with manual control, such as opening some windows and the door each morning. Motorised vents activated by sensitive heat sensors are more efficient, but may be too costly for home use.
Glass or plastic glazing?
The best glazing material is glass, as it lets 90 percent of light through, does not degrade in sunlight and, unlike plastic materials, reflects heat radiated from within the glasshouse back into the structure instead of being lost.
Toughened glass should be used for doors and anywhere where people might fall onto the glass.
Double glazing and special glasses that alter the spectral composition of sunlight are often used for conservatories, but are less suitable for cultivating plants as the reduced light quantity and quality affects plant performance.
Polycarbonate sheeting is a twin-walled plastic glazing that has the advantages of being resistant to breakage, lightweight, durable in adverse weather, and good at retaining heat. However, clear polycarbonate only transmits 83 percent of the light – considerably less than glass, and this may limit its use for growing seedlings, cuttings and plants that require high light levels.
Twin-walled acrylic plastic glazing is also available. It transmits 85 percent of the light that hits it, but is more brittle than polycarbonate, tending to crack during installation.