Greenhouse Heat Conservation
The larger the greenhouse, the easier it will be to maintain a healthy temperature. Also, a large greenhouse can be divided into two parts, one of which, the stove house, will have a temperature of about 60c F. (16° C), while the other part remains a cool house, with a temperature of 45 to 50° F. (7 to 10c C).
Try to maintain a steady temperature, but do not be unduly worried by a rise in temperature caused by the sun alone. A rise of 15° F. (8° C.) due to the sun may do no harm, whereas a rise of 8° F. (4° C.) from artificial heating can cause considerable damage.
It is very important to conserve heat in a greenhouse, but extremely difficult to insulate the panes of glass without reducing the amount of light.
The best form of insulation is a translucent plastic material such as Thermoplus (polythene), which may be fastened with drawing pins or adhesive tape to the roof bars inside the glass, leaving a gap of at least l in. between the plastic and the glass pane. Heat is conserved by this layer of static air, and it is even possible to increase the temperature by as much as 10° F. (6° C). To maintain a normal temperature, about 40 per cent less fuel will be required. However, this method does cut down the amount of light reaching the plants.
Dripping water from condensation may cause damage to plants underneath; it can be avoided by using a special plastic sheeting which channels the water to the outside edges of the sheets, to run away safely.
Another method of conserving heat is to use an outside blind made of hessian, treated with green Cuprinol to prevent it from rotting. Fix the blind to the top of the house so that it can be rolled down in the evening when the sun has gone down, and rolled up again as early as possible in the morning.
Even a 9-in.-thick brick wall will allow a great deal of heat to escape. Seal all cracks and crevices in the woodwork, glass or brickwork with a special mastic preparation such as Seelastic, which can be applied with a special Expandite gun. Sylglas adhesive glazing tape used on the outside bars acts as an efficient seal, and also as a protection if the greenhouse is built of wood. Two or three coats of thick white paint on the inside walls will seal any cracks in the brickwork. This treatment not only conserves heat, but also prevents insect pests from hiding in the mortar between the bricks, and increases reflected light.
It is inadvisable to seal permanently the slight gap that is usually found along the line of the eaves, between the top rails and the glass. This gap provides for the escape of condensed moisture, which can harm plants in cold weather, when the atmosphere in the greenhouse needs to be kept dry. However, if continuous icy blasts should find their way in on a cold night, block the gap temporarily with strips of sodden newspaper.
Rubber flaps nailed to the bottom of the inner side of the door will help to make the greenhouse airtight; on frosty nights, lay a strip of sacking outside the door.
Always put the main door of a greenhouse on the more sheltered side, as a protection against draughts and wind.