Double-glazing is the most efficient method of insulating the entire greenhouse. This is best to be done at the time of construction, if possible. Installing double-glazing is expensive but the benefits will be enormous.
Flexible plastic insulation or bubble plastic is an other very efficient method of insulation. Bubble plastic consists of double or triple skins of transparent plastic with air cells in between. Bubble plastic should last three or four seasons, so is a very cost effective method of insulation the greenhouse. A single layer of plastic sheeting is not as efficient as bubble plastic at reducing heat loss but is less expensive and not cut out so much light. Plastic sheets may be used in winter as a form of double-glazing by attaching them with clips that fit into the greenhouse frame.
Thermal screens consist of clear plastic or translucent material, such as cross-laminated fabric. They are attached to wires between the eaves and drawn horizontally across the width of the greenhouse in the evening. They are useful for conserving heat over night because they restrict the amount of heat that rises above the eaves, trapping the warmth lower down around plants. You can also use vertical screens to partition off a heated section at one end of the greenhouse, leaving the rest of the greenhouse unheated.
Base cladding on the floor of glass-ground greenhouses reduces heat loss significantly. Over winter place polystyrene panels along the foot of the glass panes to provide extra insulation. Remove them before summer border crops are planted.