Covering the plastic greenhouse with two layers of plastic instead of one and inflating the air space in between using a small fan makes the covering more airtight and more heat-retentive. Even some glass houses are being covered with a plastic outer shell to aid in heat retention. The principle disadvantage of the extra plastic layer is the reduction in light intensity.
Large amounts of heat are lost through the concrete or wooden sides of the greenhouse. The siding material may be supportive or merely provide a skirt between the glass and the ground. In either case, the addition of insulation can plug an important area of heat loss.
The insulation can be an earthen bank against the outside of the wall. It can also be a layer of insulating material such as fiberboard or foil-covered styrofoam against the interior wall. With pierced concrete block foundations, the holes can be filled with insulating material at the time of construction.