There are quite a few ways to make a greenhouse more heat efficient to make it feasible to grow either through the winter or for a large part of it.
The first thing to determine is what crop(s) you wish to grow since a number of plant varieties would be considered cold weather crops. The heat requirement would be considerably less.
Since the majority of heat loss is through the roof, the most common method of making a greenhouse more heat efficient is by adding a layer of plastic to the outside. With the benefit of a small blower there is an air pocket created. The closer that this air pocket is to “dead air” the less heat transfer that there would be. 10 to 15 cm is considered the ideal gap. When there is more then 15cm there will start to be movement of air and greater heat transfer. It is impossible to have a perfectly uniform space because of the way the plastic goes over the ridge and is fastened around the edges.
It is important not to have any holes in the plastic. The resulting air loss with a hole would result in air movement and less heat efficiency.
It is often thought that a lower greenhouse is easier to heat since there is less surface area to lose heat. The reality is that the bigger air volume of a taller greenhouse is less likely to have the temperature fluctuate.
To make a structure more heat efficient, the north wall could be insulated. Care has to be exercised in insulating ends since it is important not to create extra shading. Insulation can be put flat on the ground around the structure as a way of slowing down the cold radiating into the structure.
Another trick that some people have used effectively to reduce heat requirements is to make small tunnels over the beds of vegetables. This works well for the night time. It is important to have this cover removed in the day time to maximize the value of the sunlight.
If your intention is to only use part of a larger structure through the winter, it is wise to hand a curtain for the length of the ridge and then only use the south facing side. This maximizes the benefit of the sun and minimizes the effect of the cold wind.
With a little creativity it is easy to get benefits from the greenhouse even in the winter.