Heating, interior tunnels, air circulation, humidity control
When someone is intending to grow in a plastic covered greenhouse over winter, the first order of business is to put a double layer of plastic with air between.
This cushion of air acts as insulation and will reduce heat loss by about 30%. A secondary benefit from this cushion of air is that it reduces condensation in the greenhouse dramatically.
Even if you are not going to heat the greenhouse, it makes sense to put the extra layer of cover with air between since it will provide a significant extra margin for cold.
The small fan which puts the air between the layers is made to work against pressure. It can not “over fill” the cavity.
When installing the second layer of plastic, it is not pulled tight. This allows the second layer to puff up to about 10 cm which is the optimum space.
It is the dead air space that creates the insulation value. If the air is moving, you will not have optimum efficiency. This is the reason it is important to be vigilant about patching any holes in the plastic when they occur.
Especially if your intention is to grow without heating, you could benefit from installing and using small interior tunnels. These tunnels would only be covered during severe weather. Since the air volume is small, even the heat from a few light bulbs will make a difference.
Interior air circulation is even more important in the winter time than it is in summer. In the winter time when things are more likely to be closed up, the probability of stagnant air is high.
It is important to monitor the humidity level in your greenhouse as well. Getting rid of excess humidity will cost you some heat, but you are creating a healthier environment for your plants.
If you are using a forced ventilation system for either humidity or heat control in the winter, it is important to have the thermostat close to the incoming air so that things will shut down quickly if the incoming air is extremely cold.
Any heating system which is used in a greenhouse should have a stainless steel heat exchanger and burner. The standard aluminized units simply will not stand up long term in the moist environment of a greenhouse.