While learning about how to keep a greenhouse warm, I came across a video that talk about how air running through the green house walls can insulate the greenhouse. They do this by having two layers of polycarbonate. When air blows between them it causes an R5 insulation effect. The cost in electricity is the same as 1 light bulb.
The cost for this system as shown in the video below was about $300. This includes two layers of polycarbonate and the blower mechanism. For a tour on how this works, here is the video by Urban Farmer Curtis Stone.
About R Values – https://www.greatdayimprovements.com/insulation-r-value-chart.aspx