A second layer of greenhouse film, separated with one of the small blower fans and mounting brackets sold here has more than one benefit:  
 
Creates a dead air space that works as an excellent insulator 
Keep the heat that has been collected throughout the day in your greenhouse or high tunnel
Make your building more efficient
Lower the amount of heat needed to grow crops year round
Creates a dryer environment
 
Two different sizes available:
 
3 bolt -round mounting bracket                                                
60 cfm 
120 VAC
Pre-wired
Perfect for greenhouses and high tunnels up to up to 1500 square feet
6 ft cord included
 
 
4 bolt  -square mounting bracket                                                 
148 cfm 
120 VAC
 Pre-wired
6 ft cord included
Perfect for greenhouses and high tunnels great than 1500 square feet
 
Please note:  Blower fan and bracket are sold separately.  You will need the appropriate mounting bracket for the blower fan.  
 
3-9" dead air space created between the two layers of film.
All fans include damper, allowing amount of air between layers to be adjusted.