Double Poly Covering:

 Purpose:  Insulation value added by creating an air void between two separate layers of poly covering.

PROS: Effectively slows heat loss in winter and heat gain in summer. Its cost effectiveness depends on several factors. #1 Plant requirements, heat, light etc. #2 Average winter ambient temperatures. #3 Reduces wear of poly by buffering the wind (the inflated outer layer act as a shock absorber)

Cons: #1 The addition of a second poly layer decreases light penetration and may cause adverse effects in plants. #2 Additional cost may not be justifiable for certain plant applications.

Summary:

When over wintering plants or early grow out stages at lower temps say 55 degrees in a climate having a milder winter or summer avg.  double poly is questionable as to cost effectiveness.

Many beginners into greenhouse growing focus on saving on heat such as double poly covering or installing shade cloth over their plants in times extreme heat; without first focusing on the growing requirements of the plant they are growing. Altering the environment can end in great disappointment for those that fail to understand this

A Greenhouse is a structure where as an Artificial Environment is created that is Optimum for plant Growth. It can be either for short term season extender or long term plant production. Understand that any and every component that is added to the structure will change the environment so always consider the effect on your plants before proceeding.