Most of the heat is absorbed by plants, soils and other materials. Rays reflected off these objects are of another wave length that does not escape the plastic readily. It reflects from surface to surface causing further heating. Without wind to carry it away, heat accumulates rapidly. A walk-in greenhouse of all plastic is not efficient.
Full plastic allows much of the sun rays to pass thru.
If the north wall is wood or any non-transparent material it will be much more efficient. A solid north wall will absorb heat and also reflect the remainder back onto inner surfaces. I have been testing and improving several types of structures and materials for the past 9 years. I have learned much by trial and error. You will gain by following the evolution of some of the structures.