Twin Wall Planning

Well I decided to plant my first seeds yesterday. Looking at the temperatures for different plants I decided to just see if I could get some herb's started since it is still 20-30F outside. I'll just grow them inside until I can get the greenhouse to warm up. Speaking of the greenhouse I finally figured out a way to add an extra layer. I'm going to build a pipe structure over the top and attach it to the back wall since I can accomplish that with one piece of glazing plastic. I'm trying to get 1 1/2 inch of space between the layers of plastic glazing since that is what I have read is the optimal spacing. At this time the plan is to use 45 degree elbows, 4-way joints, and redwood strips to build the frame for this second layer. I'll post pictures as soon as I get it done. I'm curious to see what this combined with a wall of water will do for the ground temperature inside a wall of water.