polycarbonate over glass?

[rock house]

My roof is a layer of 4 x 8 sheets of the twin wall polycarbonate. Last winter I had a little trouble in Fort Worth TX keeping my 15 x 22 gh. warm enough. I am thinking of adding another layer on the bottom of the 2 x 4 rafters to create a 4" dead air space.

[raybark]

Insulation works by preventing the conduction of heat from one air space (inside) to another (outside). Part of the reason multiwall polycarbonate has internal walls is to be a barrier between the air pockets. That is, a twin wall sheet will have a greater R-value than would the same thickness if it was a single air pocket, rather than two. The reason expanded polystyrene (Styrofoam) is such a good insulator is because it is filled with millions of tine, non-communicating air pockets.

Your 4" air space would be far more effective of an insulator if you can break it into more, smaller spaces.