Gotcha.  I still think there's some value in that winter sunlight.  It might be that running lights under mylar is much more cost effective but I'd love to see numbers.

That's interesting that you're not getting condensation with mylar.  I wonder if the radiant barrier effect is doing more in terms of heat loss than I thought it would.

I Took this pic tonight of the GB with the light on inside the mylar. During the day with the light off you can't see through the mylar. After I closed the ends of the tunnel the temps were getting higher than I liked so I opened the ends back up. I'll play around with rotating the lights on and off with timers to eliminate this problem and also extend the time that they help to heat the GH.

We just had 16" of snow in 1 day and my double layer 6 mil poly roofing held up with no problems. My GH is 2x4 framed at 24" roof centers. I put one layer of plastic on the roof followed by furring strips and then a second layer secured by more furring strips. didn't even notice a sag. The walls are doubled too with a 3 1/2'" gap. Only negative is it has to be replaced every year but it's a cheap way to get started.