Granity wrote:

Isn't there a problem with polycarbonate in that it flexes in strong
winds and can pop out of it's fastening clips?

I've faced the tail end of a hurricane with 50+mph winds without losing any
panels. You do need to pay careful attention to how the retaining clips
are installed.
On the light level issue: I recently bought a light meter (I'm a
techno-geek, OK?) and when the midmorning sun measured 3200 foot-candles,
the inside of the GH (twin-wall polycarbonate, I think 5mm thick) read
2080 f.c., so about a third loss. Plants do very well, but I notice a
favorite red lettuce is more like pink in the GH, and full crimson only
when transplanted outdoors.


--
Gary Woods AKA K2AHC- PGP key on request, or at home.earthlink.net/~garygarlic
Zone 5/4 in upstate New York, 1420' elevation. NY WO G