I spend a lot of time over on the Greenhouse forum. I'm still in the dream/design phase, alternating between 12 X 16 or larger...glass vs. polycarbonate. However, many on that forum are from Western or Southern states so they just don't understand one thing:

JUST HOW CLOUDY AND DARK OUR WINTERS ACTUALLY ARE!

Maybe some one here can be more sympathetic!

I live in Akron, OH and while we don't get the heavy lake-effect snows that the counties further east and north do, we do get the cloudiness. Two weeks can go by without a lick of sunshine in winter.

I plan to grow citrus in my greenhouse. Citrus ripens in winter and needs sunshine for sweet fruit. Glass allows 90% light transmission, with no diffusion and costs an arm and a leg for double-paned insulated (which with an R value of 2 is the only way to go)!

Polycarbonate allows 75% light transmission, (with 10mm twinwall) and insulates as well as glass (R value 1.9-close enough). It also diffuses light which on a sunny summer day is great.

What I worry about is winter. With the exception of the few sunny winter days we get, we're lucky to register 400 foot-candles of light midday in January. Which with even a glass greenouse will be diminished to 370 or so and polycarb even less.

I'm convinced that either way, I'll NEED HID lights if I ever want to actually pick more than a single orange off my trees!!! I have looked at daylight intesity charts and Cleveland in May has the same light levels as Miami in DECEMBER! Which means that Seattle is about the only cloudier place in the nation in winter than here!

Anyone have experience here??