Ed wrote:
OK, our 125+ mature orchids are overflowing our attached sunroom, so we are
considering a 10' x 12' polycarbonate greenhouse for the backyard.
(snip)
I have no experience with greenhouse culture, so I could use all the advise
the group is willing to share.

If you are building from scratch (rather than a kit), I would consider
making an insulated north wall and a shed roof (south slope only).
During winter at our latitudes, the north wall of a greenhouse and the
north slope of a gable-end roof are major heat sinks and don't do much
to add additional light to the greenhouse.

If you go with electric heat, make sure you have a backup gas or
kerosene heater on hand. If you frequently lose electricity during
winter storms and are not always home, it would probably be safer to
use a gas/propane heater with standing pilot light that will keep
working during outages. Greenhouses lose heat very rapidly in freezing
temperatures.

Nick