In article , Kay
writes

If double glazing is so good
how come many of the manufacturers don't offer it?

Because most gardeners don't heat their greenhouses through the winter?
It's only cactus growers and similar who tend to heat their greenhouses
significantly (i.e. maintaining a temperature of 40 deg F or
thereabouts) as opposed to merely maintaining it frost-free.

But surely if it's cheaper then even a cold greenhouse would benefit
from double glazing, you might get away with no heating at all.
--
Janet Tweedy
Dalmatian Telegraph
http://www.lancedal.demon.co.uk