Ray remembers 1/6th in aluminium greenhouses. Our big double is 50' by
100' and has roof vents and two double doors one one side but no side
vents. Seems to work. He reckons that side vents in a smaller, domestic
set up might be a good idea but in this instance, size does matter.
;-) If we think of those pretty greenhouses seen in older house's
gardens, many of them are long and narrow and have side windows that
open. PS Why do I know these roof vents as 'lights' from my CI
childhood? I don't think I know anyone over here that calls them that.
Or does it perhaps have another meaning I've forgotten?

More delving back into history.
In Victorian times the "cold frames" were covered with very large 6ft X
6ft glazed frames known as "Lights", later called "English Lights" they
were a 2 man job to move around, they stayed in use till into the
1950's, and in fact you will probably still find some in historic gardens.
They were superseded by the smaller and lighter "Dutch Lights" which
were a single sheet of glass in a wooden frame.
These Dutch lights were also used to make glass houses, hence opening
some of the lights.
The Dutch light houses were then superseded by Glass houses consisting
of 4 sheet frames, making construction quicker and easier, then
Aluminium framed Venlo glass houses still using the Dutch light sized
glass 28 3/4" x 56" or for those metric minded 1422 x 730 mm
To see pictures of English Lights go to
http://www.pampas2palms.com/2012/06/...nd-part-1.html

David @ the cool and sunless side of Swansea Bay