In article , Paul Giverin
writes

The mention of polystyrene has given me a thought. I could lay
polystyrene ceiling tiles in the winter when its really cold and I could
lift them in spring when I'm likely to be needing to water stuff inside.
I use those decking squares as duck boards down the middle at the moment
and I could retain these all year round.

I think it's unnecessary. Most of your heat loss will be through walls
and top, so thick layer of bubble wrap, as you're proposing, is the best
thing you can do. Ceiling tiles are too fragile to walk on, and you will
want to be in there through the winter to see what is happening, and
clear any dying foliage, and from early spring when you start sowing.

The main cold problem is the cold air outside moving past the greenhouse
- the earth acts as a giant storage heater, so putting an insulating
layer between it and the greenhouse is not necessarily a good idea.
--
Kay
"Do not insult the crocodile until you have crossed the river"