I haven't tried insulating my greehouse, but if I would expect that the
spacers would improve insulation because the trapped air works as insulation
itself, much as the air in the bubble wrap does( up to a point ). If the gap
is too large, the trapped air will have room to circulate which ruins the
effect.

Personally, I'm going to avoid insulating the whole greehouse and instead
have got one of those small plastic 'greenhouses' which I'll put inside.
That should keep the tender stuff OK and the rest can take its chances.