I'm tempted to go for a double layer of the spaceboard (picked up mine last night plus spares for club apiary).
I used slab insulation like this when I first started beekeeping (as I felt it might help somehow), and have never used it since, without any difference in winter losses. I know several commercial beekeepers who are managing thousands of colonies - without block insulation and tiny losses so I'm bemused about what the point of it is?
Have I missed something - has someone proven it to be of benefit? I don't mean the anecdotal "it's insulating it must be good" is there some recent study come out or suchlike that indicates insulation is actually better?
I see you are using it in the club apiary. Given you presumably have loads of new beekeepers - are you recommending them use it too?
The only half resonable evidence I've seen is in Brother Adams books, where he describes the poor build up the following spring after excessive insulation so have always steered clear of it since.
Did I miss something (I know, Finman will be here to say heat is good, and he uses heat in the -40 degrees C of the finnish winter but here in England). For centuries it's been colder over winter and all they ever used was a tin roof.
Adam