Types of vermiculite & Hearth ?

[smuth10]

Is horticulture vermiculite/perlite the same as regular vermiculite/perlite? What is/are the difference(s) if any?

2nd ? - A couple of other sites I have been to have the insulating section of the hearth as the bottom layer and not the top layer, as I see on Forno. What are the pros/cons of doing it either way. I would think it would not be very smooth if you have it as the top layer and as such, would be harder to lay the firebricks on level.

Thanks, Scott...