[dmun]

Two inches (50 mm) is not enough vermiculite! Four inches underneath! If you need, like me, to keep your under-oven thin because of design considerations, you need to use cal-sil board or equivilant high tech refractory insulation board.

This is important! You will never get more than one chance to insulate under your oven. Sand will help level the bricks, but it has NO insulation value.

And, yes loose vermiculite is fine, if you build an enclosure to house it. You can block off the corners, to reduce the volume needed. Be sure your enclosure is tight if you use loose insulation, the powdery stuff can leak out of surprisingly small cracks.

Sorry to be emphatic, but I can't tell you how many builders have not asked this simple question before, rather than after, when their ovens are burning cords of wood and not reaching pizza temps.