Rice Hulls--insulating

I couldn't find adequate information online elsewhere so I figured someone here could shed light on my questions.

Rice hulls seem an excellent insulation source, I'm looking at using them in the ceiling under both a living roof and a small connected cedar shake roof. The only bags of rice hulls I could find come in 50lbs at the cheapest $12 a bag plus shipping. They rate at around R-3 per inch so my questions are:

1) how many pounds of rice hulls do I need for a 500 square foot roof? They come in compressed 7 cubic foot bags, but I have no clue how far each bag would go at say a 10 inch layer (R-30).
2) anyone know where I can source them cheaper? It is an ag. waste product, you would think I could find it for a bit less.
3) has anyone actually used rice hulls? experiences? results? method?

Thanks!

They are used as bedding in chicken or turkey cafos, so they are available in bulk semi loads.

But you are going to have a lot of freight to get them to Montana.