Hi Kate,
I've used homasote on top of plywood. Its a grey fiber board that is rigid enough that it needs to be cut by a jig saw (or small sections wth repeated strokes of a mat knife) but its soft enough to nail down track easily. Scrap sections can be broken and layered to create jagged rock formations. Its a bit dusty when cut. Its a good idea to paint homasote to seal it to prevent warping if you have a lot of humidity changes. I liked using it but think I'd try foam next time around.
Ralph
I've used homasote on top of plywood. Its a grey fiber board that is rigid enough that it needs to be cut by a jig saw (or small sections wth repeated strokes of a mat knife) but its soft enough to nail down track easily. Scrap sections can be broken and layered to create jagged rock formations. Its a bit dusty when cut. Its a good idea to paint homasote to seal it to prevent warping if you have a lot of humidity changes. I liked using it but think I'd try foam next time around.
Ralph