How do you hang foam core sheets from the ceiling when the exsisting joists are 30x40 inches apart?

It sounds like who ever built the structure never intended to hang anything from the joists. You are not adding allot of weight, but I would still add a joist 2 x 4 so you have 24" centers to work with. that way all of your outside edges and the center will be supported when using 4' x 8' sheets.