I’m not a big fan of bales in the roof as you likely already know. They are so heavy and there are several areas of concern in regards to using them above head.
1. The frame needs to be drastically increased in size and/or spacing to support the extra load.
2. Plastering over head is VERY difficult and tiring.This can be alleviated by using planking in place of plaster for the finish.
3. The R-Value gained is not anything better than what you can get with regular insulation materials (either blown in or batts and either natural materials like cotton or wool or conventional ones like fiberglass).
Personally, I stick with light weight insulation materials in the roof and leave the bales for the walls; however, if you want to use bales, you can use an assembly that looks something like this:
– 2×6 tongue and groove planking over beams/girders (to be engineered per the loads and spans)
– Double layer of drywall as described for boiler rooms, etc.
– Vapor barrier as required. I’m not sure what options you actually have for this and may end up with plastic sheeting as the only option.
– Bales dipped in lime plaster to provide fire protection for the bales themselves.
– 1″ Plywood
– 1.5″ air space
– 2×2 wooden nailers secured with 18″ (min) panel screws to the 2×6 wood planks and the beams below the bales.
– 1″ Plywood roof decking material.
– Living Roof Assembly with 2×6 Rim
– Underlayment and pond liner wrapping over the edge of the rim boards.
– Chicken wire over rim board edges to secure the soil and living roof in place. Be careful not to puncture the pond liner inside the rim boards. Nail the wire to the outside of the boards only and fold all sharp edges in before installing.
This system can be done, but it will be very expensive in the cost of wood or steel framing required to support the extra loads of the bales and the living roof. You will definitely need to have an engineer design this frame. Once again, I think you’re better off sticking with conventional insulation materials in the roof, especially if you plan a living roof as that is heavy enough on its own.