More Trade-Offs

In addition to the list I posted the other day...
- Initially, I had assumed I would insulate my tiny home with polystyrene, as so many tiny-home builders do (even though I have experienced the breathing and environmental horror of cutting large blocks of styrofoam before). As I researched, though, I discovered that polystyrene is highly combustible. If it is covered with sheetrock, at least the sheetrock serves as a bit of a fire break. I won't be using any sheetrock in my home, however. Should a fire start in my tiny home, rather than having the entire home erupt into a towering inferno, I'd like to have enough time to escape with my life. So...I will be using wool insulation (which is fire retardant as well as a joy to work with) in the walls and roof, even though it costs more. I may use polystyrene in the floor, since no wiring (the most common cause of home fires) will run through the floors. I will use a radiant barrier throughout, to add R value. I will also use a vapor barrier throughout. In the floor and ceiling, the radiant barrier and vapor barrier will be one and the same; in the walls, they will be two separate things. By the way, does anyone have any thoughts about air-tightness in tiny homes? Will I need to have some mechanism for pulling fresh air into my home? Do I need to worry about making my home too tight?
- Although most tiny homes seem to be sided with clapboards or board-and-batten siding, I plan to use cedar shingles. I haven't been able to decide about using spacers between the house wrap and the siding. I've read that spacers are necessary to allow moisture to dry out, and I've read that spacers cause more problems than they solve by creating a tiny micro-climate behind the siding that actually causes a buildup of moisture. Installing spacers behind shingles would be harder than it is for clapboards. Perhaps the little ridges on shingles allow enough air movement behind them?
- I thought I was set on metal roofing. Now I've been reading about recycled rubber roofing...