rruff, if you have the inclination and the supplies handy, I'd be curious to see how a luan panel with a single layer of fiberglass (6 oz cloth say) would fair against the full fiberglass panel in screwdriver test.
Done! I happened to have some very old epoxy and some Bondo cloth (~5.5oz), so I laid that on the 2.7mm luan with 15psi XPS underneath. Bomber. I can't ram a screwdriver through that either.
I also laid 3 layers of cloth and epoxy right on the XPS. That was easier to penetrate than I expected. I think what is happening is that the weak foam does not support the fiberglass very well. Either the fiberglass over the honeycomb core, or fiberglass over wood over foam is better. Still the 3 layers of fiberglass over cheap foam was significantly better than just luan over the same foam.
I'm still undecided on how I want to do this. The fiberglass-honeycomb panels from Carbon-Core should be quite durable, but the panels alone would cost $3500 with shipping. And I'd have to do a lot of fiberglass cutting and scarfing and bonding, and reinforce the panels for attachments. And I'd have fiberglass interior, which I don't really want. Option 2 would be their PVC or PE foam which would cost about half that (but ultimately no cost savings), and I do the exterior fiberglass and interior plywood myself. It might be more work but not a lot, because I'd build the window and door frames and hard points, plus a wood perimeter frame in the panels. Just screw and glue the floor, roof, and walls together when I get them done, and go over the edges with fiberglass. The upside is that I wouldn't need to do any fiberglass cutting, and I'd get the plywood interior.
My original idea was to build with cheap foam and redwood stringers in the core, with plywood face sheets, and an outer layer of fiberglass. This is similar to option 2 above except I'm using cheap foam (vs structural foam) and exterior plywood+fiberglass skin instead of just fiberglass. The downside is that the outer ply will be susceptible to rot if I get leaks. But I know this will be strong and tough otherwise. There is a big cost variation depending on what plywood I use. 3mm Okoume would end up almost as costly as the Carbon-Core options, while luan would be about $1200 cheaper. One advantage to this approach is that I can build the panels in my cold garage using PL Premium to bond everything together. I only need warmer temperatures for the outer fiberglass layer.
Decisions...
