Aww okay, so instead of bonding XPS sheets together you are gluing XPS|Wood|XPS|Wood|XPS and then skinning it all with glass/epoxy? That does sound like it would be "easier" (as in more familiar) for attaching cabinets etc and the properties of the XPS and the wood might actually work well together. Although you would lose some overall r-value.
Yes, that's right. The R value loss is pretty trivial though; wood is a much better insulator than aluminum for instance! And way less bridging than residential stud wall construction.
Any place you want to fasten to the wall you need a hard bridge between the two face sheets; either a plug of epoxy or wood. Else you will pull the face sheet off the styrofoam. I am not building solid cabinets in corners that inhibit the structure. I want them to flex a bit, and they will be fastened to the floor where there are already lots of hard points.
By the cheap stuff do you mean like the 4x8's available at home depot etc? I was referring more to the finished composite panels (maybe the correct term for that is Sandwich panels(?) FRP(?), I'm pretty new to all this. As for skills I'm fairly handy, like learning new things, but haven't taken on a project anywhere near as big and complex as building a camper from scratch.
XPS (
$32 for 2" sheets of 25psi), Ebond epoxy ($30/gal), and whatever FG cloth I can get on sale. Heavy biax I like the best, but harder to hide overlaps.
By "finished panel" do you mean where a sheet of FRP is glued to the core? Plenty of people have done that. You can build them yourself or have them built. Total Composites (and others) will make and ship you the panels and all the hardware your need to put them together. I think a pretty good size box will run ~$15k. That saves a lot of work, but you could build the panels yourself for less.
Hand layup over foam gives you more freedom for curves and rounding edges. That's the main reason I'm doing it this way. You need to prep the foam properly and get up to speed on FG layup. Make some sampels and test them. Not hard to learn.
Another option is to use thin marine ply (3-4mm) for the face sheets instead of FG or FRP. I built a camper like that years ago and it worked great. Well sort of like that. $10 luan was much higher quality than now, and that's what I used for the sheets instead of marine ply.