I am looking to build a lightweight but well insulated trailer. It will be used to live in full time, with a decent portion(~50%) of the time being off grid. In contrast to most people that live in their campers full time, we will not necessarily always be able to move to follow the weather, as we will be working travel nursing contracts. Additionally we would like to be able to stay in the mountains in the winter for recreation(skiing + ice climbing). Lightweight is important as we want to be able to tow it with one of our
existing vehicles (08 i5 H3 or 2014 2.5l boxer Outback) and want to be able to take it into remoteish locations(poorly maintained forest service roads etc).
Size wise the plan is
6.5 x 16-18ish feet interior dimensions with a goal weight of < 3500 lbs, though less would be good as it would let us use the outback. No permanent grey or black tanks and
ballpark 30 gallons freshwater tanks. Additionally possibly a small diesel tank or small propane cylinder for heating. Interior cabinets built with 80/20 aluminum frame or foam/ply laminate panels.
For the trailer frame itself, my brother who is an engineer, is helping me design a custom frame. I couldn’t find any that were long enough that weren’t designed to haul 7000+ pounds, didn’t have all kinds of extra rails etc, or had enough ground clearance.
I am coming at this with my only previous composite experience being a couple of small boats built with marine epoxy/fiberglass/ply construction.
I have seen lots of trailer builds where people frame it with wood and then fill in with foam sheets to insulate it and then put some kind of siding over it. However I would like to avoid any wood framing to avoid moisture/rot issues. In most campers the framing is small(2x2 or 2x3), indicating to my mind anyway that it doesn’t really need to be that strong. I really like the look, weight, and lack of void spaces in fiberglass “egg” trailers(Scamps, Bolers, Escapes). None of them have more than about r15 insulation though, and none are really setup for fulltime, all season use, so would require gutting and refitting for our purposes. My plan is to build something similar but with a foamcore/fiberglass shell. I have seen this threadon here and this is more or less what I am planning on doing.
However, I have not found anything quite like what I have in mind, though I am a bit unsure if what I am considering is a good idea or even feasible and was hoping to get input from you all who have more experience working with various composite materials.
I am considering using polyisocyanurate foam sheets for the core, as they insulate very well, have good compressive strength, and I can get them fairly cheaply. I am planning on 4-6 inches for the roof, 3-4 for the walls and 2-3 for between the subfloor and floor. This would let us heat and cool it using a gas heater of some sort, and solar, a lithium battery bank, and a highly efficient mini-split heat pump(assuming my calculations are even close to correct 
). For the corners, I am planning on rounding them with a decent radius to make them look better and slightly improve aerodynamics(I am under no delusion that this will improve them a lot). I am thinking of either strip planking these ahead of time or making them using a couple of stitched panels that I round the corners off of by carving/cutting. The whole thing would be then be sheathed(handlayed) on the outside and inside. Which brings us to the questions:
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I have never really seen this done before, and I know that usually if it something hasn’t been done it is for a reason. Is this completely unfeasible/inherently flawed in some way or is it a creativity/effort/cost/something else issue that has prevented others from doing it before?
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Is it reasonable to use the poly-iso foam for the core? I know the boat building guys are all like “NO, it doesn’t have enough shear strength”, “it is not structural”, “It is friable so the glass will delaminate” but these don’t necessarily seem like deal breakers in this case, as it seems most trailers seem to be built MUCH weaker than any boats are.
1+2 part b. If poly-iso is not the way to go, is there something else that would be better? Or even if the poly-iso is not strong enough by itself, could I use it as a mold that I then leave in place for the insulation? Sort of a “lost foam” technique that does not lose the foam? This is more and more how I am beginning to think about it, where the main structure comes from the fiberglass/resin and the foam is just left for insulation. I still worry about delamination though and am lost when it comes to scheduling the cloth, bringing us to:
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Most of the “egg” campers seem to be blown chopped strand mat that ends up being 1/8"-1/4" thick(not sure about this, it is kind of hard to get away with walking up and drilling holes in peoples’ campers to see how thick they are :rolleyes: ). I have also seen this thread, where he is using 2 layers of 300 csm which to my very in experienced eye seems like not a lot of not a very strong material, which is encouraging to me in regards to my build. Just based on what I have read on builds of objects that seem somewhat similar to what I am planning, if I had to pick a schedule right now to build it would be something like: foam > 1708 with mat down to help with adhesion to foam > 10oz or 17oz cloth or another layer of 1708 > some kind of layer to help smooth everything or just sand/fill ad nauseum > sprayed gel coat or 2 part epoxy paint. I know I can’t use just any 1708 fabric with epoxy, but there seems to be some debate if there is an advantage to using the mat/fabrics that DO NOT have the polyester binder with epoxy or if it just adds weight/expense. However, I am open to any and all input, and there is a certain appeal in using Jaxon’s approach of csm and polyester resin if that would be strong enough.
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Which I guess brings us here. My previous experience with epoxy has been with marine epoxy, but I am not sure if this is the best for this build and would like suggestions on other options. I think I am out on vinylester as it seems so darn toxic.
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Finally, I would like to include some extruded aluminum rails on the roof to mount solar panels and on the interior for cabinets and would like input on anchoring them. I am planning on laying them up at the same time as the other lamination using fillets and glass tape, however, if there are other better ideas out there I would be happy to hear about them.
Thanks for taking the time to read this and for any input you might have! I plan on documenting the build well and hope that will go at least a little ways towards paying back any shared insight.
Search the net and you will find plenty of overlanders and campers on trucks using this.