Box Wall construction

[svenwesley]

Hi all.

Short story; We've been travelling light for quite a while. My Vanagon has served us good and will still be my choice many years further on when I go alone. But the family grows, and the demands of better living standards slowly grows too. And after a year abroad we decided to build a bigger camper. We've been looking for an LT and even though I was more or less looking around the entire EU I couldn't find one in decent shape, the problem is that I want to get an older one to avoid paying a lot of taxes.
And then it just happened...
I ran into a guy who sold me an MB 307D from -83, with no rust at all! It's rebuilt with the biggest flat bed there is from a Sprinter, the floor area for our camper box is uge. The plan is to build a box that can be lifted off, the flat bed takes only 15-20 minutes to get back on if I need it. I will build a boogie "loose mount" in the rear that allows the frame to twist without flexing the box too much. The thing is that these kind of campers are very rare in my area, I'm only guessing that my ideas should work. So I'm seeking some advice and feedback.

I work with composites and have access to fiber glass, epoxy and paint at very little cost. My idea is to build the box like in the pictures.
On the inside I was thinking of a white painted plywood layer.
As a core material, XPS insulation with pine/plywood ribs across.
Outside fiber glass and epoxy for a strong durable surface.

Inner walls and boxes (like sofa, bed etc) will be glued and laminated to the outer walls to become a part of the structure for rigidity and strength.

All joints and corners will have an L beam protecting and reinforcing the outer edge, the inside will be filled up for a radius and then fiber glassed (see image).

Do you think I need the pine/plywood cross beams?
Is my construction ideas strong enough?
I think the inside will get a more comfy feeling with the wood even though it's painted and I have a plan to prepare the plywood with thread inserts so it will be easy to re-arrange stuff inside. But if the wood beams aren't needed I will not use them to avoid places where moist can get in. Even though they will be covered in epoxy. Just want to make sure to build a box that keeps up. We've seen quite a few delaminated constructions with XPS and it might be better to get a high density EPS instead.

Thanks in advance for all the advice, which I hope I'll get!

 

[fog cutter]

i would suggest staying away from any dimensional lumber due to its structural instability. mahogany if you must but sparingly. plywood treated with sealer (toxic or green - whatever your philosophy). 3M VHB tape may come in handy along the way.

 

[svenwesley]

My plan was to use 8 mm wide cross beams in pine, they will be glued with epoxy and will be sealed so to say. I'm not sure what you mean with structural instability though.

Here's a picture of a wall cut out I've found. Unfortunately I don't know the brand or factory who did the camper.

 

[fog cutter]

dimensional lumber will expand/ contract with humidity changes loosening fasteners and creating cracks between dissimilar materials. most readily available wood is first growth timber. it's grown quickly for harvest and the annular rings don't get time to compress and yield a dense/ stable product. from what i can gleen from the picture above, the wood is an engineered product (mostly heat cured resin held together with some wood) and is basically glorified plywood.

 

[nicknoxx]

Personally I'd skin the outside in a piece of ply too so your wall becomes a torsion box which is very rigid. This looks like the method used in the floor in your picture but you don't need to have quite so many ribs in the walls. I'd epoxy the ribs to the skin, add the insulation then the inside skin. Sealing the whole thing with epoxy will keep out all the moisture. Also you could use PIR insulation as it's more efficient than XPS. You will end up with a heavier structure but it will be strong.

 

[svenwesley]

Hermetically sealed with epoxy the ribs shouldn't move. But I could use divinycell instead to be sure a crack or damage on the outside doesn't create a problem water wise.

 

[svenwesley]

Personally I'd skin the outside in a piece of ply too so your wall becomes a torsion box which is very rigid. This looks like the method used in the floor in your picture but you don't need to have quite so many ribs in the walls. I'd epoxy the ribs to the skin, add the insulation then the inside skin. Sealing the whole thing with epoxy will keep out all the moisture. Also you could use PIR insulation as it's more efficient than XPS. You will end up with a heavier structure but it will be strong.

PIR, will check that up for sure! A common problem with XPS is degassing. The foam is releasing gas over a year after production date and the bubbles causes delamination if the board is sealed. It doesn't happen with EPS but the strength is not even half and the insulation value is also lower.
The walls will be a sandwich construction, but the outer skin will be a glass fiber epoxy laminate. I think it's a good idea though to add another material to be more ding resistant. I haven't seen a camper with plywood on the outside, is that common? I've been playing with the idea of a layer with polycarbonate on the outside to make it very damage resistant.

EDIT: Found a few PIR board distributors. Polyurethane, that's good.

EDIT 2: I just called one of the manufacturers of PIR boards. He said the boards might be too hard and brittle and can crack if they are flexed too much. He gave me some store names where I can buy the board off the shelve and do some testing.

 

[nicknoxx]

I don't know what it's like where you are but here (UK) PIR is sold as Celotex and Kingspan among others and it's definitely not brittle.

 

[svenwesley]

I don't know what it's like where you are but here (UK) PIR is sold as Celotex and Kingspan among others and it's definitely not brittle.

We all have different preferences.
I talked to SPU which is recently acquired by Kingspan. SPU is a Finnish company (I am in Sweden). The engineer told me that they make batches on order where they replace the alu foil with a paper sheet instead. After curing the paper is grinded off. It gives a raw surface which epoxy or any glue for that matter will bite into really well, apparently common way to make doors with a wood veneer. As long as it doesn't matter that the sheet is not exactly 30 or 50, rather 42 mm thick or something similar they could make a bunch of sheets for me at the same time when making a batch. That sounds like perfect to me.

I've also talked to my distributor of plastics (that we use in the workshop) and he said the polycarbonate sheets are good for painting, they do it all the time. PC is super tough to break so it will be a perfect outer skin.

With that in mind, this is the new wall construction. From left to right (inside to outside).
- plywood
- glass fiber
- PIR
- glass fiber
- thin sheet of PC (might be red when done).
Everything bonded with epoxy.

 

[ianc]

That looks heavy.
I would avoid using wood in the sandwich. Water will almost certainly find it's way in and the wood will never dry out.

 

[Victorian]

Contact Pecolit in Holland they sell very nice panels...

 

[svenwesley]

That looks heavy.
I would avoid using wood in the sandwich. Water will almost certainly find it's way in and the wood will never dry out.

There's no wood in the sandwich if we go with the latest ideas, only a layer on the inside of the box.
The box is 4,1 x 2,1 x 2,1 m.
With a 6 mm standard plywood the box will be under 300 kg.
With a 2 mm standard ply 220 kg.
With no plywood at all 175 kg.
With a light weight plywood 4 mm 214 kg

If I laminate four layers of fiberglass on each side (160 g/m2 density plain weave), skip the polycarbonate and the plywood, it ends up around 185 kg.
If I replace the PIR foam with EPS 50 mm and still do no poly or ply it will be 147 kg but probably half the strength in the walls.

Is that heavy for a camper box that size?
Like I said, I have nothing to compare with.

 

[david506th]

Fir, Douglas Fir.... Lighter, more stable and better for water compared to Pine or Yellow Pine sold in box stores. Mahogany (sapele in america) is too heavy and cost to much. Poplar is also a good choice but cost more, and do splurge for marine ply.

 

[svenwesley]

Fir, Douglas Fir.... Lighter, more stable and better for water compared to Pine or Yellow Pine sold in box stores. Mahogany (sapele in america) is too heavy and cost to much. Poplar is also a good choice but cost more, and do splurge for marine ply.

Douglas fir has the same density as pine.

 

[dlh62c]

Is my construction ideas strong enough?

Have you considered modifying a truck cargo body to suit your needs?

Here in the USA, they can be purchased in different configurations, Morgan is an example. Morgan will custom build a cargo body without a rear door assembly and supply a blank panel to fill in the opening.

The strength in the design will be in the extrusions used to attach the top, sides and bottom together. Their primary job being to prevent racking of the body. Here in the USA, there's a whole industry supporting cargo body manufacture and repair.