Teardrops n Tiny Travel Trailers

rruff wrote:

QueticoBill wrote:Probably belongs in another thread but I too am concerned about the dimensional stability of XPS in my build. I've seen large stressed skin panels - 4 x 8 and larger - literally tear themselves apart. TO be fair, they were aluminum fame (perimeter only) and the solution was to use slotted connections of the frame.

Can you describe what was happening there in more detail? What was the skin? What was tearing apart from what? What caused the change in dimension?

XPS core, p-lam skin, extruded aluminum perimeter. Skin swelled and buckled breaking bond, or pulling foam apart under bond. I didn't see inside after.

rruff wrote:What did you use to fair the foam before the epoxy and glass? Did you texture the foam first?

I have used a combination of long board sanding (for highs) and light weight (vinyl) spackle (for lows). The spackle is a little controversial and somewhat untested in this application, although others have followed suit with no complaints that I am aware of. Got the idea from foamie RC airplane techniques I read about many years ago. In some areas I also experimented with a mixture of spackle, foam dust from under my table saw, and TB2 wood glue combined into a coarse mix. I'd have to look back at my own comments, but my recollection is that it sanded much more like the foam. That was more for coarse work where I was trying to extend the use of the spackle and later top coated with more straight spackle, IIRC.

No spiking or special prep other than sanding to break the glaze of the foam extruding process (the thicker foam boards don't have the plastic film that the thinner stock sizes do).

Yes, the foam does move around a bit more with temperature compared to the wood (we're talking maybe 1/32 inch or less up against 1-1/2 tall spars), but the epoxy and glass seem to have locked that down, or at least have minimized the effect... at least under the conditions I have observe... wide temperature changes but not elevation changes, more or less at sea level give or take a few hundred feet plus or minus atmospheric changes... the atmospheric changes probably vary by greater than the physical elevation above sea level).

As GPW notes, this is all fairly unimportant if you are not too concerned with trifling appearances, or unnecessarily burdened by the disease that is attention to detail.

QueticoBill wrote:XPS core, p-lam skin, extruded aluminum perimeter. Skin swelled and buckled breaking bond, or pulling foam apart under bond. I didn't see inside after.

Was it just sitting still or stressed? What changes occurred (temperature, elevation)? I don't know what p-lam is but I can imagine if the thermal expansion coefficient is greatly different from aluminum it could pull the bond apart. Typical XPS is soft stuff, it doesn't take a lot to compress it. It may puff up a little between the stringers, but the bond would need to be weak to pull the face sheets loose.

BTW, PLP might have a little give to it compared to some glues, but it is not a stretchy material.

I made a test piece with 1" 15 psi XPS (textured with a carpet seam roller) and 2.7mm luan faces, bonded with PLP (spread evenly with a notched trowel). It makes a very stiff and strong panel with no frame at all. I don't know if stringers are needed except at the edges, but I'm going to add some for piece of mind. Actually after reading this thread: viewtopic.php?f=2&t=49207 I'll probably use epoxy fiberglass for the outer layer as KC did. Supplies should get here tomorrow to make samples.

But the DuraSip sheets might be better yet. Bonding on a sheet (especially 8' wide sheets) instead of hand layup would be easier and result in a smooth surface with less work.

P-lam is plastic laminate, such as Formica, Nevermar, etc. As far as being stressed, I think in your meaning no. It was a free standing vertical panel, so there were stresses but not like a trailer on a highway.

Is PLP PL adhesive? I believe a foam to skin joint with PL adhesive is simply more flexible and allows more movement than one with contact cement.

I don't doubt your test panel was strong but my concern is aging and many cycles of expansion and contraction, especially when restrained by other framing. All foam and skin with the right adhesive is probably fine. All wood framing and skin is certainly proven to be long lasting. It's the combination of the two working differently over time and many cycles that concerns me, and some companies that produce some stressed skin panel products.

Yes, if engineering to the nth degree, this concern about dissimilar inner and outer skins is probably a valid concern. However, when I consider how my camper is turning out compared to a more traditional stick built where little effort is put into obtaining a 100/ct bond between foam and skins, with all of the inner wood skin bonds to wood structure and blocking, and all of the epoxy/glass bonds to wood structure I figure you could just about remove the foam all together and still not see a structural failure.

Really, when you tie all of it together, the unity of the box really adds up to a very rigid structure.

Maybe it will do some weird things out in the wild once the sun is beating down on it, but that has yet to be seen.

I agree. The framing makes the foam unnecessary structurally, and far from failure.

QueticoBill wrote:I agree. The framing makes the foam unnecessary structurally, and far from failure.

Depends on your framing and skins. For most of the stuff we look at very thin skins are used (3mm plywood, <1mm PMF or fiberglass) and you'd need very close stringer spacing. Rigid foam bonded to the skins adds a huge amount to the strength and stiffness and insulates to boot. I'd be more inclined to say that the frame and stringers are superfluous. For me, they are there for piece of mind... ie the skin won't fly off even if the foam or the bond to foam gives way.

Regarding foam, XPS *is* used in some commercial structural panels made for camper builds. It is however much weaker than the PVC foam that is typically used for boat hulls. There are many examples of frameless XPS and PMF trailers, and the 10 oz canvas PMF is a *very* light skin.

I do tend to think of long life in buildings, and teardrops and tiny trailers probably don't have as long of life as most buildings I work on. I still believe any wood framing - even 1 x 2 s every 12-16" - glued to skins probably do more structural than foam adhered with PL and do it sooner - as in the foam will give and flex and the bond allow some movement before the more rigid wood framing gives.

I wish I could afford a test. (Does TNTTT have a research department and funding? ) three 4' x 4' panels - one with just foam, one with just 1x2s maybe 12 or 16" oc, and one with both. Start piling tube sand on them spanning almost 4'. Of course it would be nice to repeat with 1/4" and then 1/8" ply.

In buildings, weight doesn't matter. It's also a much larger "thing" that spans longer distances. Light construction can last a long time if it's done well and it isn't wrecked. The strength and stiffness in a sandwich panel primarily comes from the skins and how far apart they are spaced. The foam keeps them spaced apart and takes relative shear loads. But adequate panel strength and stiffness are easy to acheive on small campers. Failures are more likely to come from water intrusion, poor bonding, and skin penetration and denting.

I think you should definitely make some samples to test. It's cheap. You don't need 4x4 pieces, just make them as wide as you want the stringer spacing to be. If you feel the need to test stringers that is. I'm not even making samples with stringers because I've already demonstrated that I can jump up and down in the middle of a 48"x15"x1" foam sandwich with 2.7mm luan skins. I'm mostly concerned about penetration and dent resistance, and a good bond. You can do that with small samples.

“ I'm mostly concerned about penetration and dent resistance, “ Yes , indeed , but what about all the testing in the past where everybody proved the dent and puncture resistance ... Years now , no reports of punctures or dents that don’t heal ... and don’t think plywood is puncture proof ... ( see limb through the plywood trailer roof picture )

The original idea of a Foamie trailer wasn’t to make somnething to last as long as the pyramids , but just an inexpensive "hard tent" that didn’t rot or delaminate the first season ... Something light and easy to build for most anybody ( even Grandma’s have built these )
No rules , build what you like, and can afford ... but just remember , even the simplest of foamies have worked very successfully for years now ....

Stress test , dent resistance ... ??? I’m 6’3” 240 lb, and using a standard sledge hammer and all my might , we gave it our all .... still didn’t go through .... How strong is that ...

Pic below shows how a limb made 10 holes ( ten) in a membrane/plywood/beaded foam , and paper commercial trailer roof ... punctured the ceiling inside in several places . on limb piece on the floor ... Glad we weren’t inside ...

I don't disagree with most of what you say but you can't compare an impact on a solid surface to one in free air. I suspect that same sledge hammer on a foam roof would be as damaging as the limbs on a roof.

It'll be interesting when Rowerwet & GPW report on their friends kayak/boat building experiments. Can't quite wrap my head around the muslin/GG treatment........JG

By the way , just wanted to say the RV dealer charged $5,000.00 for that roof repair ^ ... and a little spot on the side ... fiberglass (?) .... He just threw on a regular sheet of plywood over the holes , and covered it over with a new “membrane" ( thck as a cheap vinyl table cloth) ... the fiberglass repair was really amateurish , What a RIPOFF!!! And I had wondered why he insisted we buy the trailer insurance from his company ... RV scam ...

We're waiting on my friend ( NOGC ) to get busy on his boat using the durasip material ... now he has our friend next door ( fabricator ) interested too ... we’ll take pics when it happens ...

Rowerwet is the best one to ask about the muslin ( old bedsheets ) .. He’s on Facebook too ...

GPW, I wonder if you heard some update from your friend building the boat with this skin?

OP, he has his full size 1/4” plywood templates cut out and is dry fitting them to check for accuracy… He told me he will order the material soon to continue … ( This is going slow because I think Work got in the way of his Fun … )