Sandwich everything - talk me out of it.

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Sandwich everything - talk me out of it.

Postby Squigie » Tue Aug 13, 2019 1:33 pm

I'm thinking about sandwich construction for everything - floor, walls, bulkheads, bed base (under bed storage likely), etc.; basically everything but cabinet doors (if I even use doors).

Walls would be 1/8" ply interior, 1" skeleton and extruded foam, 1/8" or 1/4" ply exterior.
Floor would probably be 1/2" ply base,1" skeleton and foam, and 1/8" or 1/4" top ply.
Everything else: 1/8" ply, 1/2" skeleton (likely poplar or pine) with 1/2" EPS or extruded foam for insulation and/or sound deadening, and 1/8" ply.
Of course, there would be blocks in the sandwiches for any attachment points.

Why shouldn't I?

As I plan my build and identify exact (or classes of) materials and components to use, I am constantly worrying about weight.
I want a light build. And I only have 2,200 lb Timbren suspension to work with.

"Hollowing out" the plywood used as bulkheads, cabinet faces, etc., seems like a reasonable opportunity to shave pounds. It's a lot of labor, I do recognize that. But I'm fighting a monster. Ounces are pounds and pounds are tons.

I'll do the math on actual weight savings for a cabinet bottom after I get home today. But even without the numbers, what are your thoughts...?
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Re: Sandwich everything - talk me out of it.

Postby Pmullen503 » Tue Aug 13, 2019 1:48 pm

I would put the thicker ply on top of the floor rather than the bottom, the weight between supports is on the top. I would only use skeletonized plywood for the sides and lumber for rectangular parts like the floor and bulkheads; less cutting to do. For non-load bearing parts, a wood perimeter only with foam infill.

As always: think airplane rather than tank when it comes to structure.
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Re: Sandwich everything - talk me out of it.

Postby tony.latham » Tue Aug 13, 2019 1:55 pm

Walls would be 1/8" ply interior, 1" skeleton and extruded foam, 1/8" or 1/4" ply exterior.
Floor would probably be 1/2" ply base,1" skeleton and foam, and 1/8" or 1/4" top ply.


There are two things in the above that jump at me.

Why the 1" skeleton in the walls? Why not 3/4" (CDX)? (And by the way, 3/4" foam is sometimes hard to find. But there's a pile up here in Salmon.)

I think that 1/2" for your floor is too much. I'm on my fourth build and the floor with this one 1/4" subfloor ply, 3/4" pine frame, and 1/4" subfloor. With foam board inside of course.

It's stiff and I've got no issue walking on it.

Image

Image

:thinking:

Tony

p.s I use 1/4" subfloor because it's better than 1/4" AC. Less voids and rated for moist environments. And a couple of bucks cheaper.
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Re: Sandwich everything - talk me out of it.

Postby Squigie » Tue Aug 13, 2019 3:30 pm

Thanks for the replies.

Thicker ply on the bottom of the floor would be for impact/debris protection. Structurally, I don't see a need for anything thicker than 1/8" on the bottom if I'm sandwiching. But from a longevity and water infiltration standpoint, I'd like to avoid holes in the subfloor from rocks, tree limbs, and other 'Jeep trail' debris getting kicked up, flipped over, etc.
This trailer will see a lot of rough road use, as its primary purpose is to give me a smaller, easier shelter to take hunting. (Rather than the larger tent trailer that's too big and inefficient for just one or two people.) It isn't intended to be an "off road" trailer in the usual sense, but won't be a pavement and campground queen, either.


The 1" skeleton in the walls was a decision based on what I can get locally. I hate EPS. It isn't as efficient as XPS, must be cut (no scoring and snapping), and makes a ridiculous mess when cut or shaped. But EPS is the only thing I can get locally in 3/4". ...Unless I buy or order at least a half-unit of XPS (64 4x8 sheets!). Then I can get 3/4".
In theory, it could also be argued that the 1" wall is a relic from considering a steel skeleton (based on square and rectangular tube already on-hand, primarily with a 1" dimension). But I've given up on that idea, now that I'm chasing pounds and ounces in very material and structure.


I ran some numbers. The answer was ... interesting. But I don't have time to share right now. I'll be back in a bit with that.
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Re: Sandwich everything - talk me out of it.

Postby Tom&Shelly » Tue Aug 13, 2019 3:31 pm

I'm building with sandwich walls, floor, bulkheads, and even the shelves for the dresser (above the legs in the teardrop cabin). Only downside is that, this being my first build, I didn't always think about where I want hard points to mount things, and have to get creative. Better planning would have helped, but I didn't think of everything. (I haven't built the galley yet, and wish I'd had more hard points in there on the bulkhead and sides.)

I couldn't find foam other than in one and two inch thicknesses, so built a hot wire cutter by stringing nichrome wire across a bow. The scrap we cut off went into the dresser shelves which are 1/8" Baltic birch, 1/4" AC ply and foam, and 1/4" Baltic birch. Probably not much weight savings there, but I used pretty much all scrap material from the rest of the build, and had fun with the experiment.

I couldn't find the good underlayment Tony is talking about. I found some I've used for router templates, but it is not waterproof. (Put a scrap in a bucket of water and you'll find out within a day or so if it's good.) the 1/4" AC ply we found (at a real lumber yard) was pretty bad, so I ended up using 1/4" Baltic birch (something like $15 a 5' x 5' sheet). I was worried about the seams, but with epoxy and fiberglass, they were no problem. So our walls are 1/8" Baltic birch on the inside, 3/4" AC ply and foam in the middle, and 1/4" Baltic birch (glassed) on the outside. That worked great!

For the floor, we used 1/4" AC ply on the bottom, 3/4" poplar boards and foam in the middle, and 1/2" AC ply on the top. Way overbuilt! It was too heavy and hard to maneuver onto the trailer. If I did it again, I'd use 1/4" material on top and probably trim the poplar boards to be thinner.

Bulkheads were 1/8" Baltic birch on either side. One had a 3/4" core; the other 1 1/2" (for sound insulation--there is an air conditioner on the other side and our sleeping heads on the cabin side). With the hot wire, we had no problem with the foam. I used some of the scrap to insulate our garage doors, and am now using more for the teardrop ceiling.

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Re: Sandwich everything - talk me out of it.

Postby Aguyfromohio » Tue Aug 13, 2019 4:02 pm

We used sandwich construction for two trailers, it worked.
Sidewalls, floors, front walls, back walls

I'm not sure I would do it that way again. It's more like manufacturing and less like craft work.

- must plan ahead for every attachment point so you have wood to screw into.
- must have a large flat surface to make a flat panel. My garage floor is not flat enough, check yours ( we built the floor first and used that as a big table for the walls)
- the glue ups are a mess spreading glue over full 4 x 8 sheets.
- you can't completely remove polyurethane or epoxy from a nice finished surface, some of my beautiful prefinished maple plywood interior wall have ugly spots.
- running a wire inside one wall and around the corner into another is a headache
- the advantages of sandwich construction end up being not much for weight or labor savings or insulation

If I had a big empty 40 x 60 ft shop with a couple flat 5 x 12 ft tables, and vacuum press gear, and if I planned to build a few exactly the same I would prefer to use sandwich construction.
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Re: Sandwich everything - talk me out of it.

Postby tony.latham » Tue Aug 13, 2019 5:46 pm

I hate EPS. It isn't as efficient as XPS...


I'm not a big fan of it either, but I've used it on the walls of two teardrops. At the most, it's a two-hour corner to turn and then it's done. And of course, 1/4" of foam won't result in any detectable difference.

Sounds like you have a plan though. :thumbsup:

Tony
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Re: Sandwich everything - talk me out of it.

Postby Squigie » Tue Aug 13, 2019 5:51 pm

Hypothetical 24"x58.5"x3/4" bulkhead: (Loosely based on Generic Benroy, but not really)
3/4 ply:
21.32 lb (Assuming 70 lb sheet.)

With 1/8"x1/2"x1/8" sandwich:
Reinforced with 1/2"x1" poplar skeleton and XPS:
11.18 lb

Reinforced with 1/2"x1" poplar skeleton and EPS:
10.07 lb

'Skinny' skeleton, 1/2"x1/2" poplar, and XPS:
10.80 lb


And, just for another point of reference, I ran some numbers to see how steel compared...
1/2" steel square tubing, 0.063" wall (0.049" would be better, but it's harder to get), with 22 ga (0.030") sheet:
16.51 lb
That would, of course, only be 0.560" thick (nominally), and insulating inside would only be reasonable and viable via expanding foam injection or assembling with a spot welder (which I don't have).

3/4" steel tubing, 0.049" wall, with 22 ga sheet:
17.84 lb


So, even steel can be used and come out lighter than plywood, while still remaining stronger ... depending upon how the part needs to perform.
There are, of course, much better ways to build a bulkhead out of steel; but I just wanted to run the numbers really quickly with a similar construction method and part layout.
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Re: Sandwich everything - talk me out of it.

Postby Squigie » Tue Aug 13, 2019 5:57 pm

So...
The weight savings is real.
The benefits and handicaps are real.

No one is really arguing with the method, just suggesting that it'll take more time than I anticipate - and may not be worth it - correct?
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Re: Sandwich everything - talk me out of it.

Postby Tomterrific » Tue Aug 13, 2019 6:34 pm

Use 1/4" ply for the walls, no sandwich needed. Use 1x2or3 around the window and door for extra stiffness. The roof should be sandwiched for strength and insulation. The floor will do well with either 3/4" or 1/2" ply. Use 1x's over the frame and if trailer does not have enough cross rails for 1/2" floor stiffness put in a couple of the 1x's between the frame rails. Glue all wood with PL.

Tt

I used weak Revolution ply and the mentioned framed battens around the Windows and doors. Very light weight. I have beat the poor camper on the roughest interstates and it held together for 6 (?) years. Sits outside all year.
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Re: Sandwich everything - talk me out of it.

Postby Aguyfromohio » Wed Aug 14, 2019 9:56 am

Squigie wrote:So...
The weight savings is real.
The benefits and handicaps are real.

No one is really arguing with the method, just suggesting that it'll take more time than I anticipate - and may not be worth it - correct?


Yup. That's a fair summary of my opinion.
While the weight savings are real, they are small. Maybe a hundred pounds or so for the whole trailer.

Maybe I'm suffering from the illusion that the grass is greener on the other side of the fence -I've never built one with a stick frame.
I imagine that it allows more ability to change the plan as the build goes along - hey lets put another outlet over here, and let's move that reading light down...
And I imagine it's easier to wire up the completed frame before the skins go on.
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Re: Sandwich everything - talk me out of it.

Postby tony.latham » Wed Aug 14, 2019 10:11 am

No one is really arguing with the method, just suggesting that it'll take more time than I anticipate - and may not be worth it - correct?


As long as you are building your walls with a ledge for the ceiling and cutting in dados for the bulkheads, the extra time with sandwiched walls is well worth it.





:thumbsup:

T
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Re: Sandwich everything - talk me out of it.

Postby QueticoBill » Wed Aug 14, 2019 10:28 am

I'm just working on my first but I have built stressed skin panels - sandwich panels of ply/foam/ply for over 40 years - and I plan to go that way - floor, walls, bulkhead, roof, hatch. Minimal farming. 1/4" floors and walls, 1/8" roof - stem to stern. With 1/4" ply, if glue and/or screws are not enough to attach to, use molly bolts or through bolt and plate. Or attach with fibreglass and epoxy. I do plan to glass the exterior.
QB
A tear with no name: viewtopic.php?f=50&t=67624
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Re: Sandwich everything - talk me out of it.

Postby Philip » Wed Aug 14, 2019 11:04 am

Look threw my build in the link in my signature.

For wall I used 1/4" A/C ply inside and out. For wall stubs I used 1/2" by 3/4" trim board. It cost more that a 2x4 but it weights a lot less. When stapled and glued between two sheets of ply. You have a very strong wall. The only place I used heaver boards was around windows or doors and at angled attachment points. Then they were 3/4" x 1.5" trim boards.

Trim boards cost more but your not dealing with large weak knots or anything. Each board is a known planed size every time. If you need them curved. It doesn't take much of a steamer to soften it up to do a bend. If your glassing the camper 1/8" ply would be more than enough.
Here is my build.

viewtopic.php?f=50&t=67073
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Re: Sandwich everything - talk me out of it.

Postby Squigie » Wed Aug 14, 2019 3:45 pm

Thanks for the replies.

As is obvious, I'm still figuring things out.

I'm waffling between aluminum skin over the exterior ply (with interior finishing undecided), versus glassing the whole thing - which I've never really done before, unless you count 'battlefield expedient repairs' on composite aircraft cowlings. (... I know I don't)

'Glass could provide a good finish and great appearance if I end up with pretty color and grain in the exterior ply. But it could also be a good base to paint.
...And that's something that I've had in mind for several years. Either one could be painted. Aluminum would be easier to skin, I believe. But 'glass should have better longevity.

Aguyfromohio wrote:Yup. That's a fair summary of my opinion.
While the weight savings are real, they are small. Maybe a hundred pounds or so for the whole trailer.

Maybe I'm suffering from the illusion that the grass is greener on the other side of the fence -I've never built one with a stick frame.
I imagine that it allows more ability to change the plan as the build goes along - hey lets put another outlet over here, and let's move that reading light down...
And I imagine it's easier to wire up the completed frame before the skins go on.

In my mind, a hundred pounds could be argued either way: Insignificant, when the labor is considered. Or quite significant, when weight is the higher priority.
I have no idea where this trailer will end up, but I'm targeting 1,000 lbs. Trimming 10% of that - or dropping 100 lbs to get to that - would be, in my mind, quite significant.


I'm also still unsure about framing. You may not have noticed, but I did not specify what the structural core of my walls would be ... because I haven't figured that out, yet.
If I do go with a 1" core, plywood is feasible but expensive.
Otherwise, it's stick-framing time.

I thought I had tracked down a supply of 3/4" XPS today via a local classified ad. "It's the blue or pink foam board insulation, not the crumbly white stuff, right?"
Construction guy: "Yes, sir. It's the pink Corning stuff. We've got about six units of three-quarter-inch to get rid of."
I showed up and was confused until I had the guy I had talked to walk the yard with me.
They had about thirty units of EPS (1/2" to 2") and a single unit of TWO inch XPS left over from a commercial construction job; no 3/4".
For the condition, price they were asking, and distance I would have to haul it home, it wasn't worth my time to load any EPS (or the XPS to cut it down).

If I can find a source for 3/4", I think I'll go that route.
Even with the 20% loss of R-value, it just seems easier to go 3/4" core.

But, right now... I'm still looking.
I do appreciate the hot wire cutter suggestion from Tom&Shelly, but don't want to commit to that yet.

tony.latham wrote:
No one is really arguing with the method, just suggesting that it'll take more time than I anticipate - and may not be worth it - correct?


As long as you are building your walls with a ledge for the ceiling and cutting in dados for the bulkheads, the extra time with sandwiched walls is well worth it.

:thumbsup:

T

Yep, that's the plan.

I've looked at many construction methods. I like the dado bulkhead slots and roof ledge the best for those joints.
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