Insulated vs. solid walls

[angib]

Because I plan on using a composite sandwich layup of either very thin plywood (not 1/8", because it is too heavy) with a ridgid foam core. The facesheets and core will be joinded wet layup using epoxy (becasue it won't melt the Home Depot foam core).

I'm a great fan of lightweight construction, but.....

I suggest you need to carry out a "ball-pein test" on these construction scantlings - how hard do you want to be able to hit it with the ball end of a ball-pein hammer? Yes, thinner-than-1/8" plywood will be strong enough in overall strength, but you may find that the hammer will puncture the plywood with very little force. Of course you won't ever hit it with a hammer, but stones from the road will - if you go thin enough on the plywood, big insects will puncture the ply or your knee will, if you turn over in bed!

'Robustness' is needed as well as 'strength'.

Andrew

[kennyrayandersen]

Andrew,
I think you bring up a very valid concern and a few impact tests are certainly in order. I may actually be able to dig up some data on that. I was present when we did some horrendous impact tests dropping a several hundred pounds a foot down with a rounded blunt-nose protrusion on the end. Man! was that loud. It was a wing, and as long as we were whacking it near the spar there was hardly any damage. we got it a little too far from the spar with a little too much weight and wham -- it punched right through the wing -- great fun!

Mine will be much thinner though, and since I don't have an autoclave -- not quite as robust of a resin system. Your right also that I hadn't planned on whacking it with a ball peen hammer, but what about when my wife is throwing rocks at me!

Anyway, I will probably end up with one or two plies more over the bottom (inside and outside) and an extra ply in the front to guard against rock damage (I probably should plan on some lil' mud flaps for the Fortwo as well). I think that the glass I was looking at was about .01+ per ply. The whole point of going with 2 plies of glass is that it would be lighter than bonding .025 aluminum to the foam core. I'm always keeping 8 irons in the fire so I'll be planning and researching for a few months (plus I have to finish sanding 10 chairs and make my wife a bed before I can start on the teardrop!). I'll have fun dreaming and scheming in the meantime.

[kennyrayandersen]

Sorry I didn't see there were so many replies -- It's very nice so many folks are willing to share and active on the board.

Just to answer as many as I can.

1/8 plywood weights about 12 pounds for a 4X8 sheet -- 1/16 about half of that. Since you would be using a foam core and plywood on the inside AND outside, plus all the shelfs and supports, my rough guess is maybe a 20 pound difference (I'll only have one door to save weight -- I'll take the door side incase the spousal unit need to get out of bed in the night, but she seems to be able to deal with that whole issue better than the old man (nearly 49!).

I think using the glass instead of the plywood maybe saves another 20 pounds. Now maybe that doesn't seem like too much but 20 here and twenty there and pretty soon it adds up to real weight.

the Okoume plywood looks very nice, but with the sandwhich construction I'd need maybe 10 sheets -- that would be about 500 bucks just for the plywood! -- nice stuff though. The fiberglass would be much cheaper and if I don't have to worry about the size of the plywood I can be a little more creative and flexible with the profile/length without having to splice anything but the core, which is easy.

The whole point of going to a sandwich construction is that you get an extrodinarily robust wall AND it's insulated. Besides the core there will be just enough plywood burried in the foam to support the door hinges, latches, and pick-up points for the shelf supports, In the floor there will be plywood inserts that the frame will mount directly to. Most of the structure will not be a frame, but the tear monocoque itself.

I plan on doing a finite element model, when I can spare some time (I'm in the garage sanding now!).

I'm trying to save weight everywhere! walls, frame (or lack thereof), shelves suspension -- you name it.

[Woody]

You're NOT building the Space Shuttle here.

HMMMMM Gosh I wonder who said that now so famous qoute............Hmmmmmmm Now where have I heard that before, let me think.................. Oh that's right it was me. It is great to be qouted by the infamous Ira. And all this time I had no idea he cared

[Model_A Bill]

ok...wait!!!

Based on all your inputs, I think my best option is to go with the sandwiched wall and core insulation. Cool, I got that.

Are you saying that I can do a frame with core insulation, with a thin ply of wood on the insde AND a sheet of FIBERGLASS on the outside?

Do they make sheets of fiberglass to fit the side without a seem?? And your saying this would make the walls light as well as strong??

Damn I'm glad I joined this forum!!!

[Podunkfla]

ok...wait!!!

Based on all your inputs, I think my best option is to go with the sandwiched wall and core insulation. Cool, I got that.

Are you saying that I can do a frame with core insulation, with a thin ply of wood on the insde AND a sheet of FIBERGLASS on the outside?

Do they make sheets of fiberglass to fit the side without a seem?? And your saying this would make the walls light as well as strong??

Damn I'm glad I joined this forum!!!

Yep, Bill... They make some stuff called "Filon" that is fiber reinforced plastic. A lot of motorhomes have it on the sides.

http://www.google.com/search?hl=en&safe ... tnG=Search

[Ira]

How would you affix Gluon to the framing for use as an exterior wall?

Just glue? (Would scare the hell out of me.) Or around the perimeters with screws, glue on the vertical framing sticks, and then cover screw heads wth trim?

[Podunkfla]

How would you affix Gluon to the framing for use as an exterior wall?

Just glue? (Would scare the hell out of me.) Or around the perimeters with screws, glue on the vertical framing sticks, and then cover screw heads wth trim?

I think I would still put on a piece of 1/4" plywood then skin it just like you do aluminum. It doesn't have enough body to it to use without a backer of some kind.

[Ira]

That's why I think you missed Bill's point there:

I'm pretty sure he got all excited because he got the impression he could use the Filon all by itself as the wall, and not merely as a skin.

Otherwise, since weight is his big issue, he would use the ply and stain/epoxy/paint anyway for minimum weight, and forego a skin altogether.

[Model_A Bill]

I'm pretty sure he got all excited because he got the impression he could use the Filon all by itself as the wall, and not merely as a skin.

Otherwise, since weight is his big issue, he would use the ply and stain/epoxy/paint anyway for minimum weight, and forego a skin altogether.

Yes Ira, that is the impression I got. I read through the link and it said the glue was pretty strong and that when it stuck, it stuck permanently. In his pictures, he did have a plywood skin but if the glue was that "permanent" I assumed it could be adhered to a sub frame that was filled with foam insulation.

If that is too risky, and I have to plywood skin it first, I guess I would just paint it. That way I have less weight and the benefits of more color options.

Wow...so many decisions. YOu guys are making me have to think here. My wife doesn't like it when I do that!!!!

[Ira]

Your lightest plan of attack for now is to go with 1/4" ply wall, 1 by 2 framing, 3/4" rigid insulation, and 1/8" cabin wall.

You can even research which cabin wall material is the lightest and best use for this, but if you just use wood paneling (added bonus of it being finsihed already!), I would look for something that's really wood and not particles.

But here's the thing and a question for others. I'm sure this has been brought up before, but I don't remember:

Has anyone ever simply made a ply, insulation and cabin wall "sandwich"--just glue/laminate the crap out of it and forget the framing all together? Wouldn't that really bring the weight down, and still be structurally sound enough?

What I'm envisioning is making the rectangular sandwich, and makiing all of your cuts at once instead of for each piece--profile, door opening, etc. (Like you wouldn't have to do your wall cuts, then your cabin wall cuts, or actually install the installation.)

And you would have the only insulated galley in the neighborhood!!!

You would have to frame the door opening after, but I'm sure there's a way to figure that out. And you could even run your wires before you make the sandwich.

[toypusher]

Ira,

You would need some framing inside the wall to fasten bulkheads to, and of course the door that you mentioned. I built mine with only framing were something was going to fasten to the wall and ran all my wiring in the wall when I built them laying down. There was alot of discussion about this before. I think they call it 'stress laminate' or something like that (dam CRS)!!

[Ira]

Ira,

You would need some framing inside the wall to fasten bulkheads to, and of course the door that you mentioned. I built mine with only framing were something was going to fasten to the wall and ran all my wiring in the wall when I built them laying down. There was alot of discussion about this before. I think they call it 'stress laminate' or something like that (dam CRS)!!

Just for the bulkheads? And did anyone actually do it the way I'm describing?

And what if you used 1/4" for both in and out? Wouldn't that make it a lot stronger?

[toypusher]

I guess that if you used 1/4" on both sides that you could just screw right through the whole wall to attach bulkheads, cabinets, etc. Or maybe just screw into the inside wall will lots of strong glue. I thing that that could be done. Why don't you build one and we will see if it's OK or not???

[Ira]

Why don't you build one and we will see if it's OK or not???

If I had a garage, I would build another. But think about it:

Just one cut for the profile, and a cut for the door.

THAT'S IT!

No cutting and installing framing, no cutting and installing insulation, not having to cut the additional cabin walls...

You could have both sides (sans doors) totally done in a DAY, easy.