Plywood camper fabrication questions (thinking of idaSho)

[ReluctantTraveler]

On the topic of thicknesses and the right mix of plywood vs. PVC foam core, I'm considering a few different combinations. Strength and insulation are both important, though I'd likely prioritize strength given the size of the rig and the fact that it will be housing a family with kids.

Thoughts on the best choice for a 8'x12' main box with 6'x8' cantilevered cab-over?

1. 6mm plywood (internal and external) with 38mm PVC foam core (1/4" ply + 1.5" foam)
2. 3mm plywood (internal and external) with 50mm PVC foam core (1/8" ply + 2" foam)

Obviously, thicker foam means better insulation. The PVC foam core is structural, but I'm unsure how how strong structural PVC foam is compared to plywood.

They're nearly identical in price.

 

[Ozarker]

No idea of the structural aspects of those PVC foam core panels are, but I'd bet the manufacture has it listed somewhere on the web. 8' is a pretty wide span so I would think you'll need framing, I wouldn't try to build anything that big with either material.

Agreed, strength is the primary concern over insulation, my bet is that the #1, 1/4 ply will be stronger as the ply would be stronger than foam.

If you are sandwiching ply and foam, I'd suggest you go thicker on the exterior, at least 3/8, 1.5 or 2 inches of foam and you can use 1/4 inside, I'd rather use 1/2' exterior, 2 inch foam and 1/8 interior, about all the interior panel does is protect the foam surface.

Also, depends on the interior structures, cabinets, any divider and furniture affixed to the structure.

1/4 plywood also depends on what kind of plywood, there are different kinds and not all are equal in "strength".

 

[ReluctantTraveler]

No idea of the structural aspects of those PVC foam core panels are, but I'd bet the manufacture has it listed somewhere on the web. 8' is a pretty wide span so I would think you'll need framing, I wouldn't try to build anything that big with either material.

Agreed, strength is the primary concern over insulation, my bet is that the #1, 1/4 ply will be stronger as the ply would be stronger than foam.

If you are sandwiching ply and foam, I'd suggest you go thicker on the exterior, at least 3/8, 1.5 or 2 inches of foam and you can use 1/4 inside, I'd rather use 1/2' exterior, 2 inch foam and 1/8 interior, about all the interior panel does is protect the foam surface.

Also, depends on the interior structures, cabinets, any divider and furniture affixed to the structure.

1/4 plywood also depends on what kind of plywood, there are different kinds and not all are equal in "strength".

I don't know how to reconcile this comment against what IdaSHO wrote here:

6mm (1/4) exterior skin, which includes walls and roof
3mm (1/8) interior skin, again, walls and roof
6mm (1/4) interior floor, both cabover and main floor
13mm (1/2) belly

All of which is a 1-1/2" core, using standard 25PSI XPS ridged foam

All that said and done, Ill say this....

If I were to do it again, I would either shed a considerably amount of framing or drop to 3mm (1/8") exterior skin.
The thing is stupid strong the way its built, so shedding a bit of weight by loosing some material might be worthwhile.

 

[rruff]

Strength has many aspects. Just to keep it simple I'd say that for a camper like this there is only one aspect of panel construction that tends to matter... that's the ability to take impact and force. This will be a function of the stiffness, strength, hardness, and thickness of the skin, along with the foam beneath it. The rest boils down to "don't do stupid things"... like fail to account for stress risers, forget that you want both interior and exterior skins to be structurally continuous, or do a poor job of gluing on skins, etc.

And then there is stiffness, which is completely different. You'll get exponentially more bending stiffness the thicker the core is, for the same skin thickness. Per weight, that is a much better way to go than making the skins thicker.

Whatever you use, I think a layer of handlaid FG is a very good idea for ply that you need to be particularly durable, like the exterior. I concur with Kenny that 3mm is likely thick enough ply, and note you are using much stronger foam than he did. Mine was actually cheap luan (2.7mm) and with a layer of FG on the outside it was unfazed running into pretty large tree branches. I used mat and polyester resin, but cloth and epoxy is the way to go. 6oz cloth should be enough. Coat your ply with slow cure epoxy first, then apply the FG before it's fully cured.

Like always, make samples and experiment!

 

[ReluctantTraveler]

Strength has many aspects. Just to keep it simple I'd say that for a camper like this there is only one aspect of panel construction that tends to matter... that's the ability to take impact and force. This will be a function of the stiffness, strength, hardness, and thickness of the skin, along with the foam beneath it. The rest boils down to "don't do stupid things"... like fail to account for stress risers, forget that you want both interior and exterior skins to be structurally continuous, or do a poor job of gluing on skins, etc.

And then there is stiffness, which is completely different. You'll get exponentially more bending stiffness the thicker the core is, for the same skin thickness. Per weight, that is a much better way to go than making the skins thicker.

Whatever you use, I think a layer of handlaid FG is a very good idea for ply that you need to be particularly durable, like the exterior. I concur with Kenny that 3mm is likely thick enough ply, and note you are using much stronger foam than he did. Mine was actually cheap luan (2.7mm) and with a layer of FG on the outside it was unfazed running into pretty large tree branches. I used mat and polyester resin, but cloth and epoxy is the way to go. 6oz cloth should be enough. Coat your ply with slow cure epoxy first, then apply the FG before it's fully cured.

Like always, make samples and experiment!

All good suggestions!

My plan had been to use marine ply and fiberglass the exterior (West System). Given the size of my rig, I am concerned about stiffness and general ability to not fall down.

like fail to account for stress risers, forget that you want both interior and exterior skins to be structurally continuous

I'm not sure I'd know how to even begin to account for stress raisers, which makes me think that paying twice as much for panels from a third-party vendor might be a better choice.

Can you explain structurally continuous skins?

 

[rruff]

You don't have to worry too much about stress risers, especially if you have "framing" in the core and use it for hard points. It's more of an issue if you attach to the skins only, in which case it's best to bond and spread the load out.

Any place there is a break in the skin you need to structurally bridge the gap somehow, else you'll get a crack there.

I can see a lot of upside to letting someone build your shell, especially since you don't have a good place to build it indoors.

 

[IdaSHO]

I don't know how to reconcile this comment against what IdaSHO wrote here:

Yeah, dont even bother with anything thicker than 1/4"/6mm unless you are using garbage construction grade (ACX or CDX) plywood.
And if you are... you likely have no business building a camper anyhow
So stick to the good stuff, and limit it to 6mm. You wont be sorry.

As mentioned above, puncture/impact resistance is pretty crucial to some (including myself, as we tend to push thru trees fairly often.
With that said, 6mm is a great thickness and its puncture resistance was a factor in my decisions, though it is still overkill.

 

[ReluctantTraveler]

Yeah, dont even bother with anything thicker than 1/4"/6mm unless you are using garbage construction grade (ACX or CDX) plywood.
And if you are... you likely have no business building a camper anyhow
So stick to the good stuff, and limit it to 6mm. You wont be sorry.

As mentioned above, puncture/impact resistance is pretty crucial to some (including myself, as we tend to push thru trees fairly often.
With that said, 6mm is a great thickness and its puncture resistance was a factor in my decisions, though it is still overkill.

would you go thicker on the foam and reduce the ply thickness? Or is 1.5” foam the sweet spot?

 

[ReluctantTraveler]

Any place there is a break in the skin you need to structurally bridge the gap somehow, else you'll get a crack there.

I had planned to use vertical studs, either 16" or 24" on-center, and join my plywood seams at a stud. I'd also planned to fill and glass the seams. Is that what you mean, or something else?

 

[rruff]

^Yep, that's good.

 

[IdaSHO]

would you go thicker on the foam and reduce the ply thickness? Or is 1.5” foam the sweet spot?

Nope.

My personal opinion is that for the vast majority of people anything more than 1-1/2" foam is a waste of both money and weight.

 

[ReluctantTraveler]

Nope.

My personal opinion is that for the vast majority of people anything more than 1-1/2" foam is a waste of both money and weight.

the only reason I’d considered going thicker on the foam is because my design is a fair bit bigger than yours.

 

[IdaSHO]

I doubt it has any relevance. Id build a camper 2x the size of ours with the same walls.

 

[Ozarker]

Nope.

My personal opinion is that for the vast majority of people anything more than 1-1/2" foam is a waste of both money and weight.

Is this a homebrew panel or are these commercially available panels?

I'm doing a trailer and agree with you about 1 1/2 inch of foam fitting the needs of most (weekend warriors) however, I'm considering a thermal break between 2 - 1" foam panels, 3/8 marine ply outside and fiberglass inside. Foam is cheap, the ply was $130 a sheet for 12 sheets and that was almost 4 years ago, so I'm using it.

Hope he builds a frame if this is with homebrew panels.

Just a comment, building a mobile unit isn't like building a house, it's better to use boatbuilding techniques, stringers rather than butted studs, just say'n.

 

[ReluctantTraveler]

Is this a homebrew panel or are these commercially available panels?

I'm doing a trailer and agree with you about 1 1/2 inch of foam fitting the needs of most (weekend warriors) however, I'm considering a thermal break between 2 - 1" foam panels, 3/8 marine ply outside and fiberglass inside. Foam is cheap, the ply was $130 a sheet for 12 sheets and that was almost 4 years ago, so I'm using it.

Hope he builds a frame if this is with homebrew panels.

Just a comment, building a mobile unit isn't like building a house, it's better to use boatbuilding techniques, stringers rather than butted studs, just say'n.

I'm still weighing the options between building my own panels and buying something commercially available.

If I DIY, the plan is to frame the camper out with 16" or 24" on-center studs throughout the walls, as well as across the floor and ceiling. Construction adhesive, with pocket screws to get everything nice and tight.

Once I get the window/door placements worked out inside the frame, install the outer plywood (more glue and brad nails), install PVC foam core (with lots of temporary cross-braces to get a tight adhesion), then the inner walls.

Glass the exterior, epoxy the interior.