Plywood camper fabrication questions (thinking of idaSho)

[ITTOG]

Sorry must have missed it, what type of foam sheet are you referring to? XPS? or something different?

My camper is built using 1-1/2" XPS foam. Easy to find, great insulation properties, and inexpensive.
And in my opinion, for the small volume of space of a camper, anything beyond 2" is massive overkill and a waste in both money an weight.
Our camper with just 1-1/2 foam easily maintains whatever temp you could want in sub zero temps with a tiny 6kBTU furnace

Typically in temps below freezing and below zero we track a bunch of snow into the camper, so moisture is an issue, that requires air exchange to minimize condensation.
Even in those temp, with the windows open to provide air exchange, we still don't have problems heating it to our normal cabin temp (between 65 and 86degrees F)

Great pic of your rig.

What is causing the three lines in the snow on the front of the cab over

 

[IdaSHO]

That's heat bleeding thru the camper at the framing.

Unless you have a thermal break of some kind, you will see the framing "thru" the skins in cold temps,
as the framing is much more thermally conductive than the insulation. Heat transfers thru it much more readily.

 

[ITTOG]

That's heat bleeding thru the camper at the framing.

Unless you have a thermal break of some kind, you will see the framing "thru" the skins in cold temps,
as the framing is much more thermally conductive than the insulation. Heat transfers thru it much more readily.

I wasn't expecting that with the wood frame construction. Interesting.

 

[IdaSHO]

Kenny! Hope you've been enjoying your winter camping, friend!

So, my concern is less insulative value and more structural strength.

My camper design is substantially bigger than yours because we've got a family of 4 plus a dog. I also plan to periodically have it off the flatbed just on jacks while occupied because we do more campground camping than you, and want to travel off campgrounn without breaking camp.

I've got a 13' box on an 11' flatbed, PLUS a king-sized cabover. 8' tall walls in the main living space to accommodate an insulated basement for water, batteries, etc.

Between the large span of the roof and substantial cabover, I really want to make sure this thing is rigid as hell when its not on the flatbed.

I had originally looked at the cheap green/pink stuff you'd buy at Home Depot (Owens Corning XPS or whatever), but it doesn't seem to be structural. As a result, I pivoted into the structural foam core you'd find in marine applications.

My cousin in an architect (houses, not RVs), and was concerned about structural integrity of box store XPS without steel or aluminum frame.

I design and build for a living, using load calcs and common sense. Ill keep my trap shut with regards to "architect" comments

What structural foam core? Have a link?
The way I've built using XPS foam and skins, all glued together creates a structural foam core (composite) panel.

Quick aside: why did you choose to frame and then foam over building panels and then combining them?

Honestly, shop space. And it makes tying corners and junctions together, without the need for anything more than adhesive/epoxy.
The way it is designed/framed and essentially every joint using lap joint, wet set in LP premium and screwed (pocket screw) then overcoated
(encapsulated) in epoxy, the structure is incredibly strong even before the foam and skins. All together is stupid ridged.
Being able to tie corners together in the framing stage plays a big part of that. Had I done entire wall sections, tying together corners/junctions would be tricky.

 

[IdaSHO]

I wasn't expecting that with the wood frame construction. Interesting.

I home construction it is sometimes referred to as "frost ghosting"
But due to the difference in construction (attic vs no attic) the principles are not quite 100% the same

Bottom line, wood is a decent insulator. FAR better than steel or aluminum, but FAR less than any insulation.
So it transfers heat better. From the interior to the exterior, making framing visible.

Doesnt happen all the time. Dew point and moisture play into it as well.

 

[ReluctantTraveler]

I design and build for a living, using load calcs and common sense. Ill keep my trap shut with regards to "architect" comments

Oh no worries at all!

I don't think he fully grasped the wooden frame part of it, because I originally asked about SIPs panels. And he fully caveated all of his responses with "I don't design structures meant for moving on the road," so I suspect he was erring on the side of extra cautious. Family and all!

What structural foam core? Have a link?
The way I've built using XPS foam and skins, all glued together creates a structural foam core (composite) panel.

Something like this: https://expeditionportal.com/forum/...thinking-of-idasho.237207/page-3#post-3090792

Or this: https://www.carbon-core.com/product/pvc-structural-foam-sheets/

I had originally planned to use this: https://www.homedepot.com/p/Owens-C...Rigid-Foam-Board-Insulation-88WDNGX/315197857

Always sandwiched between 1/8" or 1/4" marine-grade plywood from Boulter Plywood, framed on the all edges with 16" on center studs, and glued all over with SikaFlex or PL Premium or the like.

The way it is designed/framed and essentially every joint using lap joint, wet set in LP premium and screwed (pocket screw) then overcoated

I for some reason thought you said "no screws, just brad nails." It's honestly comforting to hear that the frame is screwed.

I'm slowly wrapping my head around the idea that the whole thing becomes one big structural piece, unlike how a home works.

Your build approach seems like it would make lining up doors, windows, and hatches a heck of a lot easier! More wiggle room vs. having to get it 100% right when you construct the panels.

@rruff's counter argument was that glassing is a lot easier when all of the panels can be laid on the ground, which is fair. The idea of glassing a vertical panel made me originally not want to fiberglass the thing.

 

[rruff]

I wasn't expecting that with the wood frame construction. Interesting.

You'll also get condensation inside where the wood is.

If you are building a fancy rig it probably makes sense to have a heat-exchange ventilator... or just a big enough heater that you can open windows.

So, my concern is less insulation value and more structural strength.

Between the large span of the roof and substantial cabover, I really want to make sure this thing is rigid as hell when its not on the flatbed.

Have you built any test samples yet? Ridgidity and strength are two different things. A thicker panel increases both, but how much is enough? Make a 8' x 16" piece and dance on it, see what you think.

As I recall Kenny used 6mm and 4mm (Meranti?) ply with a 1.5" core. I used 2.7mm luan with a 3/4" core for the walls and roof. Floor/base was 5mm on 1.5" core. So his walls are at least 8x the stiffness and strength of what I made! My roof was angled on the sides though, so less of a span, and the camper was a little smaller too. But if you really don't care about insulation then you shouldn't need a thick wall. Since you have a basement you can make the floor/base incredibly strong and stiff just by taking advantage of the height of it.

 

[ReluctantTraveler]

You'll also get condensation inside where the wood is.

If you are building a fancy rig it probably makes sense to have a heat-exchange ventilator.



Have you built any test samples yet? Ridgidity and strength are two different things. A thicker panel increases both, but how much is enough? Make a 8' x 16" piece and dance on it, see what you think.

As I recall Kenny used 6mm and 4mm (Meranti?) ply with a 1.5" core. I used 2.7mm luan with a 3/4" core for the walls and roof. Floor/base was 5mm on 1.5" core. So his walls are at least 8x the stiffness and strength of what I made! The My roof was angled on the sides though, so less of a span, and the camper was a little smaller too. But if you really don't care about insulation then you shouldn't need a thick wall. Since you have a basement you can make the floor/base incredibly strong and stiff just by taking advantage of the height of it.

I haven't made a test panel yet as I was still sourcing vendors/options. I will!

I suppose a lot of this is marketing, but you see companies who make composite panels talking about 2-3" wall thicknesses a lot.

I do care about insulation, as we'll likely be both camping in the snow sometimes (for snowboarding trips) and dealing with sub-zero tips when we travel down south from the north during the winter months.

However, it sounds like Kenny's plenty warm in his 1.5" thick walls! We'd have twice as many people emitting body heat in ours.

 

[rruff]

I haven't made a test panel yet as I was still sourcing vendors/options. I will!

Start with cheap stuff from the hardware store. If that exceeds your expectations (and I think you'll be impressed), then...

 

[ReluctantTraveler]

Start with cheap stuff from the hardware store. If that exceeds your expectations (and I think you'll be impressed), then...

Do you have any pics or thread of your build, BTW? I'm REALLY curious to see what you've done! You've been really really helpful!

 

[1000arms]

... I had originally planned to use this: https://www.homedepot.com/p/Owens-C...Rigid-Foam-Board-Insulation-88WDNGX/315197857

I for some reason thought you said "no screws, just brad nails." It's honestly comforting to hear that the frame is screwed.

I'm slowly wrapping my head around the idea that the whole thing becomes one big structural piece, unlike how a home works.

Your build approach seems like it would make lining up doors, windows, and hatches a heck of a lot easier! More wiggle room vs. having to get it 100% right when you construct the panels.

The foam you linked is 15 PSI (Foamular 150). 2" foam can usually be found in 25 PSI (Foamular 250), currently at about $1.50 per square foot, in 4' x 8' sheets.

https://www.homedepot.com/p/Owens-C...Rigid-Foam-Board-Insulation-52DDNGX/315197962

I think @IdaSHO only used the screws to hold things together while the adhesive cured (rather than for any real structural strength).

Keep wrapping your head around structural integrity and how powerful adhesives can be when used correctly. I suspect you are well on your way!

Remember the possibility of building up frame sections with layers of 3/4" (nominal) plywood. Three pieces of 3/4" (nominal) plywood plus adhesive should allow 2" Foamular 250 to fit within the panel skins. It would be quite easy to make door and window openings as well as "bulkheads" for mounting coat-racks. exhaust-fans, bike-racks, etc.

 

[ReluctantTraveler]

Remember the possibility of building up frame sections with layers of 3/4" (nominal) plywood. Three pieces of 3/4" (nominal) plywood plus adhesive should allow 2" Foamular 250 to fit within the panel skins. It would be quite easy to make door and window openings as well as "bulkheads" for mounting coat-racks. exhaust-fans, bike-racks, etc.

Do you mean plywood layers instead of, say, solid lumber for internal studs?

I'd already planned to use it for the skins, of course, but also for rounded windows (I want the Arctic Tern awning-style ones), inspired by how Kenny did his.

 

[1000arms]

Do you mean plywood layers instead of, say, solid lumber for internal studs?

I'd already planned to use it for the skins, of course, but also for rounded windows (I want the Arctic Tern awning-style ones), inspired by how Kenny did his.

Yes. It could be done with overlapping strips of 2" wide 3/4" plywood. It would be more work than just using studs, headers, plates, etc.

One could, with careful cutting (or pay to have it CNC cut), could, instead of just using strips, also incorporate pieces with nubs at 16" OC (say 3" on one "strip" and 5" nubs on another "strip") build up a frame section while minimizing weak points. One could even do "scarf" joints at the nubs to really distribute the loads.

The overlapping layers, especially with scarf joints, allows for a very strong frame.

 

[ReluctantTraveler]

Yes. It could be done with overlapping strips of 2" wide 3/4" plywood. It would be more work than just using studs, headers, plates, etc.

Is there a benefit to doing that?

 

[1000arms]

Is there a benefit to doing that?

Yes. I edited my post above to include that one could improve frame strength. It also makes it easy to incorporate "bulkheads" on the exterior or interior by using a 3/4" plywood "panel" as part of the frame. Just make sure to include it in 2 or more parts of the frame.

@IdaSHO 's frame is quite strong, but building the frame up from layered plywood could be even stronger than what he created. Going with a larger 2" frame (I think he used 1.5") would be even stronger, which might be what you are seeking for your rig.

Whether all the extra work is something you want to do, and whether it is truly needed for your camper?

Please do think about it, as it will likely help you with your thoughts on how to make a light and strong camper frame.