Building our box out of wood?

[s.e.charles]

maybe consider applying skins (in & out) with the face grain 90 degrees to framing members even though it will require additional blocking to support every seam.

Q: do you have the luxury of time to do this project?

Q: do you have the luxury of 100% budget over-ride?

you will most likely need a healthy dose of both.

 

[Deleted member 9101]

I'm trying to reduce thermal bridging by pushing the studs out to 24" on center, so I'd probably use thicker plywood than you did...

I'd do 12" and not worry about thermal bridging.

 

[verdesard0g]

one thing I learned about MDO*: glue only penetrates the paper layer. take something apart, and you'll see what I mean.

* i've only used "2 sides". i guess if you used 1 side & glued just the wood veneer, it would hold.

yep MDO is MDF on one side only, the outside....and it's 3/8". The trailer weighs in at just a bit over 4K lbs. fully outfitted.

 

[rruff]

what about building it out of wood, framed like a house, with plywood on the inside and outside, and everything glued and screwed. Flat-roofed.

You can use wood, but I think you are better off treating it as a "stressed skin" structure with wood reinforcement, rather than stud frame construction. That means the foam is also structurally glued to both sides. Houses don't have to take the pounding and sharp jolts that a camper does... well, not unless you have an earthquake.

I made a camper similar to that 20 years ago. The plywood was 2.7mm luan inside and out, with 1x2s and 3/4" XPS for the interior of the panels. An important feature was the layer of fiberglass that encapsulated it. I was very impressed with its ability to knock tree branches out of the way.

If you are considering having a box made, check out Total Composites: https://totalcomposites.com/ The owner is active on this forum (name Victorian). Boxes run around $20k or so... it's all the other stuff that get's expensive. They are located in BC, so from Portland you could pick it up and avoid shipping (and maybe Chinese import taxes; that's where the panels are made). It looks like they use fiberglass tubing in the panels if you need structural reinforcement. Nice thing is you should be able to mount stuff on the outside and not worry about sealing holes to prevent wood rot.

Even though you said you only need it to last 5 years, you are going to sink a lot of time and money into making this the way you want. That's true regardless of the materials you use. I think you'll be happier 5 years from now if your rig is in great condition and ready for more adventures, rather than ready to fall apart...

 

[MTVR]

if using wooden framing, its faff to worry about thermal bridging for a thin wall box whats about 40 cubic meters.

If I'm doing the math correctly, we're talking about roughly 25 cubic meters (900 cubic feet).

And I don't know what you mean by "thin wall". We're talking about walls 4" (100mm) thick, and a ceiling/roof possibly up to 6" (150mm) thick).

We want to be able to have enough insulation that we can control our inside temperatures without fighting a losing battle in terms of our energy consumption...

 

[s.e.charles]

yep MDO is MDF on one side only, the outside....and it's 3/8". The trailer weighs in at just a bit over 4K lbs. fully outfitted.

MDF vs MDO - FineWoodworking

I’m confused by the MDF and MDO abbreviations. Living in somewhat rural NH, we don’t have a lot of lumber yards and I recently ordered and received a sheet of […]

www.finewoodworking.com

have no experience with your product, but this clarifies mine:

(see page #4) https://murphyplywood.com/pdfs/softwood/HDO_MDO_Product_Guides.pdf

or: https://www.andersonplywood.com/mdo-medium-density-overlay/

or: https://www.lumberjocks.com/reviews/965

 

[IdaSHO]

If I'm doing the math correctly, we're talking about roughly 25 cubic meters (900 cubic feet).

And I don't know what you mean by "thin wall". We're talking about walls 4" (100mm) thick, and a ceiling/roof possibly up to 6" (150mm) thick).

With that wall thickness you can easily create a thermal break by staggering studs/framing.

Just replace the 2x4's with 1x2's or 2x2's, and the insualted with rigid XPS foam.

For ceiling, you could do a truss'ed design to maximize the thermal break...
Many RV companies already do this, but with a radius/arch. You could easily make them flat

Like this, but done horizontal.

I considered these early on, but my camper just isn't large enough to justify it.
Such a thick wall really soaks up interior volume, when you already dot have much!

With a large camper, I feel the benefits are there....

 

[MTVR]

We had not considered 6" thick walls, but at 8'6" wide overall, we might be able to make everything fit...

 

[rruff]

We want to be able to have enough insulation that we can control our inside temperatures without fighting a losing battle in terms of our energy consumption...

I don't believe that is an important concern. You can certainly build a box that has a high R value (and it's easy to do that with stresed skins and foam core), but you will still need ventilation for the occupants, and this ends up being a larger % of your heat load than in a house. Heating loads are ventilation requirements (heating the fresh air that's introduced) and surface area/R. In modern houses the ventilation is already a high %, and you will have a very tiny house...

The biggest reason to avoid bridging is condensation.

 

[IdaSHO]

Sorry for the confusion...

You dont need to go 6" thick. And personally I wouldnt bother going so thick.

Stick with your 4" walls, using a 4" bottom plate and staggered 1x2 or 2x2 studs, you are good to go.

 

[MTVR]

I don't believe that is an important concern. You can certainly build a box that has a high R value (and it's easy to do that with stresed skins and foam core), but you will still need ventilation for the occupants, and this ends up being a larger % of your heat load than in a house. Heating loads are ventilation requirements (heating the fresh air that's introduced) and surface area/R. In modern houses the ventilation is already a high %, and you will have a very tiny house...

The biggest reason to avoid bridging is condensation.

Understood. And thank you.

What about ventilating during the milder part of the day, and then closing up before it gets real hot (or real cold)?

 

[MTVR]

It seems to me that the roof is going to get more sun, at a more direct angle, than the sides. Maybe I could put an unbroken layer of polyiso foam boards across the top of the studs under the outer plywood.

Just spit-balling here...

 

[s.e.charles]

…. And dont over think every ************** detail.
If you do, dont tell us here. You will be ridiculed without mercy.

that's always been my theory. i mean, why take advantage of someone else's experience when you can flub things up for yourself and get the credit?

 

[Deleted member 9101]

Understood. And thank you.

What about ventilating during the milder part of the day, and then closing up before it gets real hot (or real cold)?

With such a small volume of air and two people, you will need to constantly bring in fresh air, or it will get uncomfortablely humid pretty quickly.

My house is obnoxiously well insulated and to compensate for the lack of fresh air it has an outside air intake for the A/C to pull from eveey time it turns on.

 

[rruff]

What about ventilating during the milder part of the day, and then closing up before it gets real hot (or real cold)?

Jnich77 is correct. You need a good amount of constant air exchange, and you'll need that regardless of how good your insulation is.