Is it okay to not have a vapor barrier in lightweight tiny house type construction?

Question

So, I have a bit of an odd project. It is a bit outside of the box, and I'm looking for alternative approaches that stick with the theme (if the plan outlined below doesn't work, that is).

Essentially, I'm building a camper, but think "Tiny House". It will be built on a 17-21 foot camper frame. The difference is I'm going to great lengths to keep it as light as possible for two reasons:

1. Camper frames aren't designed to hold the heavy weight that a true Tiny Home build requires.
2. This needs to be light enough so that it is reasonable to hook up for a weekend trip. "Tiny Homes" tip the scales well beyond reasonable.

I'd like to keep the "cabin" look as well with a natural looking siding. I'm also trying to keep it as basic as possible. More like an ice shanty that may never see actual ice and more be used as a bunkhouse. It will be heated, cooled, etc... and sometimes not while parked.

For the walls:
2x3 studs will be used for framing perhaps 2' off center, and rigid insulation will be used to fill the gaps. On the outside my idea is to use an exterior ply paneling that looks something like T&G. There is a product at lowes that fits the bill. This will provide both sheathing and siding in one layer.

I would use a combination of 2" and .5" insulation layers to completely fill the 2.5" depth created by the studs. Some sort of adhesive would be used generously to glue the insulation layers to each other, the studs, and the exterior paneling. A decorative interior paneling (something thin & light weight, like 1/4" paneling) would be used and also applied with generous amounts of adhesive.

So, it's sort of like I'm creating a big DIY SIP panel that includes framing, siding, and interior paneling. The purpose of all of this is to lower the materials needed, and keep the weight down, but keep it very strong by bonding all layers together. It will after all be dealing with the forces that happen on the highway.

So with all that said, I'm sure I'm being naive about many aspects - such as a vapor barrier. My basic understanding of tyvek is vapor can pass through it, but moisture cannot.

I won't having sheathing to apply a vapor barrier to because the sheathing and siding are one layer. Is this a terrible idea, and are there alternative approaches?

Window installation is another concern.

Bottom line - I don't want to build something that is going to be a rotting moldy box in a year.

Answer 1

Didn't you ask the same question somewhere else recently? This sounds really familiar.

You should stick to proven wall designs. 2x3s are going to be more difficult to work with. No sheathing will mean virtually no racking resistance against wind. You almost never want a dedicated vapor barrier, especially if you're planning to use rigid foam anywhere (that will result in a vapor trap). Your exterior non-sheathing plan is risky if you don't plan to put housewrap or tar paper under it.

If you really want to do this, here would be my recommendation, inside to out:

• Lightweight wood paneling or whatever
• 2x3s for structural framing with the cavities filled with slightly compressed unfaced fiberglass or mineral wool batts (installed well)
• Diagonal steel braces for racking resistance
• 1+" Foil-faced non-structural foam sheathing (use Polyiso if you live where it's hot; otherwise EPS) over the outside of the studs to act as thermal break, with seams scrupulously taped with high-quality tape to constitute a water-resistive barrier layer
• 1x3 or 1/2" plywood strips fastened through the foam and into the studs to act as vertical furring strips
• siding fastened to the furring strips

This gives you a safe, compact, energy-efficient wall.

Final thought: if you want a weekend camper, that's what you should buy. It will likely cost you far less money than doing all of this yourself.

Answer 2

Skip the 2x3 studs. If you are going to build "nearly SIPs" you'll get a better (lighter, stronger, better insulated) product if you just build (or buy) SIPs and build with SIP methods. Modern construction adhesives make this far more practical as a DIY project than it used to be, though getting them factory built should not be overly difficult or expensive.

Or pick up a used refrigerated truck body - which will be steel, fiberglass or aluminum-faced SIPs, and already built to take highway speeds. Learn faux painting in the time and with the money you save and people will have to come right up to it to see if it's boards or not.

Answer 3

Perhaps I'll simplify this and use 11/32 sheathing so I can apply housewrap around that, and then whatever siding I wish (perhaps T&G cedar). It'll probably add 500-600 lbs to the walls, but it sounds like it'll simplify it a bit and be worth it.