Using the trailer as part of the floor construction?

[Themontashu]

I’m building a trailer out of 2x3 and in the name of saving some height and increasing the insulation I was thinking about filling the tubes with spray foam, putting tabs on the frame about an inch down for a bottom of the floor, then insulation, then a skin over the trailer tube itself to complete the floor.

Anyone done anything like this? I like the idea of added interior height and the ability to do as much insulation as I please.

 

[celadon]

I have wondered about this too. Even skinning over both the bottom and top of the steel frame and insulating between. I imagine that the metal would still do a pretty good job of being a thermal bridge... I haven't considered it in too much detail, so I'll be very eager to see what responses you get here. Thanks for posting this topic!

 

[Pmullen503]

I'd worry about rust inside the steel tubes if moisture got in. Might be less of a problem, who knows?

The thermal bridging issue is real and you can get condensation. In my foamy, I get frost on screw heads that connect to the outside when winter camping.

Unless I were trying to shave every inch off the height. I think I would build a conventional insulated floor.

 

[Themontashu]

I'd worry about rust inside the steel tubes if moisture got in. Might be less of a problem, who knows?

The thermal bridging issue is real and you can get condensation. In my foamy, I get frost on screw heads that connect to the outside when winter camping.

Unless I were trying to shave every inch off the height. I think I would build a conventional insulated floor.

I can cap tubes, and I’d probably fill then with insulation. I have certified welders in my back pocket who do pressurized stuff so that’s not a big concern.

Condensation seems to be the answer! Sounds like I should just do it how everyone else does but I’m also wondering if using an adhesive for the floor instead of screws, I’m already planning on anchoring the walls from the sides so I can just tie into the frame there. Additionally, using C channels instead of tube would reduce a lot of that thermal bridging. Again, sounds like I shouldn’t go this route, but it has me thinking for sure.

 

[Squigie]

If you're not worried about the steel conducting cold (sapping heat), I wouldn't worry about it. Seems fine.
You lose the benefit of a monocoque and frame strengthening each other, since the overall structure is now two walls and a roof attached to a frame. But it would probably be fine.


As for internal corrosion in the steel, that can be dealt with. There are many ways.
(You replied while I was typing, and mentioned capping. But I'll leave what I wrote. It was more of a reply to Pmullen, anyway.)
Fully sealing it is the simplest concept.
But there are others.
One that I have never seen used before, and honestly have never priced the tubing, is using A588 'weathering' steel for the frame. It oxidizes to a dark red/brown and then slows tremendously, due to copper content, even if fully exposed to the atmosphere. Very stable, long-lasting stuff. Commonly used for bridges, buildings, etc. It welds wonderfully, too. I'd like to see someone use it, but don't know if it is viable price-wise. I use it in corrosion-prone locations on vehicles, when fabricating brackets, mounts, etc.


One method that I have used in the past was arguably two, combined.
Wasn't a trailer. Rather, part of the structure for a porch. (But using the same 2x2x1/8" A500/A513 square tube so common for TDs.)

I sprayed inside the tubes with a water-displacing penetrating oil. (CRC Knock'er Loose)
I let them bake in the hot summer sun for a few hours, then sprayed again.
A few more hours of "bake" time really let that oil work into the pores.
One end was welded closed, or welded to another portion of the structure. If possible, the inside of that weld was sprayed/fogged again.
Then I purged the tubing with C25 gas (75% Argon / 25% CO2) and welded the other end cap or next piece of tubing in place.

About 18 months after completion, I had to drill two holes to add another element. The hiss was very satisfying.
Until I realized that it was a partial vacuum sucking cold, humid air in. [not so smug face] So, I drilled a little drain hole* at the bottom of those two tubes and we'll check again in a couple years. (They were on a 15 degree angle.)

 

[Pmullen503]

..........

Condensation seems to be the answer! Sounds like I should just do it how everyone else does but I’m also wondering if using an adhesive for the floor instead of screws, I’m already planning on anchoring the walls from the sides so I can just tie into the frame there. Additionally, using C channels instead of tube would reduce a lot of that thermal bridging. Again, sounds like I shouldn’t go this route, but it has me thinking for sure.

I think if you take care that your fasteners don't go all the way from inside to outside you don't have to worry. But if you mean gluing the cabin to the trailer, I'd feel better if there were fasteners too, but that's just me.

One thing about insulation. You quickly get to the point of diminishing returns because of the need for ventilation. The relatively small internal volume compared to number of occupants mean you need turn over the air inside at a pretty high rate compared to a home. So unless you are using some kind of air to air heat exchanger in your ventilation system, you have to add more heat.

I just use an extra blanket.

 

[Themontashu]

..........

Condensation seems to be the answer! Sounds like I should just do it how everyone else does but I’m also wondering if using an adhesive for the floor instead of screws, I’m already planning on anchoring the walls from the sides so I can just tie into the frame there. Additionally, using C channels instead of tube would reduce a lot of that thermal bridging. Again, sounds like I shouldn’t go this route, but it has me thinking for sure.

I think if you take care that your fasteners don't go all the way from inside to outside you don't have to worry. But if you mean gluing the cabin to the trailer, I'd feel better if there were fasteners too, but that's just me.

One thing about insulation. You quickly get to the point of diminishing returns because of the need for ventilation. The relatively small internal volume compared to number of occupants mean you need turn over the air inside at a pretty high rate compared to a home. So unless you are using some kind of air to air heat exchanger in your ventilation system, you have to add more heat.

I just use an extra blanket.

I’m talking about glueing the floor down, but then bolting the walls to the metal, effectively turning the frame into the monocoque.

Adhesives (done right) are actually strong as ****, much stronger than mechanical fasteners in a lot of cases. That’s assuming it’s the correct application of course. (Here’s an example of an aircraft stud that gets glued on using a pretty damn small flange https://www.clickbond.com/product-detai ... se-stud-01) but depending on thickness I like the idea of a backup.

100% on it being a small space, it’s just like sleeping in a small tent that way. The insulation is really just to help prevent condensation. Now I’m wondering if floor insulation is just useless entirely as the foam mattress is going to have quite the R value.

 

[Pmullen503]

..................

I’m talking about glueing the floor down, but then bolting the walls to the metal, effectively turning the frame into the monocoque.

Adhesives (done right) are actually strong as ****, much stronger than mechanical fasteners in a lot of cases. That’s assuming it’s the correct application of course. (Here’s an example of an aircraft stud that gets glued on using a pretty damn small flange https://www.clickbond.com/product-detai ... se-stud-01) but depending on thickness I like the idea of a backup.

100% on it being a small space, it’s just like sleeping in a small tent that way. The insulation is really just to help prevent condensation. Now I’m wondering if floor insulation is just useless entirely as the foam mattress is going to have quite the R value.

I have through bolts that are covered by the mattress, no condensation there obviously. But not every square inch is covered by the mattress. My floor is 1 1/2" foam with a 1.5" wood perimeter and wood where the frame bolts are, covered both sides with 1/4" ply and PMF. No condensation on the exposed floor parts, so adequate insulation. Walls are 2" foam and PMF, no condensation there. Windows, door handles and a few through bolts for the roof rack is where I can get condensation or frost when it's like 25F or below.

I agree that the right adhesive can work because of all the extra surface area. But one thing to consider: After 8 years I'm thinking about some extensive modifications/improvements to my foamy. I was planning to take it off the trailer to facilitate that. Doing those changes "on frame" would be more difficult.

 

[Squigie]

Unless the mattress is closed cell foam, you can still end up with condensation under it.
It may go unnoticed.
It may never be a problem.
Or it might end up as mildew.

I have dealt with condensation under my sleeping matt/pad/mattress in a few trailers and a motorhome -- (Cab-over bed, and the bed partially on top of the generator enclosure). And also with several tents - (setting up on frozen ground, even with ground cloths). If I can take small measures to prevent it, I will. Especially when it is easily done during the build process.

Whether or not that matters depends on your intent, usage, and how much you care.
For me, it matters. As far as total number of camping days, I camp more in the Fall (sub-freezing most of the time) than Spring and Summer combined (might freeze over night, but generally doesn't). Dealing with cold and condensation matters.
But I have a brother in the same stage of life, with a similar family situation, and similar assortment of trailers. He only camps a few days a year in the cold; and when he does, he usually stays in my trailer or our other brother's trailer. He does almost exclusively summer trips with his gear. Condensation and cold are not factors.

Every bit of my TD is designed to be insulated. But if my brother was building one, he'd go for basic, uninsulated 3/4" ply walls and floor, and probably never regret it.