Enclosing the underbelly: a few design questions

We want to do some winter traveling next year - time to enclose the underbelly. We will need to stay at RV parks in order to power the heat tape -- boondocking in very cold weather is just not going to work for us. (Sigh.) The biggest downside of enclosing the underbelly is that no matter how well I seal it, it will provide a home for critters. That is a risk that can't be avoided. (Fortunately, the trailer itself is now well sealed. No more mice in the last several years of camping!)

After doing quite a bit of research, I've decided to use rigid foam panels with a coroplast coating. I have plenty of room under the trailer (since it is lifted due to an axle flip), even though the tanks hang down a few inches below the level of the frame members. The enclosure will essentially be an upside-down box, with side pieces that drop down a little so as to clear the bottom of the tanks. The sheet of insulating foam (and the coroplast bottom layer) will rest on cross-members.

Here is a rough sketch showing the general layout:

But there are a few specific issues:

First, what is the best way to secure the vertical panels of the enclosure to the existing chassis frame members? Drill into the steel frame members and attach brackets, using self-tapping sheet metal screws? Or weld drop-down brackets to the frame and then attach the enclosure to those brackets? (My son knows how to weld - I don't.) My concern about drilling into the frame is that I do not want to weaken it at all. My concern about welding is that he will have to weld upside-down, under the trailer -- it seems risky.

Second, there will be a 90 degree joint between the vertical drop-down sides of the enclosure and the horizontal bottom panel. What is the best way to seal that joint? (Not duct tape. It does not hold up well when pelted with rainwater at 60 mph. Don't ask how I know this.)

Third, is it better to use aluminum or steel for the cross-members supporting the bottom foam panel? Aluminum is light and easy to work with. Steel is stronger but heavy. The aluminum might sag. The steel might put too much stress on the attachment points of the enclosure.

Fourth, is it necessary to glue the coroplast to the foam panels? If so, how? My inclination is to use 3M industrial-strength aerosol contact cement. It's expensive, but it works. I would do the gluing before installing the foam.

As always, your advice is greatly appreciated!

JBarca
Nomad II
Feb 11, 2018
profdant139 wrote:
Some more great tips -- thanks! JBarca, by jam nuts, do you mean nylon core nuts??

So here is another question -- if I seal the bottom with coroplast, what happens if there is condensation from the outside of the water tank? Won't it puddle up on top of the plastic??

The jam nut can be a nylock nut if you want it. But just using a normal full nut will work. They do make jam nuts, they are a little thinner but it doesn't really add much. If you want bulletproof, add a lock washer under the head of the nut. The nylock nut you do not need a lock washer.

See here for more. In your case, the threaded hole in the clamp is one nut and the extra nut is the jam nut. http://www.boltscience.com/pages/twonuts.htm

My enclosed tanks has foam board insulated sides, a reflectix bottom insulation and a chloroplast bottom. I used gorilla tape around the perimeter of the chloroplast to seal the flutes so road water splash while towing does not get in the flutes.

As far as condensation, it has not happened to mine. Since the tanks are inside an insulated compartment, there are not large fast temperature changes between cold and hot or hot and cold surfaces. In the dead of winter when the camper is in storage the insulated area stays dry as inside the tank compartment is not exposed to damp air. Yes everything in the camper is at zero degrees stored outside, but when it warms up during the day above freezing, it is very slow due to the insulation. The entire camper can be sweating from a rapid sun heat rise to create condensation on cold objects but the tank compartment it not as there is no fast temperature rises or damp air.

During the summer, I do not have tank sweat either from putting water in the fresh tank. Again the insulated compartment keeps the moist air away outside from condensing on the tank. Or the cold water in the tank away from the very hot moist air.

Think of a plastic ice water bottle, in the summer if the plastic is exposed to the air, the bottle sweats due the cold water/plastic being directly exposed to warm air. Take that same water bottle, put an insulating sleeve over it and fill it with ice water. Due to the insulation, no condensation forms unless there is a hole in the insulation. Then the air can get to the bottle through the hole and the sweat is only at the hole area.

I may not be explaining it exactly right, but due to the insulated compartment, the conditions just are not right for condensation to form.

Hope this helps.

John
profdant139
Explorer II
Feb 11, 2018
Some more great tips -- thanks! JBarca, by jam nuts, do you mean nylon core nuts??

And deadticket, I like the double tape idea. The corrugated panels (coroplast) can be found in big hardware stores or online. They come in various thicknesses.

So here is another question -- if I seal the bottom with coroplast, what happens if there is condensation from the outside of the water tank? Won't it puddle up on top of the plastic??
deadticket8
Explorer
Feb 10, 2018
BTW where does one buy the corrugated plastic panels?
deadticket8
Explorer
Feb 10, 2018
I will put in my 2 cents having lived in my trailer in winter in many states. Two words- Double Up! Heat tape that is. If you don't have a ducted furnace you will be using heat tape to keep pipes from freezing yes? If one of your pipes freezes do you really want to rip the bottom open, crawling around on the frozen ground trouble shooting heat tape? Nah. Unplug #1 and plug in #2.Maybe problem solved. A little more initial expense but you saved all that money buying a 3 season trailer! Insulate any water lines after heat taping, I have used the cheap foam stuff in winter( with heat tape) never a freeze up on my supply line. I used to use copper in winter but changed to PEX with great results. You may want to put heat blankets on tanks(they make some nice ones for engine blocks) if you are planning to keep bottom unducted.
JBarca
Nomad II
Feb 10, 2018
Hi profdant139,

I'll add 2 things to help the cause.

1. Drilling or welding to the frame. Express caution here as there are OK ways to do both or wrong ways to do both. We do not even know what kind of frame you have. If you want to pursue this idea, it would be best to post pics of the frame ane how you want to weld or drill. Then we can comment on exactly what you have and are trying to do.

2. I followed Larry's link. Cool!!! If you go with the beam clamp then there is no drilling or welding and it's simple. I would add one thing if you want to enhance it. You can add a jam nut on the clamping screw if there is concern about the clamp coming loose. Larry has reported his never came loose, and no reason to question that, but he also has torsion axles can have less vibration into the frame to start unloosening. Point being, adding the jam nut really does not cost much and can help the anti back off. I have seen some of them come loose on machines that had constant vibration without the jam nut. A TT bounces down the road does vibrate. Some more then others.

Good luck and hope this helps

John
LarryJM
Explorer II
Feb 07, 2018
profdant139 wrote:
Good point, Larry! So if I understand it correctly, with your method, the aluminum support rails can go essentially anywhere, since the beam clamps can slide up and down the web on the frame members?

And when you say "bolt the panels together," do you mean the foam panels with the coroplast covers? Or do you use an "underskin" of aluminum sheeting?

Thanks again for all of these tips!

The way I did mine is each panel is bolted with nylon bolts and nuts to the adjacent panels thru the alum angle on the edges and the holes have to match so the panels need to go on in a certain way. Also almost every panel has some sort of penetration like for LP lines up to appliances, water drain lines, dump valve area, and of course the over axle panels are unique since they have to fit around the suspension supports. One reason for bolting the edges together is to get a little better "span strength" by having two 90deg angles bolted together. The foam board I put for extra insulation simply lays on the panels and there is no extra skin like alum, just the coroplast.

Larry
profdant139
Explorer II
Feb 07, 2018
Good point, Larry! So if I understand it correctly, with your method, the aluminum support rails can go essentially anywhere, since the beam clamps can slide up and down the web on the frame members?

And when you say "bolt the panels together," do you mean the foam panels with the coroplast covers? Or do you use an "underskin" of aluminum sheeting?

Thanks again for all of these tips!
LarryJM
Explorer II
Feb 07, 2018
profdant139 wrote:
I have found a few "underbelly build" blog posts where folks have used aluminum panels to span from one frame member across to the other side, using self tapping sheet metal screws into the web of the frame beams.

I am not sure that this would have the strength or rigidity to support foam insulating panels, but it is a simple solution.

What do the "four season" manufacturers use on their underbelly enclosures?

One issue with not making the panels somewhat semi-rigid at least is if you remove one it is hard to match up the screw holes in the panel with those on the frame. You have to basically jack up the panel somewhat flat so things match. With the system I used you just bolt one panel to the other and then lift the sides where the aluminum is and put on the "beam clamp".

Larry
profdant139
Explorer II
Feb 07, 2018
I have found a few "underbelly build" blog posts where folks have used aluminum panels to span from one frame member across to the other side, using self tapping sheet metal screws into the web of the frame beams.

I am not sure that this would have the strength or rigidity to support foam insulating panels, but it is a simple solution.

What do the "four season" manufacturers use on their underbelly enclosures?
LarryJM
Explorer II
Feb 07, 2018
I think I listed the thickness of aluminum angle I used for my underbelly project and IIRC it wasn't the lightest one but the heavier one. The thin angle just wasn't stiff enough when spanned over 6' and holding up the coroplast material. I think I might have paid around $15 or so per piece back in 2008/9. For me money was not much of a concern up to a limit, but added weight was a concern.

Larry

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