Aluminum land yacht insulation design... Overland related

Hey all,

Sorry if this is a bit long winded...

After every google search I was looking at linked to this forum, I figured I might as well engage with the community. I am a masters student in mechanical engineering, but realize that theoretical understanding doesn't compare with real world experience. It seems like this place is quite the compendium of knowledge.

So I am working with a friend prototyping some overland expedition vehicles. The cabins are built on f-350's with my friends being a 2015 regular cab, and mine being a 2001 crew cab. The basic concept is a pretty simple one, basically standard aluminum aircraft construction. The first one being built is aluminum square tubing (.125 wall 1.5" box) with .090 wall 6061 t6 sheeting that is riveted together. All aluminum will be corrosion proofed before final assembly, and the interface between extrusion and sheet will either be aided with sikaflex or VHB tape.

In doing some research into how I want to start out with my platform, and it seems like using box tubing is going to be challenging in terms of insulation. Using something like C-channel will enable us to better insulate the "wall studs" so to speak, and minimize thermal bridging and condensation issues. I guess my entire question here is regarding the best construction methods in order to create a good, insulating vapor barrier without breaking the bank.

So I read a lot about insulating metal hulled boats here, and it seems to me that the key is to create a vapor barrier, and that armaflex is the preferred approach. Our trucks will see very cold weather camping (-20*F or better), but likely in a much less humid climate than boat designers would experience. The problem that I am running into is simply that Armaflex is really expensive, especially in inch or greater thickness.

Is spray insulation something to consider here? It sounds like it is a terrible mess and often a fire hazard, but the upside is that it's inexpensive, and could add some structure to the build.

I was also wondering if using a thinner Armaflex that is properly adhered to the aluminum, and then attaching a flame resistant EPS to that to make a cheaper version of decent insulation.

I hope this is enough information. I have a few images here that should shed some light on the design.

Overview:



You can see the wall studs here...



And a top view cross section of my proposed wall...



Edit to add: looks like noone really posts in the metal boatbuilding section... Is this the best place for this, or would it be better moved to the general boat building section?

 

Welcome, gmacmt.
If I were doing it, I would start with coating the inside of the box with spray on truck bed liner. Then I would put a layer of insulation, type chosen by you, enclosed by a wooden panel to finish the inside. One place I have found handy is the forum called http://tnttt.com/viewtopic.php?f=38&t=40667

Keep us informed, please. I like the look of it so far.

Long live Kentucky

 

http://www.ecohome.net/guide/rigid-foam-panels-same-learn-best-used

I would go with one of the rigid foam panels as above. I was surprised to see that on one of them the R value dropped with lower temperatures. But as it starts out higher perhaps even at lower temps it would be better than the others

You can cut the panels for a friction fit but I would glue them in with either a spray adhesive or a flexible adhesive out of a caulking gun. Your foam supplier can supply a tape that you can tape the joints over if you are trying to maximize the insulating effect.

As the foams act as a vapor barrier, re the article, you will not need to provide another.

Whatever paneling that you are installing inside the unit will hold the foam in place if the adhesive lets go.

Any gaps can be filled with the "Spray Insulation in a Can" product that can be purchased at most building supply stores.

Your inverted channel stiffener will not allow fastening of a panel

 

Thanks for the quick replies.

Hoytedow, my only concern with something like bedliner is that it would make future servicing very difficult.

Barry, appreciate the insight. It seems like XPS is the cost effective option for sure, though the EU apparently wants to ban the compound (HBCD?) used to make it fire retardant, which makes it a little intimidating. Regarding fastening, I think I am going to try to figure out a solution that will mitigate thermal bridging better than something like riv-nuts into a square tube, and I have thought quite a bit about how to make that happen.

The more I look into armaflex, the more confusing the pricing scheme seems to be. I am seeing quotes ranging from 2.50-6.50/ft^2 for 1" material. I would have no problem at the lower end of that spectrum, but there's no way I would be able to make 6.50 work.

Thanks again!

 

Instead of the channel or square tubing, you can easily find a sheet metal shop that would build you some Z profiles which is easy to attach to the outside and then provide a fastening surface on the inside which will minimize thermal bridging.
Ie the profile is like a Z except the slanted vertical leg in the Z is in fact vertical and allows the insulation to go under the inside horizontal leg

Or I guess a channel would work but with one flange against the outside panel and the other flange on the inside and the web at 90 degrees to your paneling

ie turn your channel 90 degrees to the drawing in your first post

 

You are right in trying to mitigate thermal bridging. The best approach is to minimize the cross section of aluminum in the middle of your walls. IMO, You have structural overkill in that department. 1 x 1 or even 1 sided x 3/4 molded would do. The insulating tape on the inside and outside of the frame is good. I'd also experiment with putting a washer under the (closed ended) Rivnuts to get a bit of standoff to protect the tape. Believe it or not, bubble wrap is a standard outer vaporbarrier material in rv construction. You can use reinforced aluminum duct tape for joining and sealing pretty much everything between the walls. So for a wall panel, you have outer skin, bubblewrap, epx foam sealed with tape to the inside and outside of the frames, more bubblewrap, and the inner skin.

The other point worth making is that the real heat vampires are the cutouts for windows, vents, doors, and appliances. I had a 1963 Airstream trailer that could be towed wet down the interstate in -20 degree weather. It didn't have any fancy insulation, but all the water tanks and waterlines were heated by the furnace. Those are the details that make a rig usable in the winter. Make sure all the incidental heat from controllers and water heaters gets routed where it can do some good. Keep tanks inboard and above the floor. I prefer pilot lighted appliances in the winter - I don't trust electronic ignitors to keep my tanks from freezing. For really cold conditions, skirting the bottom of the vehicle is important. It's a small space and will always have some cold spots. It's going to use as much fuel to heat as a small house. I've lived in RVs for 25 years winter and summer, from Florida to Alaska.

 

Thermal bridging is eliminated by not having interior and exterior walls touching, this is done with a wider wall plate and off set wall studs for interior and exterior walls that go up to a wider top plate. This is standard practice for sound proofing, fill with spray foam as the interior wall is built from bottom up. Off set the ceiling joists as well.

Electrical and plumbing run through the interior wall studs. Use PEX water lines, near the interior wall and ceiling. You'll have a double hull, you may check on the inserts for your windows, door jams and vents.

Consider using corrugated plastic 10mm sign board, 4x8 sheets are available as well as longer stock, I believe to 12', above that it's probably special order. This stuff is very tough, usually cover with graphics for signs but it can be painted. Easily sealed, cut, glued or screwed and it's light weight. This can go over window frames and jams keeping the frame off the interior wall and trim fits over it. This stuff doesn't flame up, it melts but it would probably take a torch to get it burning.

Besides that big door in the rear, you need an emergency exit window.

Battery banks need to be inside, vented to the outside, the cold will kill battery performance.

In extreme weather, redundancy is key, have two heat sources.

Sounds to me like you're heading up to Tuk on the new highway. If you are, study military cold weather operations. You can expect -50 degrees, but even so, a good canvas tent and heater is all you need for shelter, I know, I've done it. Also expect high wind gusts.

 

The bedliner dampens the sheet aluminum in a pretty cost effective and easy way. Those big panels of AL will be drum heads, bedliner will dampen that. What future servicing are you concerned about? You planning to take this thing apart?
For your finish after your thermal insulation think about carpeting over your paneling. One of my customers makes satellite trucks, big dish with a control room / cab. Makes for a nice finish, deadens sound, easy to bounce off when navigating through tight spaces.