Wall construction quality is a litmus test for the rest of the RV. It’s rare to find an RV with a “better-built box” and shoddy workmanship elsewhere – but the opposite is also true.
If you’re about to purchase an RV, this article is so incredibly important. There are many types of RV sidewalls. Understanding how your RV is built can make the difference between a travel trailer or 5th wheel that will bring you 20 years of happiness versus 20 years of grief.
In preparation for this article, I reviewed some of the other information about types of RV wall construction online. And quite frankly, a lot of it is wrong or severely outdated. I read one article stating that the average R-value of fiberglass is between R-10 and R-20 per inch – which is absolutely bonkers. So please don’t believe everything you hear! Take it from a guy who gets paid to know this stuff.
RV Walls Versus Residential Construction
Let’s clean up some misconceptions about types of RV sidewalls and their similarities to residential construction.
How Strong Are They?
RV sidewalls aren’t “load bearing” in a traditional sense. They’re not shouldering the burden of thousands of pounds of roof trusses, sheathing, asphalt shingles, etc. At worst, an RV sidewall must support the dead weight of the roof plus a snow load.
For that reason, you generally don’t see headers, cripples, and other weight-distribution framing borrowed from residential construction. You can get away with lightweight framing like pine 2×2’s or spot-welded aluminum studs.
In fact, sometimes, the sidewalls aren’t considered sufficient to hold up the structure! Some RVs, especially “ultralites,” are designed with “structural furniture.” In other words, the furniture is partially responsible for bearing the weight of the roof and preventing the walls from bowing or caving.
Most RV walls are between ¾” and 2.5” thick finished, unlike a house where walls are a minimum of 4.5” thick when finished.
Is There a Vapor Barrier?
Unlike residential and commercial dwellings, RV walls typically have no dedicated vapor control barrier or rain screen. You won’t find Tyvek housewrap or furring strips behind RV siding. RV sidewalls are designed so that water must stay on the outside. If it gets behind the siding – either through delamination leaks, cracked butyl tape gaskets, or wind-driven rain permeating the siding interlocks – then the wall will likely be damaged.
What About the Mechanical Systems?
RV sidewalls do not contain propane or water plumbing joints. Again, this is different from your house. But per NFPA 1192 (the main RV design code), manufacturers are not allowed to conceal water supply, water DWV, or propane manifold joints inside the structure. Instead, these distribution piping systems are usually installed underneath the subfloor, where they are accessible from the underbelly, or they are routed through the furniture. In some RVs, you won’t even find wiring in the sidewalls!
Are RV Walls Easy to Replace?
Lastly, RV walls are not designed to be easily replaced. This can be an enormous source of frustration for RV owners or renovators.
In your house, the wall system is designed so that different contractors – the framing crew, the plumber, the electrician, the drywall installers, the drywall mudders, the painters, etc. – can all work together. And in the future, wall sections can be modified or replaced as needed.
By and large, this is not true of RVs. In fact, if a laminated sidewall fails, the recommended solution is to replace the entire wall, which often means disassembling that half of the RV. It’s nearly a total failure.
Some of these design standards should change. It’s no secret that RVs are prone to leaks, and the sidewall construction bears some responsibility for that. But some of these design standards are good. It’s wise to keep all propane and water joints accessible for service and safety reasons. And some features, like a vapor control barrier, simply wouldn’t do any good for most RVs, whose needs for heating and cooling and vapor drive control are far different than a 3,000-sqft box built directly on the earth.
With that out of the way, let’s dive deep into RV sidewall construction!
Stick n’ Tin
How It’s Made
If a picture is worth a thousand words, a video is worth a million. Watch this video – and then read the comments.
Yup. That’s classic stick n’ tin: fast n’ cheap. Nothing more than a 2×2 wood frame, with studs 16” or 24” on-center and filled with fiberglass batt insulation. Finished with 1/8″ interior plywood paneling and corrugated aluminum siding. The whole thing is held together by a few dozen pounds of staples.
- The corrugated siding comes in many profiles. Mesa is the most common name. The corrugations usually run in a 2”, 3”, or 4” pattern.
- You also have your choice of common colors, such as Polar White, Cedar, Skyline Gray, Teal, etc. Panel width ranges from 8 to 48”, with 8-24” being the most common.
- Length for single panels is usually capped at 26 feet.
The corrugated siding interlocks, with the bottom of one course snapping over the top of the next. You can see many varieties of interlocking joints here at RVSiding.com.
In the modern era, you can sometimes find corrugated siding made from fiberglass-reinforced plastic (FRP) or other plastic. Nine times out of 10, though, the siding is still made from aluminum, which is lightweight, easy to work with, and relatively affordable.
Is It Any Good?
On one hand, I can argue that this sort of lazy craftsmanship is almost inexcusable. Stick’ and tin construction is known as the cheapest of the cheap.
Potential problems are legion. It’s difficult to achieve a truly watertight seal around the corrugated edges. The extra surface area reduces fuel economy when towing. Settling fiberglass, warping lumber, drafty insulation, flexing walls – what’s left to love?
On the other hand, there are 30- and 40-year old stick n’ tin campers still in service. The design is easy to hurt but hard to kill. Personally, I don’t love stick n’ tin construction. However, there are some pearls among the swine.
There are ways to improve its quality. Metal strapping at critical joints, doubled headers, water-resistant plywood sheathing, etc. You’ll sometimes read manufacturer claims like these:
- “We glue our batting to the paneling!”
- “We use top-quality 2×2 lumber!”
- “We use glue, not just staples, to assemble our frames!”
These are indications that you’re at least getting a better-than-the-next-guy stick n’ tin trailer.
… Can I rant for a moment? No sawmill sets out to make 2×2’s. They are the lumber leftovers. “Top-quality 2×2’s” is an oxymoron! Any time you see 2×2 pine stick-framed construction, there’s a good bet you’re buying a camper trailer built for the bottom dollar.
Hung wall construction is like stick n’ tin on steroids. It’s far closer to the residential-style wall in your home.
How It’s Made
The term “hung wall” is normally applied to welded aluminum-framed walls made with rigid fiberglass exterior siding. As a rule, hung walls are much better built than stick n’ tin walls.
Again, a video is worth a million words. Start the above video at 4:45 for an explanation of Newmar’s hung wall construction, then come back here.* (and then follow this asterisk).
Note: Newmar goes the extra mile by installing a continuous foam board between its interior paneling and aluminum frame. This is not normal for hung walls (but much appreciated).
- Similar to conventional stick n’ tin construction, hung walls use a built-up framed wall constructed with studs, top plate, bottom plate, etc. Unlike stick n’ tin, hung walls typically use aluminum square tubing or U-channel.
- Unlike laminate construction, the exterior fiberglass on a hung wall is a thick, rigid panel, not thin fiberglass spooled off a coil. It is usually attached to the aluminum studs using a structural adhesive like VHB tape or 3M Silaprene.
Is It Any Good?
*Ok, here’s my asterisk. I love the interlocking roof-edge connection, the continuous beadboard insulation and the solid fiberglass siding. I don’t like the fiberglass batting. Either use a mold-resistant batting like mineral wool or move to CNC-cut foam board.
Hung walls are a rarity in the RV world. You can find them on some Cedar Creek 5th wheels, Entegra Class A’s, Winnebago coaches, and Newmar coaches (owned by Winnebago, incidentally). If you want to compare the Cedar Creek version of hung walls to Newmar, here’s a 2018 flyer. Construction details are at the bottom.
Hung walls are much easier to repair than laminated walls. They are resilient, robust and have stood the test of time. On long and tall spans, like a Class A motorhome, they don’t wave and warp like thin laminated walls. My only complaint is that unless you’re springing for a Class A motorhome or luxury 5th wheel, they’re almost impossible to find!
Laminated walls are the darling of the modern RV industry. Unlike a stick n’ tin wall, where siding and paneling are fastened to a wooden frame, laminated walls are a multi-layered sandwich.
How It’s Made
The core of the wall is block EPS (fancy word for styrofoam) foam. A CNC router machines grooves and slots for wire chases and aluminum tubing. Interior paneling and an exterior skin are glued to the foam core.
Laminated walls are straight, flat and incredibly strong for their weight. They are jumbo-sized panels (anywhere from 8 to 45 feet in length) that can be quickly installed in a factory setting and easily revised for design changes. For those reasons, OEMs love them.
However, they do have their problems. Before we get there, though, let’s talk about some myths and misconceptions. And there are quite a few!
Myth 1: The interior aluminum tubing is load-bearing.
In a strict sense, it isn’t necessarily wrong. Laminated walls are composite structures. The skins actually bear most of the forces, especially when bending. The interior aluminum tubing is useful for screw retention and panel rigidity, not necessarily load-bearing strength.
This is a general guide; it’s not a rule. There ARE instances where interior aluminum tubing inside a laminated wall really does act as a column to bear vertical weight or a header to distribute weight. But don’t assume that the aluminum frame always acts the same as, say, 2×6 studs in a house.
Myth 2: The aluminum frame is some sort of structural weldment.
Again, this is (usually, but not always) false. Most of these aluminum tubes have a wall thickness of about 0.045 inches. You can’t get a decent structural weld on thin-wall aluminum.
So don’t get caught up trying to figure out if the manufacturer spot welds or double welds. In a laminated wall, the internal aluminum frame is welded to ease assembly, not necessarily to reinforce the structure.
Myth 3: All laminate walls are wood-free.
In many cases, RV manufacturers insert square wooden dowels into the hollow aluminum tubing for better screw retention. This is especially common at floor and ceiling joints.
Obviously, this wood is partially protected by being encased inside an aluminum tube. Severe, persistent leaks can still cause mold, though.
Also, some manufacturers use aluminum tubing around their windows, doors and other cut-outs; others don’t. Instead, some route out the edge of the hole and lay in 2×2 sticks. Some don’t do anything! They rely only on the compression strength of the foam core to hold its shape when clamped.
Myth 4: All laminated walls use the same material.
That’s completely false. Material choice and fabrication process can vary widely from manufacturer to manufacturer.
A cheap laminated wall may have a 1-inch foam core, loose-laid aluminum (or even wood!) frame, no tubing around windows and openings, lauan interior plywood, luan exterior plywood, and a mighty thin 0.030-inch fiberglass skin. It might be fed through a pinch roller and delivered to the assembly line 10 minutes later.
A high-quality laminated wall may have a 2-inch foam core, spot-welded and glued-in-place aluminum frame, tubing or galvanized steel strips around all windows and opening, Azdel interior and exterior substrates, and 0.060-inch or thicker high-gloss fiberglass skin. It may be vacuum-pressed and allowed to cure before being used.
Here’s a video showing some of these differences!
Is It Any Good?
Again, not all laminated walls are created equal!
Let’s look at three big differences between laminated walls:
- Bonding method
- Choice of substrate
- Exterior fiberglass
Bonding Method: Pinch-Rolled or Vacuum-Pressed
Many laminated walls, especially those used in cheaper travel trailers, are fabricated using a pinch roller.
A pinch roller is exactly what it sounds like. An assembly line squeegees, rolls or sprays adhesive between all the layers of a laminated wall. Then the full wall sandwich is fed through a roller with a gap ever so slightly smaller than the wall thickness. The layers compress, the glue sticks, and a finished wall comes out the other side ready for routing.
A more expensive alternative is a vacuum press or mechanical press. The layers are assembled and the glue is applied inside a large enclosure or vacuum bag. Then the enclosure is sealed and all the air removed. At full vacuum (30” Hg perfect), 14.7 PSI is applied to the entire wall surface. The layers squeeze together, a bond forms, and the wall is ready for use.
A related alternative is to use a hydraulic mechanical press rather than atmospheric vacuum pressure. Similar physics. Mechanical presses are rare because vacuum presses are much more affordable.
Neither process is inherently superior to the other. But two big problems with a pinch roller are A) achieving even pressure throughout the glue line and B) eliminating air bubbles. Any leftover bubbles become potential delamination origin points.
For that reason, vacuum-bonded walls seem to last longer in the field. You see better glue coverage, more consistent bonding, and fewer voids.
Still, roughly 75% of RV trailer walls are made with pinch-rolled sidewalls. Only about 25% are made with a vacuum press, although this number continues to rise.
Panel Substrate: Azdel or Lauan?
Your exterior skin and interior wallpaper must be bonded to something to make them rigid. Generally, this is either lauan plywood or Azdel composite (although a few other composite manufacturers are starting to make headway).
In some cases, such as around a curved surface, you may find fiberglass laminated to a heavy-duty cardboard-esque substrate like Unicore. Unfortunately, as you can imagine, paper-based products don’t survive long in the humid/dry hot/cold conditions of an RV interior!
Anyways, back to lauan. It is also known as Philippine mahogany. But don’t get distracted by the name; it has no relationship to your grandmother’s 120-year-old dining table. It’s cheap stuff – used to be somewhere around $6-$10 per 4×8 sheet when bought in bulk. It’s so cheap it’s often used for packaging!
Lauan is prone to problems common to all plywoods, such as delamination, rot and mildew. It is not a marine-grade panel; voids are allowed in the plies. The adhesive used to bond the plies is not a true WBP glue, either.
Now, what is Azdel? Azdel is a relative newcomer to the block. It first debuted on RVs in the early 2000’s. It’s a woven polypropylene sheet, something that looks and acts kind of like bendable plywood. But unlike plywood, it can’t hold moisture, doesn’t rot, and weighs quite a bit less.
I can’t lie: I think Azdel beats Lauan hands-down. Azdel can survive a leak; Lauan can’t. Azdel reduces weight, improves soundproofing, and reduces thermal bridging. Personally, while I could accept using lauan for interior wall paneling, I think it has no business as the substrate for exterior fiberglass
What’s On the Outside of Your Wall?
Fair warning: This is a very deep rabbit hole. We are not going to climb all the way down to the bottom.
You don’t want naked fiberglass on your RV. It’s rough, matte, and would fade in direct sunlight in a matter of weeks. So instead, you get fiberglass + some final exterior coating.
That final coating could be any number of things. It could be a gel coat (there are many formulations). It could be a one-part coating or catalyzed automotive paint plus clear coat. It could be some combination of all those things. It could be something I haven’t listed.
But most of the custom high-end formulations and processes are found on Class A coaches and some luxury 5th wheels. In that world, you can generally choose between a high-gloss gel coat exterior or full body paint (FBP).
Not all gel coats are the same.
- Chemical bases include polyester, vinylester, modified acrylic, epoxy, phenolic, and urethane resin.
- Premium finishes can be 3x thicker than bargain-basement finishes. “Gel coat” finishes can be as thin as 5-10 mils!
- Some formulations can be prone to pitting or spiderweb cracking.
The more you spend, the better you get. High-end FRP has a thicker gel coat and smoother surface than the run-of-the-mill stuff. When it comes to mainstream RVs, your choices are limited.
I’d like to make a quick mention of Filon. Filon has become an eponym for FRP (aka GRP) siding, but it’s really just a trademark by Crane Composites. There are several grades of Filon, ranging from G3 (basic industry-standard) to Max to Gold. And the company makes other types of composite siding, including rigid Filon Reflections. So don’t assume you know “Filon” unless you know what grade and type you’re inspecting.
What About Delamination?
Now, let’s address the white elephant in the room: delamination. If you’ve spent any time learning about RV walls previously, it’s a term you’re intimately familiar with.
The Achilles Heel of all laminated walls is delamination – where the layers begin to separate. This may occur because of water intrusion or a manufacturer’s defect. The most noticeable instance is when the 1/16-inch fiberglass skin separates from the lauan or Azdel backing. If the delamination cannot be repaired, there is no solution but to replace the entire wall.
Delamination was a huuuuuge problem when laminated walls were first introduced. Seemed like everyone and his mother had a delaminating camper. With modern RVs, the problem has been reduced – but remains a thorn in the side of the industry.
Some campers have skeletons. In the same way that you and I depend on our skeletons (and not our skin) to carry our weight, so these campers depend on welded frames to do the heavy lifting.
How It’s Made
FYI, this style of construction is also known as full perimeter framing or fully boxed framing.
Funnily enough, you find campers built with cage frames at opposite ends of the RV spectrum.
You find a lot of cage frames amongst mini travel trailers, teardrop campers, expedition trailers and 4×4 truck campers. The more rugged 4×4 campers may use steel frames to survive rollovers, but regular camper trailers normally use aluminum to save on road weight.
You also find cage frames used for 5th wheel toy haulers and big ol’ Class A motorhomes. Brands include ATC and Luxe. Eschewing the traditional single-piece sidewall design, these companies fabricate the main body frame and then build inside and outside from there.
InTech RV is one company bucking the trend. All their frames are built from tubular aluminum, fully welded. InTech RV actually takes the idea one step further and integrates the chassis frame with the body superstructure.
Check out the details in the video below:
Is It Any Good?
My engineering self loves a fully welded frame. Overkill at its finest. Hard to kill, easy to fix.
But there’s a reason you don’t see it on most mainstream RVs. As you can imagine, the labor required to design and build a welded skeletal frame would shame Sisyphus.
- The skilled labor and materials required are incredibly expensive.
- The strength of the cage is overkill for most recreational vehicles.
- Significant structural revisions for any layout or floorplan changes.
- Welding the body to the chassis is an irreversible process.
The custom nature and shape of a fully welded frame does not lend itself to an industry that relies on standard components and modular design
I like cage frames. Done correctly, they’ll last nearly forever. But there’s a reason you see them on premium 5th wheels and rough-and-tough expedition campers. Be prepared to pay the piper.
Molded fiberglass is an entirely different beast than anything we’ve talked about. There are two basic types of molded fiberglass campers: Single Wall and Double Wall.
How It’s Made
Solid molded fiberglass campers have earned a cult following. In fact, they have their own online forum: FiberglassRV.com. Here, you’ll find some of the most classic names in RV history, like Scamp, Boler and Oliver. Airstream even took a crack at one recently: the Nest.
Most molded fiberglass campers are small travel trailers or truck campers, but there are a few fiberglass 5th wheel manufacturers out there as well! Check out the full list of RV manufacturers here at Changing Gears.
These guys play by their own rules. The shell (also known as the hull, a borrowed nautical term) is solid cast fiberglass, so everything has to be run through furniture or underneath the floor. There’s no interior insulation beyond foam-backed carpeting, usually. They’re water-tight, easy to maintain, easy to clean, and look as cute as a button.
The hulls are assembled in pieces (usually two). Scamps, Casitas, and most other manufacturers use a top half and a lower half, but some manufacturers, like the vintage Burro, use a left-hand and a right-hand side.
Double-wall fiberglass RVs are a true rarity. The commanding market presence is Oliver, a relative newcomer founded in 2007.
In a double-wall hull, the distribution systems – like plumbing runs, HVAC ducts, electrical harnesses, etc. – are enclosed between an outer and inner molded fiberglass shell.
Double-wall designs offer far superior thermal and acoustic insulation compared to single-wall designs. If properly bonded, the finished hull is incredibly strong yet fairly lightweight. Water leaks are also less likely to penetrate interior spaces. They are not, however, true 4-season-ready.
Is It Any Good?
As you can probably tell, I’m somewhat partial to molded fiberglass shells.
- When built correctly, these hulls are incredibly tough, resilient and weather-proof.
- There are molded fiberglass campers older than me that look younger than me!
- They are also easy to renovate, which is one reason they remain so popular.
The big downside is that you’re pretty much limited to smaller travel trailers, and there’s only one fiberglass 5th wheel to my knowledge!
Solid Plywood Wall
Yes, some campers are built from common sheets of 4×8 plywood! However, as you can imagine, such trailers are few and far between – and some are no taller or longer than a single piece of plywood!
How It’s Made
Solid plywood walls are an endangered species. They hail from the ‘50’s and ‘60’s, the original RV heyday after World War II. Tiny travel trailers, some no bigger than four feet wide and eight feet long, appeared in Sears & Roebuck mail order magazines. These travel trailers had walls spliced together from sheets of 4×8 plywood.
Today, a few custom manufacturers still use solid plywood walls. Teardrop trailers, canned hams, vintage Scotty’s, and other Lilliputian-sized campers still use ½-inch to ¾-inch plywood side walls.
Is It Any Good?
There’s nothing intrinsically wrong with solid plywood walls. However, in vintage campers, the steel fasteners used to splice sheets together may rust away to nothing. In new construction, plywood walls offer little sound-deadening or thermal insulation. Plywood is also quite heavy and expensive.
For those reasons, plywood sidewalls are virtually unknown outside of teardrop trailers, canned hams, and other micro trailers.
I may as well have named this section “The Airstream Chapter.” The only two manufacturers I know of who build aircraft-style aluminum monocoque structures are Bowlus and Living Vehicle, both of which are much, much smaller than Airstream.
How It’s Made
Gosh, they’re beautiful, aren’t they? If you want to know exactly how Airstreams are made, watch this YouTube video:
You can get into even more details with this 2016 Airstream brochure.
Airstream construction is inspired by aircraft fuselage design, with a lightweight aluminum frame and buck-riveted aluminum exterior panels. Airstream has been building campers this way (with some modifications, of course) since 1931!
Is It Any Good?
Now, I wish I could tell you that the Airstream design is perfect and never leaks. Unfortunately, Airstream owners will tell you differently!
The sealant used between all lap joints breaks down over time. Eventually, though it may take 15-20 years, your beloved Airstream will need to be resealed. Veterans seem to agree that older hulls need to be resealed about every 5 years.
Effort is also required to maintain that beautiful, lustrous exterior. While the aluminum comes clear-coated from the factory, the clearcoat will eventually wear off. The aluminum will begin to oxidize. First, it will dull, and then, it will chalk.
If you own an Airstream, you better enjoy polishing and waxing. Or have enough money to pay a dealership to do it (then again, you do own an Airstream …). The company recommends you wash and wax every four weeks, people. If ignored, aluminum can become so severely corroded it’s almost impossible to repair!