RV Construction

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by Jeff Johnston


Potential RV buyers are often faced with more choices than anticipated. If you think buying a car is involved, imagine combining the concerns of purchasing a new car and a new house. Obviously, the selections go much further than model, fabric color and floorplan. Lurking below the RV’s skin are many different construction materials — some of which are easy to understand, while others may leave a buyer’s head spinning. Regardless of the RV type or its price point in its range, there is rarely such a thing as a “bad” construction material. Most of the truly unsatisfactory materials have been weeded out and discarded through the years, and we’re down to proven combinations that work well — plus there are some new materials that show a lot of promise.


Even the least-expensive construction materials can be assembled into a durable, cost-effective RV that can provide years of satisfactory service. At the same time, top-end materials assembled by inadequately trained or unmotivated employees can result in disaster. It’s all in how the manufacturer approaches training, equipment and quality control. So don’t let anyone tell you a wood-framed trailer with corrugated siding isn’t a good idea. If it’s built well, it could turn out to be a great buy.


Framing

Most RVs have steel frames that are welded, huck-bolted or riveted together. There’s not much to say about the relative merits of these systems except that each manufacturer will promote the advantages of each process. Now and then an aluminum frame does show up on the market, quite often originating with a European RV model, but such instances are uncommon.


What’s more important is how the metal is protected from a variety of elements. If it’s just painted black, it’s not going to last as long as if it’s galvanized, powdercoated or treated with one of the many automotive undercoatings available today.


Wood, metal and, to a lesser degree, fiberglass, are all used to frame an RV’s walls, floor and/or ceiling. Wood is inexpensive, lightweight, flexible and durable, and has been the material of choice for many companies for decades. Both low-end and extra-high-end products are built with wood.


Aluminum and steel are commonly used for RV framing, but due to the extra costs involved with the material and fabrication they usually apply to mid-range-and-above products. Aluminum is found in all types of RVs, while steel is normally applied to motorhomes. Steel is almost always welded, but you may find aluminum that’s welded, screwed or riveted together. All such systems work.


Metal framing doesn’t have the potential rot problem that wood framing can have if leaks are present, but the surrounding wood paneling will still decay in a damp area. This is where attention to detail at the factory plays a part. Properly installed windows and other items that require weatherstripping go a long way toward reducing leak problems and subsequent damage potential, regardless of the framing type.


A few RVs are built with fiberglass bodies molded in a couple of pieces like a boat hull. These are usually very small trailers or truck campers, and some Class B or Class C low-profile motorhomes. In these examples, the fiberglass body serves as both the main framing element and the exterior skin; the manufacturer adds molded-in wood or metal reinforcements to serve as mounting points for cabinets and the like. An all-fiberglass body is strong and certainly corrosion-resistant, but it can also be somewhat heavy if built to durable standards and is costly to manufacture.


Built-Up vs. Laminated

RV construction falls into two general categories: built-up or laminated. Built-up construction refers to the process of assembling the walls, floor or roof structure a step at a time during the construction process. For example, the wall framing and interior skin are assembled and installed on the floor, then the insulation and wiring are added, followed by the outer skin as the last step. This process is still in common use for mainly lower-end RVs, although it can be found across the spectrum of RV prices and quality levels. Sometimes a “built-up” rig can be easier to repair because the components aren’t fused together, which means they’re easier to separate for repair and replacement.


Laminated construction refers to a process by which, for example, the interior wall surface, framing, insulation and exterior wall are joined together into a single unit by a construction-grade adhesive. The unit, be it a wall, floor or roof, is assembled layer by layer, with adhesive placed between each of the layers. A mechanical process is used to apply pressure to the entire unit until the adhesive dries and bonds, after which the unit is moved along the assembly line to be installed as a single large section. The pressure application may be via a pair of large pinch rollers through which the assembly is passed, a mechanical press or a vacuum process, which uses a large press that applies pressure as a vacuum is applied.


The result is a structure that’s very strong, typically fairly lightweight for its strength and as durable as any RV-assembly process can create. The most significant problem is delamination which, although almost a thing of the past, can still occur if adhesives are improperly used or the assembly process is not done right. Repairing such an assembly is challenging because it’s virtually impossible to re-laminate a wall, for example, once it’s on the RV.


Skin Deep

The question of which is better — aluminum or fiberglass — as an RV skin will always be steeped in controversy. Both materials have advantages and drawbacks, and both are used on RVs from lower-end to the very best products made. Both materials are durable and effective as RV skins. You shouldn’t write off a certain RV model if it has what you think is the wrong skin material as long as the rest of its features are just what you want.


Aluminum comes in two forms: as a corrugated panel stock that’s assembled in interlocking sections and normally used on lower-cost RVs, and large, smooth aluminum panels usually found on higher-end models. The corrugated type is fairly easy to repair because a panel can be removed and more easily replaced, whereas a flat aluminum skin is more of a challenge. If it’s painted, standard automotive bodywork can be done, but if it’s polished-aluminum, an entire section may need to be replaced, possibly calling for expensive riveting and other costly assembly processes.


Fiberglass is available in several styles, including corrugated and smooth. The corrugated type is most often used on affordably priced RVs, while smooth fiberglass is found on all kinds of rigs in all price ranges. Manufacturers have improved fiberglass durability and resistance to ultraviolet light damage and fading such that today’s fiberglass-skinned RVs look good a lot longer than they did in years past. Filon is a brand name of one such durable fiberglass material.


Fiberglass can be either fully painted, including the graphics, or the graphics can be tape appliques. Some RVs use fiberglass skin with molded-in color with either tape or painted-on graphics. As a rule, the more paint is used, the higher the unit is priced.


A painted RV is more resistant to fading than a straight fiberglass surface with the color molded in. However, new fiberglass varieties are much improved in that department, and smooth-finish varieties such as gel-coat skins have some remarkable lifespans.


Roofing Materials

You’ll encounter many different materials used for RV roofs. The roof receives a lot of weather punishment (in the form of heat, ultraviolet rays, rain and hail) and other abuses such as tree-limb abrasions. When it comes to long-term durability, the stronger the roof skin, the better.


Aluminum is low-cost, fairly easy to repair and lasts a long time. It’s usually the material of choice for lower-cost RVs, although painted aluminum is occasionally used on high-end units as well. Its worst drawback is that it creates dirty streaks down the RV’s sides as the rain washes oxidation off the roof.


EPDM rubber roofs are found on RVs of all price ranges, including lower-cost models. EPDM is durable, flexible, easy to work with and generally cost-effective. Contrary to common belief, EPDM will also produce dark streaks on an RV’s side walls — but to a far lesser degree than aluminum. Gluing EPDM to the substrate can be a challenge, and it sometimes works loose and causes bubbles during travel due to the rig’s aerodynamics. The bubbles are generally deemed harmless by manufacturers, and they can usually be easily repaired using commercial patch kits. Another relatively new and popular roof material is Thermo Plastic Olefin, or TPO. This is sold under the name Brite Tek and is similar to EPDM, but it features a plastic base rather than a rubber base. It’s installed and used much like EPDM, but is said to be lighter weight than EPDM. Some high-end products use a fiberglass roof that’s virtually identical in makeup to the rig’s side walls and end caps. This is sometimes done with flat sheeting, and sometimes with a molded roof that fits the roof’s contours — including the curved interface with the side walls.


Fiberglass has all the advantages on a roof that it does on side walls — as well as being free of the dark streaks that can result from other roofing types. It’s also probably the most expensive type of roof to put on an RV.


Insulation

Spun fiberglass, as is used in residential construction, and polystyrene foam, better known by the name “Styrofoam,” are the most common insulation materials used in RVs.


Foam-Cor is essentially a very thin sheet of polystyrene, perhaps 1?4-inch thick, faced on both sides with kraft paper. This is sometimes used as an insulation upgrade on wood-framed, aluminum-sided RVs in that it’s applied to the wall structure — already insulated with fiberglass — before the siding is applied. It’s too thin to be of much value by itself, but can be of some modest help as an add-on.


Fiberglass insulation has been used in RVs since the start of the industry. It’s inexpensive, easy to install, durable, fire-resistant, works well when properly installed and doesn’t lose its insulating qualities over time. Fiberglass insulation is not a structural component, so any RV that uses it — from a low-end economy model to a high-end coach — relies on wall framing and other components for structural integrity.


Polystyrene costs more than fiberglass, both to buy and to install, but it’s extremely effective as insulation. It’s used as a structural component in laminated-type construction, and it adds lightweight strength to a wall, floor or roof assembly. You can’t go wrong with a rig insulated with polystyrene.


Pane Threshold

Some RVs are offered with a dual-pane-window option. While not an RV construction matter, it relates closely to insulation.


A dual-pane window is one of the best insulation upgrades you can add to an RV for the same reason that dual-pane windows are used in residential houses. A single-pane window has very little insulation value, so no matter how well the walls, ceiling and so on are insulated, the window remains a giant “hole” that allows heat and cold in and out, depending on the season. Dual-pane windows offer an insulating air space between that cuts down on both heat and sound transfer.


If you have the choice, mark the dual-pane window check box on the option list. You won’t regret it.


You decide

Finally, you need to look past the surface gloss and polish to get an idea about how the rig is built.


Ideally, it helps to take a factory tour at the plant that builds the model you want to buy, but that’s not always geographically practical. A tour gives you insight into how the rig is assembled — in ways you can’t usually can’t see once it’s a finished product. If you can take the tour, it’ll be a big help.


With or without the tour your next step is becoming familiar with the nooks and crannies of the RV. Get down and take a close look under the vehicle. Check how the visible wiring and plumbing are routed and supported, and look at welds and other joints. Is the undercoating complete? Are edges neatly trimmed and finished?


Inside the rig, open the cabinet doors — especially those under the kitchen and bathroom sinks — and look inside. Remove a drawer or two if need be. It doesn’t take long to see how carefully the manufacturer installs plumbing and wiring, and how well the cabinets are built. Lifting the wood cover beneath a dinette seat to view the water tank or other appliances housed there can also be revealing.


A few minutes spent exploring an RV you’re considering purchasing is a good investment in time. A bit of hands-on homework will reveal a lot about that rig, and help to make your buying decision easier. RVBG

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