Shower Pan

16 gauge stainless steel shower pan

One of the primary reasons I purchased the Clipper was the desire to have a bathroom with toilet and shower. The Boles Aero I renovated earlier has what I think of as an emergency toilet. It's a porta potty located on a slideout platform under the rear bed. Because it's in the way when deployed and also used the limited open floorspace, the only time I'd consider using it would be if it were the middle of the night and it was raining cats and dogs outside. Under any other situation I'd use a campground facility.

So I've been thinking about the bathroom and it's driven many, many of the Clipper design decisions.

I'd decided on a small wet bath design a long time ago. More recently I decided that I'd make it very small to reduce the footprint as much as feasible. From a practical point of view this means the floorspace needs to be no less than about 32" x 24". Because of the rounded roofline extending the width past 24" even a small amount yields a great deal of headroom making taking showers more comfortable for someone above average height (ie me). But there's a hard limit of about 29" because of interference with the shower sump floor access panel.

In the end I settled on dimensions of roughly 32"x28" as the maximum.

There are a lot of options to sourcing a shower pan. There are a lot of inexpensive plastic type pans available. There are few standard stainless steel pans available, most at 3+X the cost of a plastic pan. You can have a pan custom made for 5+X the cost of a plastic pan. Or you can make your own.

I decided against the plastic pan simply because they do break occasionally and the entire bathroom is built around the pan. If the pan breaks then replacing it is a monster repair.

Bespoke stainless steel pans are really expensive ($1000s). And the off-the-shelf stainless steel pans require making a lot of compromises and they are still quite pricy (~$700), so I investigated making my own. Eventually I decided to make my own.

I designed a stainless steel pan using FreeCAD and then using a fabricator called OSHCut to cut it out and bend it to my specification. Arranging this is an online process: you upload the digital design, select a bunch of options including material, they automatically run a bunch of design checks and then, assuming it passes the checks, you pay. They then fabricate the pan and ship it to you.

It sounds a lot simpler than it really is, but all things considered it's amazing that you can actually do this. It took me about 3 days part-time to iterate the design before I hit the "order" button. Most of that was the learning process. Now it'd probably take me about 2 hours.

The design includes a threshold that's wide enough to accommodate a retractable shower door that are popular in RVs (one brand is Nautilus). The threshold also includes a front return that will provide a front finish. And the design has vertical returns on 3 sides with 1" offsets to accommodate shower walls and water runoff from those walls.

Freecad Techdraw Spec for Shower Pan

Shower Pan from OshCut.com

One note on the shower pan design. You'll note that it doesn't include a slope towards the drain. They recommend a 1/4" per foot slope to channel water towards the drain. I chose to ignore this to simplify the design. I saw many stainless steel pans that used a X-patten break centered on the drain. I think this approach may help drainage but I couldn't design this in (forming the X-break is a manual operation). I also think it has more utility as a means to prevent "oil-canning" than it does to facilitate draining since it doesn't really add a meaningful slope. For my application it would actually introduce an installation complexity since I intend for the pan to rest directly on the floor and an X-break would prevent a flush fit.

In reality I'm not really sure in practice whether the 1/4" per 12" slope (about 2% or 2 degrees) will be inside my ability to level the trailer. As it turns out from what I've read this is around the same slope where humans can detect an uneven floor. I guess this is one of those decisions that time and usage will judge.

A few notes on the design/process:

• You really need to select the material thickness up front in the design process. this brings with it a bunch of material characteristics that you will likely need to input into your design tool.
• The fabricating service (in this case OSHCut) will have some processing considerations that you need to account for and input into the design. Things like bend radius (influenced by the choice of bending die, default or other) and bending gap tolerances (how close together seams can be).

OSHCut has a really nice tool that simulates the bending process on your uploaded design so that you can see how it will be fabricated. You don't specify the bending process, the automated process does. The simulation will also show you when something can't be bent the way you've designed it and then flag it as an error. While this tool is really cool it is a bit frustrating because the error messages are often not specific enough to point out the specific flaw in your design. For me this led to a great deal of iteration guessing on design adjustments, uploading the modified design, and running the simulation to see if your adjustment removed the error condition.

After a bit of trial and error I began to see a pattern of design flaws that led to particular errors flagged by the simulation. After many, many iterations eventually I had a design that worked with their process. The flaws in my design were mostly centered around allowable seam gap tolerances resulting from interferences (collisions) in the folding process, i.e. edges coming into contact with one another. I also needed to adjust the bend relief allowances. This latter influences the size of holes left for the welder to fill. This is an area I'm sure I could improve if I ever do this again.

There are a few more steps required to get a functioning shower pan. This first is to seal the seams. OSHCut does the sheet metal cutting and bending but this leaves some open seams in the corners of the pan. These seams need to be welded and OSHCut doesn't provide a welding service. (I have found subsequently that other services do provide welding and charge by the lineal inch, RMFG is an example).

I could have probably welded it but I don't have the skill to make it look good and I also don't have the equipment (100% argon tank). So I reached out to the welding instructor at the local community college and he welded the seams for me. Following welding the seams required some grinding to clean up which I accomplished with a Dremel and carbide grinding bits. The seams didn't turn out as pretty as I'd dreamed but I'm probably the only person that will notice. I think mostly people's eyes will be drawn to a difference between the brushed surface finish and the weld surface which ends up looking more polished than brushed.

The next piece of the shower pan puzzle is swaging the recess where the drain assembly sits. This allows the drain assembly to sit slightly recessed so that water can more fully drain out (otherwise there would be a small lip). It also keeps your feet from constantly brushing the edge of the drain assembly trim.

Swaging is done by pressing the pan between two parts of a die set. Because there aren't a lot of people making shower pans these die sets aren't readily available. The ones that can be found are very expensive. But I found a couple online forums where DIYers had tackled the challenge in slightly different ways. I took the option more suited to my skillset.

I used FreeCAD to design a shower drain swaging die set. I then used a 3D print service to print the dies using tough PLA+. The PLA+ material is not a durable approach but it's really only a single use item for me. If I were planning to use the dies repeatedly I'd probably have a machinist turn the dies from steel on a lathe. The 3D print is fairly cheap and has a quick turnaround time for PLA+ material (~$20 + S/H, 1 week).

3D Printed PLA+ Die Set

Die Set with Backing 1/4" Steel Caul Plates and Test Stamp in Scrap Aluminum

I designed the dies with a "close fit" (0.6406") center hole for a 5/8"-11 grade 8 bolt. (With hindsight I should have gone with 20mm close-fit, ie 0.827", to be compatible with commonly available hydraulic punch press.) The intent is to use an electric impact wrench to tighten the bolt and stamp the dimple into the steel. I ran some calculations based on 0.060" 304 stainless steel material and assuming a standard 3" dimple die shape and it showed that 150 ft-lbs (203.4 Nm) of torque should be sufficient. This is well within the range of a typical impact wrench. If the impact wrench approach fails then I can use a 10 or 15 ton hydraulic punch press but I'll need to drill out the center hole to 20mm since that seems to be the standard for hydraulic punch tools.

The biggest questions in my mind was: 1) will the PLA+ die stand up to the abuse; and 2) how bad the metal pan will be distorted. I've read that if the die "bottoms out" then there should be little or no distortion.

I borrowed an impact wrench from a friend and I was able to use the PLA+ 3D printed dies to swage the drain recess. I used a liberal amount of anti-seize grease on the 5/8"-11 grade 8 bolt and the die where it interfaces with the shower pan. I think it turned out well. There is very slight distortion in the bottom of the shower pan, basically none that's obvious to a casual glance. The PLA+ die also showed little sign of fatigue/wear and I'd say it could easily be used again. If I were to start over I'd probably tweak the die to provide for a slightly larger bend radius and also provide a little more offset to accommodate the material.

Laser cut 2.1" diameter drain hole in pan

3D printed die with backing plates inserted in pan 

Drain recess after swaging process

Drain test fit (shipping plastic still on drain cap)

The final step was to grind the welds, remove the weld oxides, and re-passivate the areas heated by the welding process.

I ground the weld crowns (not flush) using a combination of a new 120 grit flap disc on my angle grinder and some tungsten carbide die grinder bits with my Dremel rotary tool. The flap disc works well but the inside corners are the big challenge and the primary reason I needed the die grinder bits. The new flap disc is necessary so that the ground area isn't contaminated with addition iron that might be stuck in an old disc.

The grinding and flap disc remove much of the weld oxides but I needed to use new stainless steel wire cups using both my right angle drill and the Dremel tool to get at the inside corners. I also used some grinding stones with my Dremel to smooth out the finish left over from the sanding disc and die grinding bits.

Following all the sanding and grinding I cleaned the weld areas using acetone to remove any residual grease.

Welding can result in stainless steel losing it's non-corrosive properties so you need to re-passivate it after welding. For 304 stainless steel the standard way to do this is to bath the item in a 10% citric acid solution at 120-150 degrees for about 20 minutes, rinsing off the acid, and then letting it air dry. This removes the surface iron and allows a protective oxide layer to form. The chromium atoms will also eventually migrate back into the metal lattice to help prevent corrosion.

The alternatives to a acid bath are to use "electro polishing" or use a citric acid gel that clings to the item. While there are advantages to using electropolishing it requires additional equipment. So I chose the acid gel approach.

I made up a 10% acid gel using distilled water, citric acid powder and xanthan gum paste (87%, 10% and 3% respectively by weight). All the ingredients are inexpensive and available at Walmart. You can easily google the recipe. I made the gel a little thicker than normal to cling better to vertical surfaces. I used the gel to cover the corner seam areas that had been heated during welding, dabbing on a thick layer and then covering it with cling wrap to keep it from drying. And let it sit for about 60 minutes as my google search recommended.

10% citric acid gel paste

acid gel paste passivation of welds

After rinsing I let it dry for a day and then did a "distilled humidity wrap test" to test the passivation. Basically you wet paper towels with distilled water, wrap them on the area, wrap the area in cling wrap to keep it from drying out, and let it sit for 24 hours. If there's no rust then the passivation worked.

Distilled humidity wrap test

The results of the first test showed a trace amount of oxidation (some faint rust colored staining showing on the paper towel) on one weld area so I cleaned them all up and reran the acid gel passivation process (which is what Google suggested). This time I let it sit for a few hours (vs 60 minutes), then cleaned off the gel, and reran the distilled humidity wrap test with no visible oxidation.

Shower Sump

Below Floor Rule 98B Shower Sump

One of the main reasons for taking on the Clipper project was to have a toilet and shower in our trailer. Our '48 Boles Aero is too small for such conveniences but we both feel they would make using the trailer a lot more attractive (the middle of the nights trips to a campground bathroom just aren't fun).

The Clipper has a very low roofline so there's really no extra room to elevate the shower to provide direct drainage to a grey water tank. To solve the problem I planned to borrow from the boating world and install a shower sump. It's a small tank with a pump and float valve that can sit below the shower and pump grey water back up to the top of the grey tank.

When I constructed the replacement trailer frame I found an appropriate location for a shower sump. I added a small platform between the bellypan and the subfloor just rear of the grey water tank. Shower sump pumps generally are not self-priming and need to pump uphill with no possibility of a dip in the line to the grey tank. So it needs to be located close to the grey tank to avoid a long run that might sag. I also needed to plan for periodic maintenance (or replacement), so I'd need a large hatch in the floor that can support people walking on it.

Shower Sump located near rear of Clipper

After a lot of research I chose a Rule 98B shower sump. It's been around a while and is widely used, fits in the available space, has more than sufficient pump capacity (800 GPH which should more than handle ~180 GPM from a standard RV water pump), and appears to be more serviceable than other manufacturer's models. There are spare spare parts available for the Rule 98B and I plan to buy a set to have on hand.

I was a bit concerned about pump cycling hysteresis from grey water running back into the sump after the pump stopped. I measured the pump cutoff volume (it can't pump 100% of the grey water from the sump) and the pump trigger volume and then compared that to the volume of the pipe from the sump to the grey tank. There is a lot of margin between the two so I decided that pump cycling wouldn't be an issue.

Rule 98B Shower Drain Pump System

I am a bit worried about water damage from a sump failure so I added some mitigations. First, one of the nice things about the Rule 98B is the seal on the lid - not all sumps that are available have this. Second, I mounted the sump in a small stainless steel overflow pan similar to what is done with water heaters. Third, I ran the sump overflow port to a drain point below the bellypan. The overflow port is not a standard size so I needed to find a barbed hose connector that was close in size and then ream it out to fit the overflow port. Finally, I'm also planning to install a simple water detection alarm in the overflow pan so that I'll be alerted if there is a spill.

I plan to have the sump pump switched on with the same power switch used for the fresh water pump. Currently I'm planning to run a 12VDC control switch to a 2 circuit relay that will provide power to both the Rule 98B and the fresh water pump.

I found an aluminum boat hatch trim ring that was large enough to provide room for service and replacement of the sump when/if needed (Crownline Boat Trim Ring 32012 from Great Lakes Skipper). By sheer luck the lip of the ring is about the thickness of the vinyl composite tile (VCT) so it can sit flush with the tile when installed. I made a hatch cover using a scrap of the Advantech subfloor trimmed to fit using a template I made from the hatch ring. I covered the hatch with leftover VCT from the flooring. Once I installed the VCT I needed to trim the bottom edge of the hatch so that it would sit flush with the floor. I coated the bottom of the Advantech hatch with penetrating epoxy to seal it from any moisture coming from the sump.

For now the hatch cover sits snuggly enough that I'm not going to install a latching or retention mechanism (I will if I need to). But I did need to install a means to list the hatch cover out of the floor-flush recess. I found some stainless steel "hidden recessed furniture handles" on Amazon that sit flush but pop up a pull ring when you depress a tab. I used a router to counter sink these into the hatch cover so you can pull it out when needed.

Flush Hatch Pull

Shower Sump Hatch Installed

Flooring

 After much deliberation I ordered Tarkett VCT II Ivory Tower (582) vinyl composite tile (VCT). These are 12"x12" vinyl tiles that are glued to the subfloor. I used VCT in the Boles and it's done well in the desert heat (no apparent shrinking or lifting) and it has a great reputation for durability. It's easy for a DIYer to install and it's one of the more inexpensive choices available. I also like that it looks period correct, maybe not as good as sheet linoleum ("Marmoleum") but close.

Tarkett VCT II Ivory Tower (582)

I chose a color (Ivory Tower 582) that I hope will complement the blue gray color I plan to use on the cabinets and the mahogany wood tones from the cabinet doors and trim. The mat beige color will also help hide the inevitable campground dust.

I worked out the quantity needed by marking off a template I made long ago for the front/back area into 12"x12" squares and accounted for the wasted tiles along the edges. I came up with 23 sqft needed for each end. Add to that the 12'x7' main body area (84 sqft) and you get 130 sqft needed. So I ordered 3 boxes of 45 tiles which will provide 135 sqft.

One issue with VCT is shipping. Each box weighs 63 lbs (!) so it's expensive to ship. Additionally it's a bit brittle when not adhered to a subfloor so my earlier experience makes me leery of shipping (I had 2 separate failed attempts at shipping for the Boles Aero). So I wanted a "ship to store" situation so I could simply not accept the tile if the box is damaged. Tarkett tile is not available through Home Depot or Lowes so I found a vendor (Floor City) that ships to warehouse locations for pickup in SoCal. I picked it up at their warehouse in Irvine, CA. It was delayed about a month from the original delivery date, but I wasn't ready for it so it didn't matter.

I thought about just tiling the exposed floor area, i.e. not under the cabinets. The reason I'd do this is weight savings. Each tile weighs about 1 lb 6 oz. I figure I might save 50+ lbs. In the end I didn't have a solid enough cabinet layout so I decided to just tile the entire subfloor.

Before I could tile I need to fill the seams between the subflooring sheets and also the countersunk recessed where the elevator bolts are located to hold the subfloor to the frame. After asking for recommendations on the Facebook Airstream Renovation group I ended up using the same Bondo used for car body repair. It's a 2-part filler that is mixed just before use and cures quickly. It was a little difficult to work with because the warm temperature made it cure very quickly. I ended up having to mix about 20 small batches to fill all the seams and recesses. I then sanded the areas smooth with 80 grit sandpaper on my handy "prepping weapon" (a nicely made sanding block). This set for about 48 hours before I started installing the tile.

Seams and Elevator Bolt Recesses Filled with Bondo and Sanded

I purchased tile glue (Roberts 2057 VCT Adhesive) which was available off the shelf at the local Home Depot. Based on the instructions for my estimated square footage of tile it should have taken a little less than 1 gallon, but I ended up buying a 2nd gallon and using about 1/4 of it too.

Before you apply the glue you need a plan and layout lines. At the recommendation of more than one YouTube I chose to center a run of the tile along the central axis of the trailer and also the doorway. I snapped a chalk line 6" offset from the trailer centerline and another perpendicular and 6" off the centerline of the entry door. This provides a full tile for the highest traffic (vs a seam between tiles).

The glue is easy to apply: you trowel it on using a 1/16" square notched trowel (also available relatively cheaply at Home Depot). There are YouTube video that demonstrate the application. Once it's troweled out you let it dry to a tacky feel (about 45-60 minutes in warm summer conditions). Then you proceed to lay down the tile. The glue has a open time of several hours once it reaches the tacky point so there's plenty of time to lay the tile.

The tile adheres to the glue immediately so once you lay down a tile you can't easily remove it. In fact with VCT you'll probably end up ruining the tile in the process of removing it. It's also difficult to achieve any minor adjustments of the tile once it's placed. So you need to be careful aligning the tile correctly before you lay it down.

I troweled out glue on 1/2 of the trailer lengthwise. Let it dry to a tacky feel and then laid the first central tile. It forms the "cornerstone" for all the remaining tiles so you need to get it right. I laid tile outward lengthwise from that first tile until I couldn't lay anymore full tiles. Then I did the next row and the next. It's really easy to do full tiles - it took about 10 minutes. The hard part is the edge pieces that need to be fit one-by-one. VCT is easy to trim by scoring with a utility knife. There's plenty of YouTube videos that show how to measure, mark and trim VCT. It took me about 1.5 hours for each 1/2 of the trailer I did.

Troweling out VCT Glue (bottom right)

VCT Glue (on right) Nearly Ready to Lay Down Tile

First Half of VCT Installed

After laying down the tile I used a linoleum roller (rented at a local tool rental outlet) to make sure the tiles were securely pressed into the glue.

The next day I repeated the process for the remaining half of the trailer.

2nd Half of VCT Installed

I still need to trim out the shower sump access door. It'll have the same tile on it but I'll need to dado the underside to sit flush with the aluminum door trim ring.

Electric Outlets and Switch Plates

Beware, this is a deep dive into outlet covers. I spent way too much time investigating them. I don't know why but it's surely a window into how my mind works.

On one of my countless searches through eBay's endless listings I came across some electric outlet cover plates that looked unusual. Although I knew nothing about them I thought they looked very midcentury so I watchlisted one. Later as I was planning the Clipper project I dug a little deeper and decided the style would fit well. The design is generically referred to as "Uniline".

Uniline Outlet Cover from eBay listing

Here's what Google has to say about it (prompt: "history of uniline switch plates history"):

"The Uniline series of switch plates, introduced in 1949 by Victor R. Despard, are iconic mid-century modern wall covers. Renowned for their distinct Art Deco-style vertical ribs, early versions were crafted from Bakelite, while later production runs in the 1970s and 1980s used durable thermoplastic.

History of the Uniline Series 

• The Originator: Victor R. Despard, an influential electrical engineer and inventor, designed and patented the Uniline switch plate and modular interchangeable devices (Despard lines). 

• Patent Designs: There were two primary Uniline designs patented: the 1947 patent (flat raised vertical center with two ribs on each side) and the iconic 1949 patent (raised vertical center with ribs separated by three fine lines). 

• Manufacturing Powerhouses: Because Uniline was a design standard rather than a single manufacturer, the ribbed covers were licensed and produced by major electrical brands of the era. Common makers found on the back of Uniline plates include: 

• Bryant (the most common) 

• Hubbell 

• Pass & Seymour (P&S) 

• Arrow-Hart & Hegemann (Arrow H&H)

• Production Timeline: The original rectangular border key design was in continuous commercial production from around 1950 until 1983. 

• Evolution of Materials: Early mid-century plates (the 1950s) were made of compression-molded Bakelite in dark chocolate brown and ivory beige. As plastics evolved, later 1970s and 1980s iterations were manufactured in smoother, non-ribbed thermoplastic varieties.

Collecting and Sourcing Today 

Today, these ribbed switch plates are highly sought-after by mid-century modern enthusiasts and vintage home restorers. 

• Vintage Original Hardware: Because millions were produced, you can frequently find new old stock (NOS) and salvaged ribbed Bakelite plates. 

• Sourcing: You can explore secondary markets like eBay or vintage craft marketplaces like Etsy for vintage P&S Uniline plates, Hubbell, or Bryant pieces."

There are quite a few different switchplate covers from the time period that look very similar to the Uniline design. It makes sense that other manufacturers would want to take advantage of the popularity of the style.

Sierra Electric Corp

"Tuxedo" Style from Eagle

I personally think the "Tuxedo" design best fits what we now, in retrospect, consider as "Mid Century Modern" design because of it's slightly outwardly angled center area. Because of that I probably would have chosen this style but it's difficult to find with only a few examples available on eBay.

Along the way I noticed some unusual versions of the Uniline switchplates. They were often referred to as "Despard" switch plates. 

Uniline "Despard" Type Switchplate Produced by Bryant Electric Co

Here's what Google has to say about them (prompt: "history of despard switch plates"):

"Despard switch plates, commonly known as interchangeable or stacked switch plates, were patented in the 1920s by Victor R. Despard. Designed to save wall space and maximize functionality, they allowed up to three small electrical devices—such as toggle switches, pilot lights, and outlets—to fit tightly into a single-gang wall box.  

The Evolution of the Despard System 

• 1920s Innovation: Patented by Victor R. Despard, the system introduced modularity to electrical wiring. Instead of wiring large, single-function fixtures, electricians used a metal mounting strap that could be loaded with various combinations of switches and outlets. 

• Mid-Century Peak: Despard switches reached their highest popularity during the 1940s and 1950s. As homes gained more appliances and lighting but maintained smaller rooms, this system was heavily used to pack light switches, extractor fans, and convenience outlets into concentrated, space-saving clusters. 

• Versatile Features: The modular components included not just standard toggles, but specialty pilot lights, 4-way switches, and uniquely keyed AM radio antenna outlets. Unlike modern toggles that flip vertically, traditional Despard toggles operated horizontally. 

• Modern Decline: By the late 20th century, architectural trends shifted toward larger, more expansive walls. The need to conserve switch space diminished, and standard Decora (rocker) or larger toggle plates became the industry norm. 

How to Identify a Despard Plate 

Despard plates are unique because they do not have a large rectangular or toggle-shaped cutout in the middle. Instead, they feature one or more sets of three perfectly aligned smaller holes. The plates are attached to the mounting strap via screws positioned at the top and bottom of the plate (rather than the middle)."

Although the selection is small there are still companies providing new equipment that supports the Despard switchplates. Legrand's Pass & Seymour's brand has switches, outlets, mounting straps and some modern-looking outlets. And a retailer, Kyle Switch Plates, has a large selection of Despard compatible items. 

I really like the Despard-style switchplates and plan to use them in one application: the various control switches required for thinks like hot water heater power, water pump, etc. I don't like how every trailer appliance has a different switch and how vintage trailer renovations are cluttered up with these modern looking switch panels. These are typically a very low power DC relay switch and easily substituted. The one exception is the water pump which is several amps. For that application I plan to use the Despard switch as a low amp DC relay switch and locate a 12VDC power relay remotely to switch power to the pump.

I found a "stacked" 2-gang Uniline Despard switchplate on eBay that I plan to use for the various control switches. I'll need to figure out a labelling approach to distinguish the switches. And I'll also need to come up with some inserts (3D printed?) that I can use for LED indicator lights (e.g. typical water heater "lockout" light).

"Stacked" Despard Style 3 and 2 Gang Switchplates

Insulation

 

Insulation

Summary

This is a long post so here's a short summary of what I used to insulate:

• Primarily 1.5" Extruded Polystyrene (XPS) Foam Board (R value - 7.5) in the walls
• 1.5" XPS under the subfloor
• Tecvan "Duck Liner" 3/8" foil-backed non-combustible Neoprene foam sheet (R 0.89) layered with 3M Thinsulate SM600L (R 5.2) on the endcaps
• 1/16" thick "high density" Neoprene self adhesive foam tape applied to the ribs and crossmembers before the interior skin is attached

I used about 8 sheets of 4'x8' XPS on the interior walls. Getting the 1.5" thickness required a 90 mile drive (the 1.5" thickness isn't available at my local Home Depot) so I was very careful to layout the cuts to maximize yield and that took a lot of extra time. There was very little scrap left over so 8 sheets was the absolute minimum required.

I used about 80 square feet of the Duck Liner and Thinsulate on the endcaps. Duck Liner comes in 32.5'x3.25" (105 sqft) rolls and is available online from several retailers. Thinsulate is sold by lots of people online and typically comes in 5' wide rolls of standard lengths. I bought a 20' roll (100 sqft) and ended up with about 4' (20 sqft) left over.

Main Insulation

I didn't deliberate too much on the insulation material I planned to use. I'd done all the research when I renovated the '48 Boles Aero model 14 I own. I did check to see if there were any new options and there really weren't. So I planned to go with the same 1.5" thick extruded polystyrene foam board (XPS) I'd used on the Boles.

XPS is a closed-cell foam that barely absorbs water. Of all the major insulation types it is the least absorbent. Of the readily commercially available products is a close second to polyisocyanurate (Polyiso) foam board for insulation properties, but polyiso absorbs water a bit more readily than XPS. The price and workability of XPS and polyiso are very similar. Both are readily available in a variety of thicknesses at big box home stores. 

The biggest problem with foam board insulation in the Clipper is that foam board is very rigid: it doesn't want to bend and the Clipper is all curvy. Fortunately the curves are not compound so woodworking techniques for bending wood, principally "kerfing" can be used. Kerfing is just cutting deep parallel cuts in the material so that it can bend easier. The more cuts and the closer they are spaced the more the material will bend.

I found that the tightest curve I needed to insulate was the wall to roof transition area. It's something like a 48" radius (+/-, I'm not precisely sure). I used some scrap material to experiment and found that parallel kerfs spaced 2" apart with a depth of 1 1/8" worked really well to fit the curve when I bent the material with the kerf towards the inside. If you bend the material with the kerfs towards the outside it can split along the kerf, and even if it doesn't split you end up with a lot more volume that is uninsulated.

Kerfed XPS Foam Board 

Why 1 1/8" kerf cut depth? Well you'll see from the discussion below on insulating the endcaps (where I didn't use foamboard) that there's some technical rationale for choosing an insulation thickness between the exterior and the interior of at least 1/4". Basically it will help prevent condensation inside the trailer. I figured 3/8" of insulation gave me some margin for error.

Another problem associated with rigid foamboard is that you can't simply measure the space between the ribs and cross members and then cut a piece to fit. The ribs and cross members are aluminum formed into ~1.5" C-channel so the space inside the C is less than 1.5". And the foamboard doesn't compress so you can just wedge it in.

The solution is to relief cut a bit of material from the edges of the foamboard to create a "tenon" that can slide into the C-channel. It's really pretty easy but tedious.

Relief Cut "Tenon" in XPS Foam Board

The bigger problem associate with the C-channel is that depending on how it's oriented (which way the opening of the C is pointing) it requires from 0-4 tenons, and worse the piece can be "trapped" so that you can't slide it in.

For all but the simplest C-channel orientation the solution is to cut the board into smaller pieces so they can be "puzzled" into place. The number of pieces required and solution to the puzzle is related to the orientation of the C-channel. I found the worst case to be 5 pieces:

So how do you run electrical through the foamboard? The simple answer is to cut a channel with a hot knife designed for use with foamboard. There are a lot of different models available online but based on the reviews I chose one with a built-in fan that allows the knife to be used for extended periods. Originally I thought I might use the knife to do all the foamboard cutting but it became apparent that the table saw was the faster and more precise way to go. It's somewhat messy (all that foam "sawdust" sticks to everything) but I have decent a dust collection system so it wasn't too bad.

Channel cut in XPS foamboard for electrical cabling

Subfloor

I also used 1.5" XPS to insulate the subfloor where I was able. I did this while I was installing the subfloor on the frame. Here's the process I used:

• I cut out the subfloor and placed it on the frame. 
• Then crawled under the frame and used a Sharpie to trace the outline of the frame onto the subfloor pieces.
• I pulled up the subfloor and using the Sharpie-drawn outlines I cut XPS pieces to fit between the frame members.
• I attached the pieces using 3M 78 foamboard spray adhesive and stainless steel "foam board mounting gaskets" attached to the subfloor with stainless steel screws.

XPS Insulation Applied to Underside of Subfloor

Some installation notes: 1) the 3M 78 spray is costly and doesn't add much so I wouldn't do it again; 2) I used tongue & groove subfloor so installation required me to rotate the pieces down as I installed them. This required some relief cuts along the edges of the foamboard; 3) there is no foamboard where the fresh and gray water tanks are installed because it would take up too much vertical space; 4) there is no foamboard in the area where the shower sump is installed.

Endcaps

The endcaps presented a different challenge. I had no idea if there would be a constant 1.5" gap between the inner cap and the outer skin, in fact I doubted it. So using the rigid foam board might be problematic. So I chose a different path.

I decided to use a thin layer of closed cell neoprene sheet insulation adhered to the outer skin coupled with a layer of traditional batting insulation. The closed cell sheet insulation will, in theory prevent condensation against the outer skin and the batting insulation will provide additional insulation while also being able to adapt to a variable gap between the inner and outer skin. This is a technique many van conversion experts advocate and it made sense to me as a way to address my installation.

To prevent condensation the "dew line" needs to fall inside the closed cell sheet insulation so that no moisture can build in the batting insulation. There's a pretty good article that covers this at Faroutride.com. What they don't cover is how thick the insulation needs to be to ensure the dew line falls inside the closed cell insulation.

To find out thick the closed cell insulation needs to be I used a "Professional Insulation Thickness Calculator" provided by the Armacell insulation company. It's called Armawin and can be found at Armawin.

The tool is designed for use to determine insulation thickness needed to prevent condensation on cold pipes in an interior location, but the problem is the same as a cold exterior wall exposed to a higher-than-ambient relative humidity found inside a trailer (or van). There are a lot of variables you can plug in depending on your use case, cold temp, hot temp, relative humidity, material in use, insulation material, etc. 

I chose to evaluate external temperatures of 32F, internal temp of 75F and an internal relative humidity of 50%. I thought this would reflect my most likely and concerning likely use case: cooler spring and fall weather. Colder temps and higher relative humidity will increase the minimum thickness necessary.

In any case the tool indicated that when using a typical closed cell foam sheet neoprene insulation the required minimum thickness is 0.14" (3.6mm). I made multiple runs with different conditions and decided that the minimum thickness I'd consider was 0.25".

Sample Output from Armawin Tool

I ended up using a product from Tecvan they call "Duckliner". It's a 3/8" foil-backed neoprene closed cell insulation with an adhesive on one side (you peel off paper backing to reveal the adhesive). Neoprene foam sheet tears easily but the foil backing improves the handling qualities significantly. I suppose in some applications the foil backing might provide some radiant barrier properties but sandwiched between two sheets of aluminum in the Clipper walls it would be imperceptible.

As far as the batting insulation goes I read a lot of articles with pros and cons of all the different types that are readily available. They all have the downside that even if they are technically "waterproof" they can still trap moisture between the fibers. In the end I chose 3M Thinsulate SM600L insulation. It's widely used in the automotive and outdoor garment industry. From my perspective for a bat-type insulation the only only real negative I read was price - it's more expensive than most of the alternatives.

Installation was quite a process. First I made templates for the exterior roof segments. You only need one half of one end because they are replicated.

Template for Neoprene Sheet Insulation

A Bunch of Templates

Using the templates I'd made, I cut out the Duck Liner using an utility knife. Even though the adhesive is protected by paper it is REALLY sticky.

Using a Template to cut the Duck Liner

After the Duckliner was cut out I stuck it piece by piece onto the Clipper endcap ceiling. Some people recommended using 3M 90 adhesive spray on the ceiling but I did some tests that convinced me it wasn't necessary. That adhesive is really sticky.

Duck Liner Segment installed on Clipper Endcap

From there you just "rinse and repeat" until all the endcap segments are installed. Of course there was a lot of trimming as I went along because the templates weren't perfect and repeated transferring a trace and then cutting the trace leads to oversized pieces. Additionally the neoprene foam stretches a bit as you put it in place. But it wasn't too bad.

Duck Liner Fully Installed

After the Duck Liner was installed I set to work on the Thinsulate layer. In an attempt to make things go faster I made new templates that combined a few of the segment templates I'd made and used with the Duck Liner. I recommend getting some easy removal doublestick tape and testing the templates against the inside of the endcap. I didn't do this initially and they proved to be a little too large (which required additional trimming).

I laid out and traced the template onto the Thinsulate with a Sharpie, I oriented the black "scrim" side down mostly so the sharpie line would show. I could have also used something like a soap stone to mark on the black scrim side. Note that there is also a slightly more expensive "double scrim" version of Thinsulate that would be black on both sides.

Tracing the Pattern onto the Thinsulate

The Thinsulate can be cut with a sharp pair of reasonable quality scissors. For kicks I also tried using a fresh utility knife and it was hopeless.

After cutting out the Thinsulate, I sprayed the non-scrim (white) side of the insulation with 3M 90 adhesive spray and also the corresponding area inside area of the trailer. When they dry to a tacky feel (really quick in warm condition) you can carefully place the Thinsulate into place. It worked best for me to start at the highest corner and then work down the seam and then across. This allows gravity to keep the two glued surfaces separated until you press them into place.

It's important to note that you'll need to flip the template to trace the insulation for the opposite panel.

Thinsulate Installed on Half of the Inner Endcap

I've read that the 3M 90 adhesive may begin to fail if you leave the Thinsulate hanging "too long" on horizontal surfaces without some support. Some sources recommend that you be prepared to install the inside walls/skin shortly (days) after installing any hanging Thinsulate to prevent it from falling down. 

In light of this I delayed installing the Thinsulate until I'd removed the paint from the inner endcaps and then polished them (I'm tentatively planning to leave the inner skin bare) so that were ready to reinstall. Like everything else on this project, preparing the endcaps turned into a tedious, dirty and time consuming process so I'm glad I waited for this to be complete to install the Thinsulate.

Cleaned and Polished Inner Endcap Reinstalled

Rib to Skin Thermal Barrier

The Clipper originally had a thin felt-like material glued to the ribs and crossmembers where they contact the inner skin. I assume this was used to create a thermal barrier to slow thermal conduction from the outside to the inside (or vice versa when it's cold). This heat conduction is a problem for metal framed trailers as metal (and aluminum in particular) is a great heat conductor. If you touch the ribs when the sun is hitting the outside walls you might get burned. If you don't retard this heat transfer then it will simply conduct straight through to the inner walls and all the insulation will be a waste. So somehow you want to break the metal-to-metal contact to create a thermal barrier.

I spent a lot of time trying to figure out what Airstream currently uses as a thermal barrier in the new builds. There are a lot of forum threads on the subject but I could find no definitive information. So I found out what other people use, why they used it, how it worked out and then I selected a path.

In the end it seemed like a high density neoprene or EPDM foam tape would be the best bet. It comes in different thicknesses and thicker is better as a thermal barrier, but thicker foam would also lead to dimpling at the connection points. In the end I chose 1/16" thick tape. There are a lots of different material choices available for foam tape but not all foam is created equal. For example polyethylene (PE) foam has good thermal properties and is inexpensive but breaks down relatively quickly over time and has reduced thermal range when compared to neoprene.

Neoprene Tape Applied to Ribs Prior to Inner Endcap Installation

Neoprene Foam Tape

Bumper (or Brush Guard?)

 I haven't posted in a long while and I've accomplished a lot that I need to go back and cover, but I just finished this and I wanted to document it as reference for me in the future.

Installed "Bumper"

One of the prior owners (PO) of the Clipper removed the "bumper" and associated interior and exterior mounting brackets. These items were not provided in loose collection of parts that were strapped into the Clipper when I drove it home, so I needed to fabricate replacements.

I call it a "bumper" but it's really more of a brush guard. It's really not very sturdy and it's only connected to the frame at the rear center tube. Each forward end of the bumper is bolted through the skin onto a bracket that is bolted through the subfloor (originally plywood and in my Clipper Advantech OSB). And the midpoint mounts between the end and the rear center frame mount were just bolted through the skin. The design could lead to significant structural and body damage under any significant collision. So my conclusion is that the bumper is really just there to keep people or campground brush from rubbing the trailer skin (e.g. when backing into a campsite).

Interior Bumper L Bracket - Facebook Group

Flimsy Bumper - Vintage Trailer Boneyard

Based on many online images and some more detailed images and measurements from member of the Facebook Silver Streak Clipper group I set out to fabricate and install the bumper. This needed to be completed before the insulation could be installed because the interior mounting brackets actually reside inside the rear wall cavity.

I repaired a small damaged area with a patch curbside where the forward part of the bumper attaches.

Damaged Forward Curbside Bumper Attachment Point

I then fabricated some simple L brackets from 1/8" steel I had on hand, basically welding 2"x4" coupons into a L bracket. I drilled 1/4" holes (for 1/4"-20 bolts) and welded on 1/4"-20 nuts to receive bolts to mount the bumper from outside. This means the bumper can be removed for future service if necessary. I read a couple accounts that said that bolts were originally mounted from inside the walls to the outside rather than vice-versa, but that meant you needed to remove the interior panels to replace any damaged or rusted mounting bolts (or potentially bumper) in the future. I thought inserting them from outside in made a lot more sense.

Note that I also needed to drill a hole for the marker light wire to feed through since I'd already routed those through the subfloor. I also cut a rubber grommet to place between the bracket and the aluminum skin to provide a seal and protection against galvanic corrosion.

Fabricated Forward Bumper Attachment Bracket - Streetside

Fixing the brackets to the subfloor proved to be a challenge since at this point in the build process I didn't have access to the underside of the subfloor to attach nuts and washers (or better yet a backing plate below the subfloor). There is limited space behind the subfloor so the choices for blind bolt fasteners is limited. In the end I decided to use toggle bolts (Toggler Snaptoggle Anchor Bolts). I checked the product specs and they have decent retention strength, probably stronger than the subfloor itself.

Toggle Bolt Install - Curbside

As I mentioned previously, the midspan attachment appears to have been bolted directly to the skin with fender washers on either side of the skin. The skin was reinforced/doubled with an extra sheet of aluminum, but there was no bracket attachment to the subfloor. I'd seen vintage pictures of people standing on the bumper and even using the bumper as a jack point (yikes!) I thought it would be prudent to try to provide some added vertical support so I made some brackets similar to the ones described above.

There is virtually no clear space under the subfloor where these brackets would exist so I couldn't use the toggle bolts I used on the forward brackets (they need 1 7/8" clear space to insert). After considering the alternatives I ended up with a poor solution, but one that is better than nothing at all. I used wood insert nuts. These have relatively low retention strength in plywood and OSB (much better in solid wood). I watched several tests on YouTube where they managed ~300lb of lateral pull force and up to 1000lb of vertical load which isn't much, but any vertical load will be spread across the bumper mount points so they should provide some function.

Midspan Bumper Attachment Bracket Install

Note that I inadvertently cross-threaded one of the wood insert nuts so I had to drill out the hole and use a larger one (5/16") with associated larger mounting bolt. Hopefully no one ever has to examine that detail in the future!

On the exterior the midspan attachment points use a metal standoff to hold the bumper away from the frame. I used ~2" wide 1" flange C-channel cut to make two standoffs that are 2" wide. Base on pictures I believe the C-channel is what the factory used for standoffs but I don't know about the size or thickness. I decided to use 2" because it was close to the rear frame tube standoff distance and I thought it would work with the arch/bend of the bumper.

Midspan C-Channel Standoff as installed

As I mentioned previous I am nervous about any significant loads on the bumper. Just having one may invite people the stand on it or use it as a lift point, in fact I noticed that the Airstream "brethren" of the Clipper has no bumper. Walley Bynum, the designer of both the Clipper and early similar Airstream, seemed to improve the design as he went along and this might be one design feature he later deleted in Airstreams as an improvement (or maybe less problematic). 

In any case my wife Becky made some prominent "NO STEP" warning labels for the midspan standoffs.

The bumper itself was actually pretty straight forward. I purchased two 6' sections of 2" wide 3/16" mild steel bar and welded them together to create a 12' section. I had a welding shop bend the bar into a half hoop with diameter of 84". Originally I thought I might bend it myself manually but in the end I decided the welding shop would do a better job (although more costly).

I measured and drilled holes for the rear frame attachment and bolted on the half hoop bumper. I then measured to the midspan attachment, drilled holes and bolted on the bumper. Then I performed a similar operation on the forward attachment points. There is a standoff at the forward attachment point that is formed by two nuts used as spacer.

Bumper Attachment at Rear Frame

Forward Bumper Attachment (curbside) looking down from above

Each of the forward attachment bolts is basically a ~2" all-thread stud with two jam nuts that function both a standoff spacer and allow the stud to be screwed in or removed. On either side of the jam nuts are fender washers. There is a neoprene washer between the skin and fender washer that both seals the hole and prevents galvanic corrosion. The bumper is held in place with a washer, split lock washer and acorn nut. The acorn nut provides some protection from scaping against the end of the stud as well as being more decorative than a typical hex nut, but it does require more attention to installed dimensions since the acorn nuts have limited depth.

To finish the bumper, standoffs and interior mounting brackets I machined off the millscale, sanded, primed with self-etching primer, dry sanded, and sprayed on finish coats. The finish coats for the interior mounting brackets is some black Rustoleum hammer tone spray I had on hand. The finish on the bumper and midspan standoffs is a Rustoleum automotive gloss black enamel (3 coats).

All the bolts, nuts and washers (with the exception of the one 5/16" midspan bracket fastener mentioned above) are 1/4"-20 stainless steel. The wood insert nuts are brass so there is some chance of galvanic corrosion, however the blue Loctite thread locker I used should provide some additional protection.

A future upgrade I'm contemplating is to modify the forward mounting studs so that they more readily shear upon any impact. As discussed above any significant impact likely to damage the trailer body and subfloor. If these forward attachments would shear off easier it might provide some damage prevention while still providing the function of a brush guard. Making the stud shear more easily could be as simple as carving a notch around the shaft of the stud where the fender washer meets the bumper rail. I've read that this technique is used for devices like snow blowers and snow mobiles where high torque parts might suddenly transition from snow to hard ice.