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