After weighing all my options (see part 1), I decided to go with Polyisocyanurate rigid panels for insulating the Airstream. Both Lowe’s and Home Depot carry PIC, but I like the Home Depot brand better (Rmax, made in the USA). The PIC panels are far superior to styrofoam and extruded polystyrene in just about every way (except price, but they’re actually only a few dollars more per sheet). PIC is certainly more fire-retardant (though not fire proof), and definitely equally if not more moisture resistant. They don’t call it moisture proof, but I assume that is for legal reasons only.
I actually did a test where I fully submerged pieces of PIC in sealed jars full of water and let them sit for extended periods of time. After three days submerged, the PIC came out weighing exactly the same as when it went in (indicating that it took on little to no moisture, or it would have been heavier).
After two weeks the PIC did weigh a bit more when I first removed it from the water, but after letting it sit for around an hour, it returned to its original weight. I did another check at 6 months and got the same results. Maybe not the most scientific test in the world, but good enough for me.
I went a little crazy with the actual application of the PIC panels. I decided the best method is to first cut the 1/2″ thick PIC panels into small pieces and then glue them with TremPro 635 to the exterior skins. I tried a bunch of different stuff for adhesive (TremPro 635, Vulkem 116, Silicone, 3M VHF tape, etc.), but I think the TremPro 635 is best. It works whether you’re applying it in hot/cold weather, and it’s definitely going to last.
At first I was cutting the 1/2″ thick PIC into tiny 1.5″x1.5″ pieces (see photos in gallery below), but that was taking too long, so I started using 1.5″ strips (at whatever length was good for the location). I then cut sections from 1″ thick PIC panels to fit between ribs and cross members. This leaves an air gap against the skin for airflow and moisture runoff (and the possibility of running more wire if I ever need to). This method also makes it easier to repair damaged exterior panels if that dreaded moment ever arrives (vs. spray foam, which is crazy-hard to remove when repairing the exterior skins).
In places where there is a curve in the wall, you need to score the PIC to allow it to conform to the curve. After I scored my PIC panels, I resealed the cuts with foil tape. This is pretty easy where it’s just the simple curve of the upper side walls, but it gets tricky for the compound curves of both end caps.
Finally, I sealed everything up against the ribs and cross members and window frames with Nashua 324A Foil HVAC tape (see photos in gallery below). Nashua 324a foil tape is my best friend. Be careful though, it will slice you if you run your finger down the edge!
I purchased two 3″ wide rolls at Goodwill for $10 about a decade ago. When I ran out of that, I was worried because all I was finding online was a 60yd. roll for around $40, but luckily after checking the big box stores I found it at Home Depot for $17/roll (60yds., 2.5 inches wide). It’s rated to hold from -25F to 325F. That wouldn’t work if it were applied directly to the exterior skins (they actually get hotter than that in the sun), but it’s fine for using to seal the interior of the PIC panels. It sticks in the extreme cold, and it sticks REALLY well (even during application, which works all the way down to -10F). I’ve got a roll of some other non-marked foil tape (thought the cardboard roll does say Nashua), but it may as well have NO adhesive whatsoever compared to the 324a.
I also decided it was a worthwhile endeavor to put a thermal barrier between the ribs and the interior skins. The interior skins are aluminum like the exterior. The ribs are also aluminum. That means when the sun is baking the exterior of the Airstream, all that exterior heat is transferred via the ribs to the interior skins and basically turns your Airstream into a toaster oven. I have NO IDEA why Airstream doesn’t utilize a thermal barrier on the ribs from the factory. I did some research on materials (using EPDM rubber, rolled polysterene, etc. –I even researched aerogel) and found that the best bang-for-the-buck solution is the blue polystyrene you use when laying framing on a foundation. It’s only $6 a roll at the big box stores, but I often find new rolls at salvage places for $2. It’s light, it’s thin, rivets obviously go through very easily, and it makes a HUGE difference to the interior temperature. I used an infrared temp. gun to take some readings of the interior skin over the ribs with and without the polystyrene barrier, and on an 80 degree, sunny, Colorado day, there was up to a 120F difference!
Regarding the idea of a moisture barrier… The PIC is foil faced on both sides and then sealed with foil tape on the interior. I think that is plenty for the “exterior” moisture barrier, not to mention the gap between the exterior skin and PIC panels for moisture to shed. Regarding the idea of this causing too much moisture to be trapped inside the Airstream (especially when using the propane appliances and heat)… I guess time will tell how bad it could get. One can always open vents/windows, so we’ll see how it goes. Luckily I’m not too worried about moisture collecting in/on the PIC panels, after doing my fully-submerged panel tests.
Finally, I am using two 2″ layers of PIC with air gaps between the panels and at the decking for insulating the belly. In addition to acting as a thermal gap, the air gap between the underside of the decking (floor) and the first layer of PIC also helps eliminate floor rot from moisture collection.