Insulating an Airstream (part 2: practical application)

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.

10 thoughts on “Insulating an Airstream (part 2: practical application)

  1. Hi there,

    I have just read this article and I think it’s fantastically helpful! I am about to go through the same process of insulating my Airstream and I think this looks like the best way to do it (and obviously very well researched). I was just wondering what it costed you to use this method of insulation? My caravan is 25ft and I know that it’s all relative, but just having a ballpark figure of what you spent would be really useful.

    Thanks in advance

    1. I am not sure on total cost. My trailer is the same size as yours, but I have been doing everything over an extended period of time (as the money comes in), so I don’t have a solid “all at once” figure on purchased materials. Being able to install over time and doing one section at a time was a major reason to go with the PIC panels though. I get a little cash, I run to Home Depot and buy a couple sheets of polyisocyanurate to install. When I get more cash, I do it again! [wink]

  2. What would you think of applying the surface foil, or a vapor barrier product like Stego Wrap, or the bubble wrap against the exterior skin, spraying closed cell foam against that and using the (v clever) idea of sill seal on the “studs”? I am not sure I understand the runoff theory if there is condensation on the outside skin, inside – doesn’t that vapor sit on the floor eventually? We experimented in a house with closed cell foam and the bond is quite adequate to the plastic materials if applied by a pro.
    I so appreciate you thinking of this stuff for all of us! Its been more helpful than you could know.
    Mike

    1. Regarding spraying foam over a vapor barrier or “bubble wrap” placed against the skin… you lose two major benefits of the spray foam in this scenario: sealing the skin seams (and other tiny holes, cracks, slits, etc.) and any added structural integrity offered by the rigidity of the foam when adhered to the skins and ribs. Regarding the vapor run off… that condensation *should* run down the interior of the exterior skins, collect in the C channel that sits on the sub floor, then exit the trailer through the weep holes in the C channel.

  3. Hi,
    I just found your website. Wonderful knowledge.
    I am beginning to insulate my 69 Overlander now and read your article. Loved it. I was going to use Rockwool Comfortbatt until I read your article.
    How do you like your decision? Are you happy? Does the polyisocyanurate squeeze when you walk about in the trailer?
    Any other pointers I need to know before I just jump in?
    Thank you for your time!
    Enjoy!

    Sincerely,

    Alva LaChance

    1. I am definitely happy with the PIC panels. I too was a little worried about “rubbing noise” (styrosqueak) when moving around, but I have not experienced any of that kind of noise, and we’ve made some long trips and parked in some pretty wonky places (uneven, not very stable ground). Just make sure all your panels are stable (smashed in there tight and then taped well with the foil tape) so they aren’t flopping around or able to move against anything that would allow them to squeak. Again, I chose polyiso because it afforded me the opportunity to go in sections and not do it all at once. I am hoping to try spray foam on the next one (remember, it needs to be a cold application, as hot might warp the skin!), but that requires having everything all planned out and set up beforehand, especially in regard to electrical and plumbing venting in the walls. Not to mention paying for it all at once!

      I was fairly precious about using every last bit of polyiso thinking it would help with cost. For sake of getting things done, I could have been a little more cavalier with my cuts and installing. Also, I think *strips* are easier (and probably better) than a bunch of tiny little squares for the stand-offs. You need to have those air gaps between solid panels, but I think cutting and gluing strips is quicker and easier (you can still use scrap, just cut into strips instead of squares).

      Finally, the thermal barrier at the ribs (interior side) is key! That blue sill plate foam works great, and makes a HUGE difference in the temperature of the interior skin on a hot, sunny day. Just make sure it stays put during the install and doesn’t slip off the rib when you rivet the interior skin.

      Getting ready to do the belly very soon, and so happy I’m not filling it with rockwool or batt (which is basically just a sponge to hold moisture against the frame and subfloor!).

  4. One more thing I forgot to mention… while PIC does provide better opportunity to fish wire through the gaps *after* installation if you need to, try and get all your wiring run first so you don’t *have* to do it later. If possible, run plastic conduit with some extra space in it that will allow you to run additional wire if you need to in the future. Bonus points for including a string inside the conduit to facilitate pulling a new wire in the future!

    And don’t forget… your plumbing system needs vents from the pipes and tanks all the way to the roof! Run pipes for the plumbing vents before you install the insulation! I didn’t do this on my first build, and had to figure out “clever” ways to vent properly without being able to take the vents up through the wall cavity.

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