Insulation: Adding Comfort in a Bubble with Double Bubble
Heat loss and heat gain in these trailers is significant. Essentially you have a double walled envelope of two thin layers of aluminum sheets over aluminum ribs. This provides a 1 5/8” airspace or cavity for insulation. The floor has a deeper space of 4” between the outside (bottom) aluminum skin and the 5/8” plywood deck. In addition, the 25 foot Tradewind has a nice amount of window area, all of it single pane. This is not a great structure for combatting heat transfer from inside to outside or vice versa.
Airstream used fiberglass batts as insulation. I wasn’t going to be using fiberglass batts. Critters seem to love the stuff and when they move in they contaminate it. Plus, trailers have moisture issues, like leaks and condensation, and wet fiberglass makes a pretty unhealthy condition especially combined with any dirt and animal contamination. I’m content, for now, that this trailer is without leaks and that the way I’ve got it set up I will notice a leak and be able to fix it. Still, I purposefully avoided materials in the rebuild of the trailer that absorb moisture or inhibit it from drying out quickly. The insulation value that a 1 1/2” thick batt of fiberglass provides was, for me, not worth environmental risks that come with it.
My priorities for deciding what to do for insulation were:
- understanding that the structural system of the trailer was just not going to allow for high insulation values, so admit that and use tactics in addition to insulation for heat transfer issues
- avoiding systems/materials that were affected by water or that retained water
- keeping the solution realistic in terms of materials cost and skills needed for installation
- eliminating or at least not promoting pest habitat
- flexibility for accommodating the electrical wiring in the walls and ceiling areas of the trailer
- easily reversible: allowing future access for trouble shooting , maintenance and repairs to wiring and structure
I ended up going with Reflectix Double Reflective Insulation. It consists of two 96% reflective layers of film bonded to two internal layers of heavy gauge polyethylene bubbles (total thickness 5/16”).
Here is how my thinking went in chosing it:
There are 3 types of heat transfer that need to be dealt with: conduction, radiation and convection. Reflectix provided a reasonable way to address all three. The film on both sides of the material is highly reflective and was efficient in creating a radiant barrier reflecting in both directions. By using spacers I could construct a sandwich of airspace/reflectix/airspace. This configuration provided some R value and also helped in sealing against air infiltration. So the material configured this way addresses heat transfer by radiation, convection and conduction.
I ended up by using a configuration of two layers of Reflectix with an airspace between the layers and an airspace between the exterior skin and the outward layer of the double bubble material. Theoretically there is a tiny airspace between the inside skin and the inward most layer of the double bubble, but realistically the interior skin and the Reflectix are in contact. I used 1/2” thick insulation foam board cut into roughly 1 1/4”strips as spacers. I’m not sure adding the extra layer of the Reflectix was worthile, but if nothing else it enhanced sealing against air infiltration in the envelope cavities.
The double bubble material is crazy easy to work with. A sharp knife and scissors for cutting and a sharpie and straight edge for marking were pretty much the only tools needed. Cutting the spacers out of the 1/2” foam board was the most labor intensive part of the process, and the least pleasant. I now notice on the Reflectix website that they have spacer products you can buy. Had I seen that when I was doing the insulation I definitely would have tried them out.
The work flow was to prepare the spacers and affix them alongside the structural ribs using adhesive. Then measure and cut pieces of the Relfectix to fit in the “bays” between the ribs. Using the 48” wide material (primarily) meant that I didn’t have any areas that required seaming, as all pieces broke at a rib. I affixed the double bubble material to the spacers with adhesive and to the side of the ribs with foil tape from Reflectix. Pretty straightforward except for making patterns for the pieces at the front and rear end caps.
I make no claims about what sort of insulation value my approach achieved. I know that there are places where the airspaces are violated by one layer insulation coming into contact with the other and, as I mentioned above, insulation coming into contact with the interior skin so the whole installation was not optimal. But I also know that as I worked along the temperature difference of surfaces was dramtic in areas where there was no insulation, and then one layer and then two layers. The insulation work stretched from mid May through mid July because I did wiring and other things at the same time. I wanted most of the wire runs to be located between the two layers of Reflectix, and the wiring was slow going for me as I taught myself 12 volt systems as I went. This strectch from spring into summer let me appreciate the difference in heat transfer, especially heat gain, from a cooler season into the hot season.
The finished trailer is quite comfortable, especially as regards keeping warm even in freezing temperatures (more about this in the heating episode). Heat gain is still an issue when the temperatures get into the nineties and there is direct sun hitting the exterior. Best thing to do in hot sun is get some shade. I keep noodling on how to do an easy up shade tent over the top….