Floor progress

Floor support rails installed. They are cut down pressure treated 2×4’s to 2″ tall, to accommodate 2″ foam insulation. The fasteners used are 1-3/4″ self tapping hex head screws with a washer head. They are countersunk into the wood. I’ll be applying some sealing primer to the untreated surfaces of the strips, as well as the underside of the plywood that will be laid on top.

Foam insulation update

The value of insulation is rapidly becoming apparent. It is currently 32 degrees f outside with 30 mph sustained winds. I haven’t started insulating the floor or end caps yet. The walls and rear are getting close. With the small electric radiator I can maintain 65 degrees from the waist up. The first 10 inches off the floor are about 45 degrees. Air temp at the ceiling is about 75.

If I turn on the propane heater buddy it gets too hot to work in about 20 minutes.

I hope my choice of foam panels over spraying doesn’t bite me in the ass later with condensation issues. As a test I pulled a panel back to the metal and it was really cold but dry. Within about a minute condensation started to form on the metal.

This was a skin panel that I originally plopped foam against without even bothering to wipe the condensation off originallly, so I was wondering if it would just get wetter or dry out.

I have three inches of polystyrene panel with foil to the outside and inside.

Cold bridge insulation experiment.

As a science experiment, I tried applying my sealing foil tape directly to the rib on the right, and on the left, the actual setup, which is 1/8″ acrylic foam tape applied to the rib, followed by the same mylar tape.

As you can plainly see from the photo, there is significant condensation of moisture on the right, while the left is completely dry.

Some numbers:

time/date/location: 23:30, November 7 2014, Seattle, WA
outdoor relative humidity: 90%
indoor relative humidity: 60%
outside air temp: 41 F
inside air temp: 65 F
vehicle exterior surface temp: 41 F
surface temp of 2″ foam, taken from center of panel: 64 F
surface temp of rib with acrylic foam covered by mylar tape: 59 F
surface temp of rib without acrylic foam covered by mlyar tape: 41 F

As you can see, the cold bridge just rolls on through the steel structure like it doesn’t give a damn. Even 1/8″ of cheap foam insulation tape makes a significant improvement.

If you are spraying foam directly over your ribs and structure, you’re set. Unfortunately spraying 1000 square feet of surface area to a 3″ depth was out of the cards for me, price-wise.

Instead, I’m laying down foam boards, and I’ll need to seal all the gaps. Not ideal, but it shows how much thermal loss there is in a vehicle that retains the original structure inside without modifying it if you’re planning on occupying for longer periods of time.

Thoughts on foam insulation for a metal bus

Some folks were asking about different foam types, I’d say go with the polyiso, it’s a better insulator. The reason I went with the EPS is more to do with cost than anything else.

There is a rule of diminishing return on engineered or installed R value for a given structure, and for my needs, I felt my money was better spent on other items. For example, I plan on insulating the basement from the underside, and the doors for the basement with the same “cheap” foam, and also applying a bed liner cover over the insulation in those areas to keep up durability.

If I had gone with the polyiso, the funds for doing that would have gone to just the foam board itself. Additionally, R value is one component in many to make the insulation effective, and I think the returns on the R value difference will not be all together that much different.

Here’s a toy to play with some numbers:


There’s a huge body of work related to this. I am not a civil engineer, I just stayed in a holiday in express one time.

I also purchased stock in the Dow chemical company buy buying about 30 cans of “good stuff” spray foam (red can). With the nozzle I am spending time injecting spray foam into the crevice and gaps around each panel installed in the bus. Then, I’m shaving it down flush with a wire wheel.

After that, I will tape the seam with foil tape, and apply an elastic foam tape that’s about 1/8″ thick. Then, on top of THAT, I’ll be installing 1″ eps foam board (foil facing inwards) jointed to the wooden support boards. If I’m feeling jaunty I may spray foam the gaps between the wood boards and the 1″ boards, but most likely I’ll just do a close fit, then foil tape the 1″ boards to the wooden support boards you seen in the pictures. I fully expect the interior to look like a giant jiffy-pop pan when done.

On top of that, I’ll install the finish panels, which I’m trying to decide between white ABS plastic with a fine sand texture finish, or .060 FRP panel with the same texture finish. I’m leaning towards ABS due to it’s ease of install.