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.
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.