This is along the lines of my thread about insulation over existing ply walls and then putting up another interior wall, space and weight not an issue compared to the basic construction of SIP walls.
I think insulation is really over done in cargo trailers, except the roof.
My Army days, at Ft. Drum in January, with a windchill of 52 below ZERO, my snow cave was comfy, we had a GP medium tent, a canvas tent, with a diesel heater, it was 65-70 degrees. Today, the Army has insulated tents and many ride in AC vehicles.
I've seen hundreds of conversions, listened to lifers on the road, most everyone will say my heater will run you out of the trailer and that the A/C gets cold enough to hang meat......so they claim.
Does that mean they have huge heaters and zillions of A/C BTU's? No, it means the trailer is well insulated, might be too well insulated, which can have other issues, like mold, mildew and pharts that hang in the air for 30 minutes.
Right, your foam insulation is M/M resistant, what about all the other material items left in the trailer? The better insulated a trailer is, the greater the issue with moisture. (Thats a period after moisture) How many campers have water damage and don't have a leak?
Since I'm not paying a big utility bill for my trailer (like a house) I don't really care, basically, if the furnace is half way or three quarters of the way up, and often you'll open a window to let fresh air in while heating the place. Seems odd that some folks use a window as a thermostat.
Then you stick a vent fan in the roof, usually left operable so that negates the effort of insulating the roof. Just a layer of Reflex can cover that.
My 6 x 14 cargo V Nose is silver, on a hot day it seems a bit cooler as it's generally in the shade. It's warmer in the winter, just sitting there. There is an air space between the metal and the interior plywood walls, painted and wood joints covered. The ramp door isn't insulated, probably 3/4 ply with a rubber flooring material and it seals pretty well. The uninsulated RV door almost has to be slammed shut, that's because it's fairly air tight as it sits.
Unless you're a ski bunny or a summer surfer, most seem to travel with mild weather. But, always be prepared, last year Texas had ice storms, weather extremes will be more common.
A friend of mine is a P.E., his opinion is that 1 or even a half inch over my existing plywood with another interior wall covering and 1 1/5 " in the ceiling constructed the same way would allow me to survive just fine with 5000 btu A/C and 10,000 btu propane heater. Basically, it would be a poor man's SIP wall and ceiling build.
God didn't give me a fur coat but he gave me a brain to know when to wear clothes and shoes.
