The spray foam is a REALLY big problem if you every need body work. The foam sticks to the aluminum and will need to be disolved out of the wall with nasty chemicals before a segment or panel can be removed. This removal can add enough time and cost that an insurance company might total the trailer rather than repair it.
We had a 1969 Safari which someone had replaced the original under-floor fiberglass with spray foam. The chassis was badly corroded, in part from the water which worked its way between the foam and the steel and stayed there. In order to do any welding repairs we would have had to disolve large areas of foam with chemical solvents. It was decided to build an entirely new chassis to replace it.
Fiberglass in the belly is an exellent nesting and food storage place for critters. See our
Common Defects page for some ugly photos. Fiberglass in the belly also collects gallons of water in wet or snowy driving conditions which, even in a dry climate. will take months to dry out in the relatively closed environment of the belly pan.
The mylar bubble wrap is fine for reflecting infrared wave heat but does very, very little for the prevention of thermal heat transfer.
In our
rebuilds and
new buildouts we use Polyisocyanurate closed cell, aluminum faced, board insulation of 1.5" thickness and tape the seams with an aluminum foil tape. Critters and insects will not burrow or nest in this material and it will not collect or hold water. If repairs are needed it is easy to remove. It can be scored on the backside and bent to fit in the compound curves of the upper body. In these locations we use the foil tape to make a complete seal to act as a vapor barrier which prevents the accumulation of condensation on the inside of the walls during cold weather use. In the upper bodies we also offer formaldehyde free fiberglass but it does not form a vapor barrier and will collect a great deal of water from condensation. Most people who can afford the polyiso foam board opt to install it.
About condensation during cold weather use: The water vapor travels throug the seams in the inner liner and condenses on the inside of the outer skin. It then soaks the fiberglass insulation which reduces the effectiveness of the insulation. It also runs down the inside of the skin to the sill and the plywood deck causing wood rot. Where does this much water come from? Cooking, burning propane in the cooktop and oven, local humidity, water vapor from showering, water vapor from washing dishes, and, the biggest contributor, people breathing. The average human body gives of 2 liters of water durning an 8 hour sleep period. We have had fiberglass insulated trailers come into our shop during winter use which have had to be dried out by bring them inside, raising the building temp, opening all of the trailer windows, vents, hatches, and doors and running large electric heaters 24 hours a day for two to three days.
We have the benefit of reverse engineering many trailers of all vintages each year and seeing the benefits and problems with many different methods of insulation methods, chassis construction, window design, maintenance, uses and operation which have been used over the years. With this experience we work with our vendors to come up with better methods and material solutions and attempt to solve the problems in the present and future.