[Motor31]

They use fiberglass because it's cheap and easy to use. It requires no special tooling to install. Just unroll some of it, cut it to fit and slap it in the wall. No one will see it again as long as the wall isn't opened up. In a horizontal installation it will settle as vibrations from road travel help shake it down making the roof less efficient as well. With only fiberglass batting in the roof they can get away with a commode room fan venting into the roof rather than to the outside. That air pressure then makes it's way to the vents placed into the roof in other locations since there is nothing to stop the circulation.

In regards to the bit about 3.5" of fiberglass in a 2.5" wall, yup compressing the fiberglass reduces the insulation capacity of the material. The insulation comes from creating "dead air" that does not move and maintains it as a barrier to transfer of heat or cold. Once you compress the fiberglass insulation you make it more like a solid and it will transfer temperatures better losing it's insulation value. Foam creates the dead air by trapping the air inside the spaces of the cells of the foam. There is a reason why ice chests, refrigerators (stick homes and RV style) have foam insulation. It lasts and is far more efficient than fiberglass. DRV knows this they just don't want to tool up for a foam wall style of construction. It cuts into the profit margin.

Want a real treat? On a hot day open one of the storage cabinets at the top of the wall and feel the temperature difference vs one at the bottom of the wall like the pantry next to the outside wall. It will really be noticeable if it's on the sun ward side of the rig. The settling down of insulation (fiberglass) will be really obvious as the insulation is still in place at the bottom but not at the top of the wall. In a foam insulated wall the stuff stays in place in the sandwich since it has structural strength by itself. The lamination process also glues it all together. In the roof it won't compress either maintaining the original R value.

Don't forget the insulation in the floor. It's all based on fiberglass laying on the corrugated bottom liner under the trailer. Lots and lots of open air between the flooring of the trailer and the fiberglass on an unsealed corrugated surface. Plenty of room for air to move around and that circulation is how they keep the plumbing and tanks from freezing. Using your furnace to warm the bottom of the rig under the living area.

Speaking of R value, who did the analysis of the R value for DRV? Was it an outside lab checking a complete wall with windows or a guestimate based on a wall with no openings? Their brochure didn't say on our rig.