I have written before that barn doors provide better seals than rollup doors, and that remains true. But as a consideration in choosing door type, I would not assign much weight to the "better seal" factor.
On a recent reefer run from New York to Washington (state), we ran with another reefer truck. Our truck had a rollup door. The other truck had swing doors. We were hauling the same freight on the same temperature set points, with both reefers set on continuous run. We ran the reefers steady from Friday afternoon to Monday morning.
On Sunday night, out of curiosity, we took our our thermal guns (infared thermometers) and studied the doors. The inside box temps remained the same on both truck through the run. The door exteriors registered the same temps, as did the metal components on both trucks...hinges on his swing doors and the door frame on our rollup door. Temps around the outside edges of the doors and along the bottoms, where the doors met the floors, also read the same on both trucks.
The difference was the rollup door outside metal frame has greater surface area than the metal hinges on the swing doors. So, in theory at least, the rollup door has greater thermal leakage than swing doors.
Another difference is the temperature reading in the extreme upper corners of the rollup door read lower than other areas. That makes sense since the seal at that part of the door is not held tightly shut by the door itself when the door is closed. On some trucks, if the seals are in poor condition, daylight can be seen coming in through those corners if you are standing inside the box with the door closed.
So, while it remains the case that swing doors provide better seals than rollup doors, how important is that really? Both trucks easily maintained temperature for the entire run.
If it is maintaining the thermal integrity of the freight that matters, the rollup door is the ONLY way to go. That is because of the ease with which freight is quickly loaded and unloaded with a rollup door.
With these trucks at the same pickup, the rollup door far outperformed the swing doors. We backed up to the dock with a pre-cooled box, waited until the shipper was ready, opened the rollup door, and wheeled the freight in. I then followed the freight into the box, pulled the rollup door down behind me, secured the freight, rolled the door up just far enough to get out and closed it behind me again after I exited. In all, the door was not open for more than two minutes.
The other truck's driver also entered the facility with a pre-cooled box. He then opened both swing doors before backing into the indoor loading dock. He then waited a few minutes for the shipper to arrive with the freight and wheel it into the box. He then secured the freight and did not leave the loading dock until the paperwork was completed. Finally, he exited the building and closed his doors. In all, the doors were open for 15 to 30 minutes.
Needless to say, the required pre-cooling he did before arriving at the shipper was done in vain, and the fuel used for it was wasted. We loaded after he did but were long gone before he was able to bring his box back down to temp and leave the facility.
On deliveries, the same advantantages apply. If a shipper instructs you to back up to a dock, it does not matter how long you sit there. With a rollup door, freight temperature will be maintained until the last moment before the door is opened and the freight is unloaded.
With swing doors, you might find yourself at a dock with your doors open and an unexpected delay unloading the freight. That is the kind of thing that can lead to bad temperature readings while the freight remains on your truck and a freight damage claim dispute.
Yes, there are ways to protect yourself; such as documenting on the BOL and on Qualcomm the instruction to open your doors, the time it was given, and the name of the person that gave it. But why bother with all that if you do not have to? With a rollup door, you don't have to.
On runs, rollup doors perform as well as swing doors in maintaining inside box temp. At pickup and deliveries, rollup doors protect the freight (and you) best.
Yes, it is true swing doors provide better seals. But how much does that really matter at the practical level?