This Is What That Little Door On The Back Of Truck Trailers Is For (2023)

I think we’ve pretty well established that our readers spend a lot of time driving behind trucks, staring at the backs of trucks, and this meditative process of contemplating truckly posteriors naturally leads them to ask questions. Like why the quilted metal doors? Or, as I want to explore today, what’s the deal with that little, inset door on the back of some trailers, most often refrigerated trailers? Is it a pet door?

If it is a pet door, are there truck cats?

Right up front I’m going to just tell you that, no, it’s not a pet door, and while there may be Trucker Cats, this little door is not for them. It was actually a bit tricky to confirm exactly what this door is for, because there seems to be two competing schools of thought about the door if you just try to Google it.

The first theory is that the door is used for inspecting the contents of the trailer, as explained by a number of internet people who claim to be truckers. Here’s one that explains how they use the door:

I’ve worked with refrigerated trailers for years and have never used it for any of the cited reasons. The main purpose as I see it is to check the status of the load without having to open the full door and losing temperature... I think the biggest reason is for a highway patrol or shipper/consignee to see if a trailer is loaded without losing temp in the trailer.

Huh. Okay. Another guy had a special name for it:

On refrigerated trucks and trailers, the small rectangular door is called a fruit door. It’s mainly used for temperature control and monitoring. It also can be used to help air out the trailer.

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Fruit door? That seems to be the only reference to it being called that, so I’m a bit skeptical.

Other sources also support this idea:

To view inside the trailer without opening the full access doors. used when container has a shipping tamper proof seal (manufacturers can tell if the large doors were opened when they should not have been).

and

This little door allows the driver to see how the pallets are stacked and the overall contents of the trailer without having to unseal the truck. It also prevents too much of the cool air to escape out of such a big door opening.

While this sounds plausible, especially the part about not wanting to open the full doors of the refrigerated trailer, which would waste energy and cause the trucker’s dad to pop up out of nowhere and yell “what are you trying to do, cool the whole neighborhood?” there’s still another explanation that needs investigating.

The other explanation is that the little door is used for ventilation purposes:

That is a vent door. There is usually one on the top front of the trailer as well. This is a reefer trailer. It is used for hauling refrigerated items and the interior has to be free of odor when loading at shipper. Usually, after hauling a meat load, the trailer will be washed at a trailer wash, then the vent doors are left open to allow quicker air drying and prevent mold or mildew.

This also seems plausible—you know how weird your fridge can end up smelling if you leave it off for a while, right? A refrigerated trailer is pretty much the same thing as a giant, wheeled fridge.

Other truck drivers supported this explanation, too:

These vent doors are sometimes used to allow air to circulate to dry out warming refrigerated trailers but can also be used on dry van trailers to provide air flow to non refrigerated produce (potatoes, onions, etc.) and plant (garden plants, trees, etc.) loads to prevent death and early spoilage caused by hot stagnant air.

There is one located at the top driver side at the head of the trailer as well as the rear passenger bottom as pictured here. The locations provide cross trailer airflow.

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There were many examples of both explanations online, so I realized it was time to stop seeing what randos on the internet thought, and to reach out to a real source to tell me what’s up with The Little Door: is it an inspection hole, or a vent? Or both?

I called a number of trucking companies and manufacturers of refrigerated trailers to find out. This actually proved a little trickier than I would have expected, since some companies were oddly unwilling to talk.

In fact, when I spoke with one company, a major manufacturer of refrigerated trailers, the person on the phone actually said this to me:

“You’re a journalist? I need you to tell me THE TRUTH about why you’re asking these questions.”

The person’s tone was quite hostile, like she was expecting me to say “we heard they were called devil doors and are used to poke children packed into the trailers for transport to Satanic rituals with harpoons” or something like that.

Eventually, though, I did get through to some experts, and I got my answer:

The door is called a Vent Door. It’s used for—you guessed it, ventilation.

I spoke with American Trailers of the Carolinas, manufacturers of many sorts of refrigerated trailers, and got a very good explanation.

The door at the back is paired with another one at the upper part of the front of the trailer, and when open, they provide flow-through ventilation. If you see a trailer driving with the little door open, I was told that most of the time that means the trailer is empty, the cooling unit is off, and it is being aired out for its next load.

I also asked them if the door was ever used for inspection, and was told it wasn’t, and then the American Trailers of the Carolinas employee used a pretty vivid example of why it’s useless for inspection:

“If you opened that door, what would you see? If the trailer is loaded, just a cardboard box. Maybe you could see where it said “DEL MONTE” or something. But you couldn’t see anything else. The whole front half of the trailer could be full of cocaine for all you could tell.”

Damn, he’s right. It really wouldn’t be useful for inspection.

Next, I spoke with a person in the technical department of the same large manufacturer that was suspicious of me, as I mentioned earlier. To their credit, they decided I wasn’t writing a hit piece, and they had this employee contact me, on condition of anonymity, for some reason.

Whatever. The guy knew his stuff.

He confirmed it’s a Vent Door, used for ventilation, and also mentioned that they’re likely to be used for what’s known as “backhaul” loads, where the truck is filled with cargo to take back to its point of origin, so it’s not just being driven empty, which is wasteful.

On a refrigerated truck, that backhaul load may not require refrigeration, so the cooling unit would be turned off. And, again, like your fridge when you turn it off, mold and mildew can grow and smell terrible, so to combat this, the vent doors are opened, and the trailer is aired out while hauling the non-refrigerated load.

I also asked our Secret Trucking Technician about the possible use of the door for inspection, and he also shot this down, adding that even if you wanted to, you really couldn’t since these doors have a stainless steel screen behind them.

Just to be safe, I spoke with another trucking firm, and they corroborated the story, leading me to confidently report to you that the little door is called a Vent Door, and it’s used for ventilation and airing out the trailer.

If anyone claims to be a trucker and insist that it’s used for inspection or, worse, calls it a “fruit door,” you have my authorization to call them a moron who doesn’t know Jacqueline Squat about trucking.

Another mystery solved!