How To Build A DIY Coolbot Trailer

Having a large enough space to keep your veggies adequately cool until delivery is incredibly important. With all of the painstaking labor that has gone into planting, protecting, harvesting, washing, and packing your beautiful produce, the last thing you want is for it to spoil due to an insufficient cooling space!

There are many options to choose from when you are considering how you want to keep your product cooled and preserved on your farm. You can go with a standard refrigerator, or a commercial refrigerator, or take it up a notch with a walk-in cooler. Better yet, you can create your own DIY walk-in cooler with the revolutionary Coolbot! As with many other considerations when starting a farm, it always comes down to your specific context, and the size of your enterprise.

When starting out, many growers will pick up an inexpensive second-hand refrigerator or two. This will only last a very short time, as the capacity is so small for storing any kind of volume. There is also the danger of overpacking a refrigerator which interrupts the air circulation and can lead to freezing your greens (yes, I learned that the hard way when I was first started out–what a gut punch!).

If you can find one for a reasonable price, a two-door reach-in cooler can be another option for a small-scale enterprise, or when you are just starting out. These are the commercial refrigerators you will often see in a deli or convenience store with two glass doors that either slide or open outwards, and adjustable shelves inside. 

The next step up would be a commercial walk-in cooler. As these are extremely expensive, most growers will look for a restaurant going out of business, or find a used unit on eBay. However, even the smaller walk-in coolers can still be upwards of $10,000 when purchased second-hand. The other issue with traditional walk-in coolers is that they have complex refrigeration, compressor, and evaporator systems that require specialized (and expensive) service when they break down.

The company “Store It Cold” came up with an answer to this problem with the invention of the CoolBot. The CoolBot is a small but brilliant device that wires into a standard window air-conditioning unit and tricks it into maintaining a space at refrigerator temperatures. Out-of-the-box window AC units will only cool down to 60° F. With the addition of the CoolBot, the same unit can safely and reliably cool a space down to 34° F. 

This allows small-scale farmers and gardeners to create their own walk-in coolers at a fraction of the cost and eliminates the need for costly repairs on complicated compressors. CoolBot AC units can be installed into an old walk-in cooler housing (effectively replacing the old refrigeration unit). Many farmers will choose this option as its far more economical than fixing a broken compressor. 

You can also create your own CoolBot walk-in cooler by insulating a closet, utility room, storage shed shipping container or utility trailer. The Store It Cold website is filled with resources on how to sufficiently insulate your space with either rigid insulation board, or closed-cell spray foam insulation. 

At my previous farm in Petaluma, California, I was leasing the land. Not wanting to invest in a permanent structure on someone else’s land, I decided to convert a utility trailer into a CoolBot walk-in cooler, allowing me to simply tow it away with me when we moved. I had previously helped convert a small 4’x 6’ enclosed trailer into a cooler at an urban farm I managed in Tennessee, and really appreciated the versatility that it provided. Not only did it serve as the on-site cold storage space for the farm during the week, but we could also hook it up to the van and tow it to the farmers market on the weekend to keep our produce cool on blazing hot Tennessee summer days (powered with a small Honda generator).

The added bonus to this system was that it eliminated the step of transferring totes of produce from a walk-in cooler on the farm to a delivery vehicle for transport. Once the product was washed and packed, it wasn’t moved again until it arrived at the place it would be sold. We also had our CSA pickup spot designated at the farmers market, so it was one stop shop!

The one thing I didn’t like about our Coolbot trailer in Tennessee was its size. Being just 6’ deep, 4’ wide, and 4’ tall, we were constantly running out of space, and always hunched over getting in and out of it. 

When I was starting Winding Road Farms, it was a clean slate–an opportunity to reflect on all of my previous experiences of farming and reimagine more efficient ways of doing things. As such, I decided to go with a 6’ x 12’ enclosed utility trailer. It was an “Interstate Victory Cargo Trailer” that I found on Craigslist in brand new condition, and it had always been stored in a warehouse. I learned they sold for $4,000 new, but I was able to get it for $2000. There would be no more bent-over backs in this trailer! I could comfortably stand up and walk inside it! I intentionally bought something a little bigger than I needed, to account for growth in the future.

The smaller trailer we converted in TN was done with rigid insulation board. While it would be a little more expensive, I wanted to try my hand at the spray foam insulation with this new build-out. From what I had read, it was far more efficient, especially when it came to filling in cracks. When using RMAX insulation board, it requires a double layer of 2” insulation board to reach the minimum recommendation of an R-25 insulation rating (with each sheet of 2” insulation board being rated at R-13). It also requires a lot of measuring, cutting, gluing, and taping of seams when using insulation board. I imagined the spray foam would be a lot faster to apply, in comparison to doing the job with all insulation board. With spray foam, I could also spray the undercarriage of the trailer, as opposed to layering in 4” of insulation board on the floor and losing 4” of precious head space!

With all of this rationale in mind, I decided to go for it with the spray foam! I took the recommendation of the Store It Cold website and went with a company called Energy Efficient Solutions to purchase my closed-cell polyurethane spray foam kits. Store It Cold also offers a discount code for Energy Efficient Solutions, which was really helpful. In keeping with my OCD research tendencies when tackling a new project, I read every piece of literature and watched every video I could find on how to correctly apply this product.

Spray foam sets up in minutes, so you only have one shot with no margin for error. The kits come with an A component and a B component chemical in two separate tanks. You have to ensure both chemicals are being mixed at the correct ratio while applying, and it has to be applied within a certain temperature range and at a certain thickness. Having never worked with the material before, I realized this method was going to be a stressful undertaking. This material wasn’t cheap either, so a mistake could be very costly!

The first step was to determine how much foam I would need to do the job. Spray foam coverage is measured in what they call “board feet.” One board foot equals 12” x 12” x 1” thick. In an effort to not purchase way more foam than I needed (and to save on some cost), I decided to do the inside of my doors in rigid insulation board, and the ceiling, interior walls, and undercarriage in spray foam. According to my measurements, I needed a total of 290 square feet of spray foam. With 209 bed feet at 1” thick, I would need four times this to hit the target insulation rating at 4” thick: 1160 square feet. Energy Efficient Solutions sells a 600 bed-feet kit. I bought two kits which would give me 1200 square feet of coverage.

I’ll admit it was hard to start tearing apart such a beautiful trailer, but with the finished product in mind, and the pressure of needing more space for my produce, I was motivated to take action fast! 

The first step was removing all of the interior wood paneling. After that, I measured the hole I would need to cut in the front end of the trailer to accommodate the 12,000 BTU LG air conditioner I had purchased. There were lateral steel studs on the front of the trailer that were not very far off from where the edges of the AC unit would be placed. I decided to frame out a box that would support the AC unit, and also be bolted to the frame. I wanted it to be rock solid, and to avoid having to install support arms under the unit on the front exterior.

Next, I cut my hole for the exterior plug which would power the trailer. I then wired that into a four-way outlet. I needed one plug-in for the AC unit, and one for the CoolBot, and then I figured two auxiliary plugs would be nice to have if needed to power other things. In the winter it’s nice to have a small space heater to prevent the space from dropping below freezing. Make sure to account for that when designing your own walk-in cooler. After all of my electrical was wired up, and the AC unit was installed, I then mounted the interior outlet, and the CoolBot on 4” blocks of wood to account for the coming 4” of spray foam. 

I then drilled a hole for the wire that would power the WiFi jumper for the CoolBot Pro. The CoolBot Pro comes with Wifi capabilities and allows you to monitor the temperature from your phone. In the event of a power outage or some kind of system failure, this could be a game changer for saving a trailer full of thousands of dollars in produce before it’s too late. For both the exterior plug and the Wifi jumper, I installed weatherproof exterior outlet boxes to keep them safe from the elements. 

I then installed the two layers of RMAX rigid insulation board onto the interior of my doors, wrapping the edges in HVAC foil duct tape. Side note: I stumbled upon a cool little hack for effectively cutting 2” insulation board. After attempting to cut the boards with my harvest knife (and failing), I knew there must be a better way. Using my Work Sharp knife sharpener, I sharpened one entire side of a 2” putty knife to be razor sharp. This worked wonderfully!

After the doors, the next task was the floor. Since I’d be spraying the undercarriage of the trailer with foam, rather than layering insulation board on top of the floor, I could simply glue in a piece of vinyl flooring. I went with a high-traffic moisture-resistant vinyl floor.

As mentioned earlier, applying the spray foam can be stressful if it’s your very first time working with the product. You’ve only got one shot at this, and once the chemicals are being mixed you have to keep moving. Here are a few tips to keep at the forefront of your mind:

• When you first get your tanks of chemicals, make sure to keep them stored above 50°F and below 100°F to make sure they remain viable until you are ready to do the job.
• Your tanks must be “conditioned” between 70–85°F for a minimum of 1-2 days prior
  to spraying.
• Shake each chemical for at least two minutes before use.
• The wall cavity temperature should be above 40°F to ensure the foam bonds properly.
• An infrared thermometer gun is really helpful to have on hand for checking the temperature of your tanks, and your walls.
• If taking a break longer than 30 seconds, replace the tip on the spray gun.

Rather than doing one giant 4” coat of foam, I decided to do two passes, with a target of a 2” depth per pass. Getting even coverage definitely takes some finesse. You will inevitably have areas that are more shallow, and then areas that end up deeper (typically corners). For the most part, I was able to get an even 4” of depth. 

Equipped with my hazmat suit, a respirator, goggles, and latex gloves I managed to execute the project. I will admit that doing the undercarriage while lying on my back was pretty miserable, but I got it done. I had the trailer on risers to give me more clearance for that part of the task. 

The CoolBot trailer project was probably the one I was most proud of at the end of it. It turned out beautifully. If I had to do it again, I would seriously consider outsourcing the spray foam portion of the job to professionals (if financially feasible). It was doable but extremely stressful. Having ample space to keep all of my products safe and cool until delivery made all of the difference in the world, and I loved the ability to monitor the temperature on my phone, even when I was away from the farm.

For some video walk-throughs of the finished product, CLICK HERE!

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