EverGood Farm

How I built a CoolBot Refrigerator Trailer


I'd first read about CoolBot many years ago and when doing my farm planning documents I always included a CoolBot designed refrigeration unit in my costing plans. Then a couple of years ago I saw an article in "Growing for Market" where someone had used a CoolBot to build a refrigerated trailer unit. The thought of joining the costs of a market trailer and refrigeration unit were too good to be true and I decided instantly that this was the way to do it. We're in our first year of farming on our land now and this spring I got my chance to build my CoolBot Refrigerator Trailer.

I am by no means a skilled DIYer. I like to have a go at most jobs around the farm but generally they end up looking like a non professional did them. I found the nuts and bolts of converting the trailer was not above me. It took much more time than I expected, due 90% to insulating the trailer, and don't forget to leave time at the end for materials to cure before you put food into it.

Following is a cost break down and a photo journal of how I built my trailer. After a few wrong turns and a little more work than I expected I ended up with a refrigerated trailer that I think works really well and EASILY can be kept to 40 F or below. At home I just use an extension cord to power the A/C unit. At markets I use a generator. My produce is kept cold wherever it goes!

Cost breakdown

6' X 10' Stealth Trailer with V Front $2,380
CoolBot $299
LG Window Air Conditioning Unit (12,000 btu) $315
Foam Insulation - 2" x 4' x 8' sheets (20) $400
Reflective bubble wrap insulation $100
Other materials (gap fillers, screws, power cord, sealants, plywood...) $350
Total $3,844


I ended up using the following tools:

Placing A/C unit:
Power Drill and Impact Driver

Hand Saw to cut foam and plywood
Circular Saw for some plywood cuts
Tape measure, marker pens
Caulk Gun
Power Drill and Impact Driver

Photo Journal

First of all I wanted to install the A/C unit and CoolBot and get it all running. After the initial apprehension of putting a great big hole in your brand new trailer this is actually by far the easiest and most rewarding part of the whole build process.

I decided to make the trailer as level as possible. Mostly I ended up taking measurements relatively but when it came to trying to find the studs at the insulation stage I sometimes needed to use a level to try and hit the stud and this payed off.

Step 1: Create a hole for the A/C unit

Remove the front panels of plywood and store in a safe place. (I reused all the original plywood in the final layer)
I measured the A/C unit to be 15" high and 21" wide without the top piece where the window would normally butt up against. I made the top of my hole 6" from the top of the trailer estimating 4" for the insulating + a little for the plywood. In hindsight when insulating this got a little too close. I'd leave an 8-10" gap from the top of the trailer next time. I measured the hole as best I could and then drilled holes in the walls big enough for the sawzall. I then cut one side of the wall out, then the other, and then the center strut.

To test the fit, the A/C unit can be taken out of its outside casing. It's easier to use just this outside casing to test the size of your hole and make further adjustments where needed.

Step 2: Frame in the supports for the A/C unit

I had some 2" square metal piping laying around and I decided to use that to support the A/C unit. Originally I was worried about how to support the unit as they're around 80 lbs in weight but this didn't prove to be a problem. By the time I'd added the supports and then insulated around it the unit was very solidly in place. You could easily just use 2x4s to support the A/C unit if you wished and save yourself some time.

I didn't end up framing the whole unit as I originally planned. I ended up just making a ledge basically for the unit to sit on. The outside cover was then screwed onto the ledge and the unit slides into that and is screwed to the casing. It seemed very secure.

Your A/C unit will have specific instructions for how it is supposed to sit and these should be adhered to as closely as possible. Because of the v-front I ended up covering over the top vents so it would not be drawing in the cool air from the trailer and this hasn't appeared to have affected the unit's performance.

As my trailer has a v-front I put a support post vertically down the center for the two horizontal beams to sit on. The horizontal beams were attached to the trailer and vertical post with metal brackets.

Next, a third horizontal beam goes across where the front of the unit is going to sit. Each end needs to be cut at the propper angle and again I used metal brackets to secure it to the previous two horizontal beams. I did not think I required a vertical support of this beam and liked that I had a clear usable space under the unit. Perhaps if you're using 2x4s you may be happier with an additional vertical support.

Finally the cover for the A/C unit is placed and screwed down onto the frame. The A/C unit is placed inside it's cover and screwed in. You can decide whether or not you want to seal around the unit now or wait until the insulation stage.

Step 3: Adding a power cord and testing the A/C and CoolBot works

To supply power to the inside of the trailer I ended up just buying a 2' tri-source plug. This left me with three sockets on the inside and a male connector on the outside I could connect my extensioin cord to.

To fix it to the trailer I cut a hole big enough for the male connector to fit through and placed the three socket end on top of the horizontal beam I made to support the A/C unit. In hindsight I should have left more cord so the sockets were further from the wall. I had to cut into my last layer of insulation to allow for the plugs from the A/C unit and the CoolBot. There's no real reason I can think of not to leave plenty of cord to play with.

I refilled the hole around the plug with a spare piece of the metal from the trailer and some sealant.

Now to test it's all going to work! I temporarly screwed the coolbot to a place under the A/C unit. Then following the instructions with the CoolBot was able to hook it up in less than 5 minutes I would guess. It really is very easy and logical and the A/C unit I chose didn't have any of the possible problems (i.e. no extra or metal sensor).

Plug the CoolBot and A/C unit into your tri-source plug then plug the outside male connector to your extension cord. Adjust the A/C unit to it's lowest temperature (probably around 60 F) and highest fan speed. Adjust the CoolBot to around 40 F if it's not already set there. I was amazed at the frigidity and amount of cool air coming out of the unit. Even with the doors slightly open the air temperature dropped to in the 40s within 5 minutes.

Ok, this really is going to work!

Step 4: Insulating

This is no doubt the most important stage and it definitely takes some time. I spent a lot of time insulating around the A/C unit and the front "V" of the trailer. In hindsight I'd consider a flat front trailer next time in hopes of saving time on the build. Other difficult areas are the tail lights and around the rear doors. In the end though it's a case of jamming enough stuff in there, using the gap filler and calling it good enough. How ever tight you get it there's always going to be weak spots around the rear doors and the A/C unit itself is not built air tight.

First of all I padlocked the side door closed. I'm going to insulate over it so it won't be being used again.

Secondly I took off all the existing plywood. I saved this and used it at the end to help recover the inside of the trailer. My original intention was to put the insulating foam right against the outside wall and cut around the metal joists giving me an extra 1 1/2" usable room each side. I started by insulating around the tail lights and above the door. By the time I'd finished that I looked at the rest of the trailer and couldn't imagine how long it would take to do something similar to the whole trailer so I went to plan B.

Plan B was to attach reflective bubble wrap around the inside of the trailer first and then attach the rigid foam insulation over that. Per square foot the reflective bubble wrap was cheap and depending on how it's installed could add quite nicely to the R-Value (I couldn't work out exactly what R-Value I was getting with how I installed it). Then I'd add the rigid foam. I could only find 2" foam with an R-10 value. So 4 inches would add another R-20. Hopefully I'd be getting somewhere near R-30 but atleast R-25 I'd guess in most places.

To attach the reflective bubble wrap I found that professional grade duct tape worked well. Eventually it would be sealed behind 4" of foam board so this should keep it in place fine. Try to leave a gap where ever possible between the bubblewrap and the side of the trailer. The bubblewrap is designed to work with an air gap and that's how it gets its higher R-Values. I overlapped it slightly and taped along the edges to try and get a nice seal. I used two 25' x 4' rolls and two 25' x 2' rolls. The 6' of width between the 2 rolls covered the areas nicely without having to do any long cuts of the bubble wrap.  Eventually I covered the entire trailer except the rear doors. (I'm not sure now why I left the rear doors uncovered, perhaps I forgot when it came around to insulating the doors).

Make sure you keep track of where the studs are with a marker so you can find them again when attaching the rigid foam and plywood.

Once I bubble wrapped the ceiling and walls I put the first layer of rigid foam insulation on the ceiling. I used 3" self tapping screws to hold up the foam for the first layer. I needed to drill a pilot hole into the metal stud first for the screw to be driven into.

To reduce the amount of gaps I insulated the areas in this order.
  • Layer 1: ceiling, floor, walls
  • Layer 2: ceiling, floor, walls
  • Rear doors both layers
Once the ceiling had it's first layer of foam I bubble wrapped and put rigid foam on the floor. I kept this in place also with a few screws into the flooring.

Next, the walls. These I managed to cut fairly tight and was able to jam in between the ceiling and the floor so I didn't need to use any screws in this layer. The tricky part here is making the cuts for the V-Front and around the airconditioning unit. Once done fix up the gaps with sealant and gap filler and leave for a few hours or over night as you'll probably have to do some trimming of the gap filler. It's really important with each layer to try and get it as airtight as possible so don't be shy filling those gaps.

Then you're on to the next layer which I did in the same order as the first layer. Remember to overlap the foam panels so the gaps do not line up. Things should go much the same as your first layer. Hopefully you've kept track of where your studs are. For this layer I used 5" self tapping screws and again had to predrill into the studs. I used some glue on this layer but I didn't find it that useful as it took too long to set.

After you're all done again fill the gaps with sealant and gap filler and leave for a few hours or over night.

Next step is the doors. When I closed the doors I noticed a 1" gap between the door and the floor insulation. When making my first layer for the door I cut a 1" x 4" high gap for the bottom of the doors. When I closed the door I found it took a little effort for the door to close. Perfect. The second layer for the doors can just be added over the existing door from the floor to the ceiling.

To allow the door to close some of the center area of the door where the doors meet must be cut away. Be careful not to cut too much so that you allow for enough meeting of the insulation on each door to give some insulation when the doors are fully closed.

Step 5: Adding the layer of plywood and water sealant

You'll need to buy some extra plywood as you'll be covering the ceiling, floor and doors where you previously haven't removed any plywood.

I started with the floor using a decent thickness of wood. This was easily laid down and secured using the 5"screws.

Then it was on to the walls using the old materials I'd salvaged. Of course the measurements are all different now so there was some cutting but all in all the sides went back together well. I used the 5" screws and tried to find the studs as best I could. By now I was missing quite regularly but found enough to keep the whole thing secure. I used the insulation glue also to help keep the plywood flatter against the inuslation foam. I then went around and filled the old holes with wood dough and pumped some more sealant into any new holes I'd created when I missed the studs.

For the ceiling I used a 1/8" ply (same as the sides) and again used 5" screws and attempted to hit the studs. Definitely a 2 or 3 person job. I was also quite liberal with the glue here in case I really had a hard time finding the studs.

Lastly was the doors. These were the easiest part and I nice end to the construction part of the project.

Again I went around with gap filler and filled all the gaps. This time it's more for moisture than for insulation.

After you're happy with that and its dried it's time to seal the plywood against moisture. I searched around for a long time about sealants you can and can't use next to food. After finding it hard to get a straight answer from my local stores I ended up on a woodworking sight where the question was answered. Basically after they have cured shellacs, lacquers, varnishes etc are safe around food. After all people coat bowls and chopping boards etc with these sealants so the inside of the trailer (which is never going to come in direct contact with food) should be fine. If you wanted to be really safe you could go with a type of oil. These however tend to need regular reapplication which may not be possible if used all the time.

I applied 3 layers over 2 days. Depending on the product you use it may take 1-2 weeks for the product to cure properly.

That's it!

September update

Our first summer into using the cooler and the cooler has been working great. For June we had the cooler on only a few days a week when we had produce in there. We left the doors open when it was not in use which was great for drying it out.

For July and August the cooler has been running continuously. The cooler seems to easily get down and keep down to 40 F. I ended up just parking the trailer in the shade of a tree to help with the electric bill. It stays fairly damp inside the trailer but I have not noticed any degredation in the plywood or insulation. When we're all done this year I'll let it dry out and then apply more coats of varnish to the plywood.

We feel the trailer is worth its weight in gold:
  1. Our produce is always kept cold so we can store it for longer and still sell it.
  2. We can take more to market and not worry if we don't sell it. Then if we have a really busy market we can take advantage of having more there.
  3. As our produce is stored better (at home and at market) the customer gets a better, fresher product which helps distinguish us from other vendors. Customers have definitely noticed how long our produce lasts in their fridge.
  4. Consequently, we have thrown away very little harvested produce this year.
  5. It seems to be very economical to run, between $1-$2 per day.
  6. (It's also great for parties - storing food and beer)
Also, we have not needed to buy a generator for market use. In the end we were able to hook up to an outlet at all our markets.

We have only had two issues.
  1. The sensor fell out of the A/C unit while driving and the unit froze up when we plugged it in again. After allowing to thaw out everything worked fine. Now I always check that the sensor is in place before restarting the unit.
  2. The A/C unit stopped working entirely at one point. The fuse on the plug would trip instantly. After much consternation we worked out it was moisture in the actual plug. The A/C unit is obviously not designed to be in such a humid environment on the inside. I took the unit out of the trailer and put a fan on it. Once it had dried out and the unit was working again I taped all seams and points moisture could enter the plug with electrical tape and I have had no further problems.

4 years on

Now in its 4th year of use and the trailer is still going strong. For two years we used it solely as our on farm cooler and market trailer so between June and October it was almost in constant use. Last year we added another coolbot refrigerator unit to the farm and are now able to give the coolers a break occasionally.

Other than the issues mentioned above there have been no other problems. I still use the same coolbot and air conditioning unit that I originally bought.

Next time I make another refrigerated trailer I would choose a trailer with a flat front end (The angled front end does not provide any more area really and made cuts difficult), wouldn't put paneling on the ceiling and would choose a durable paint rather than coating with a poly.

Good Luck!

7 years on - Interior Fix

Well, my CoolBot Refridgerator Trailer is in it's 7th year and the only regret I have is using Plywood to line the inside so I've torn all that out and replaced it with a waterproof washable plastic wall.

On pulling off the plywood there was a lot of rot and grime behind the walls and also once the water got under the polyurethane it tended to stay wet and rot even worse. I would not recommend using plywood as the liner to the trailer due to too much moisture.

 The foam insulation is in perfect shape however and I am still using my original air conditioner and coolbot unit. I have no issues with keeping the trailer cool in summer still.

This interior upgrade cost me around $300 for materials consisting of:

9 x PolyWall Panels @$20 each
6 x Packets of Drive Rivets
12 x PL 550 (This product didn't work too well so look for another adhesive)
18 x 8' lengths of joins and edges to cover seams etc

You'll also need a utility knife and straight edge to cut the PolyWall, a drill with 1/4" bit and a hammer, measuring tape etc.

And that's it! There's also a more rigid PolyWall product with fibre glass in it.

I managed to put the new walls and ceiling on by myself with the only tricky part being the ceiling which I did by cutting the sheets into 4' lengths due to the PolyWall wanting to fall on my head. Otherwise the PL550 acted fast to stick the PolyWall to the foam insulation and I put a few rivets in to make sure the piece held. I left the walls to dry overnight then came back and did some caulking etc. This is probably a 2 day job, less with help.

I just finished this a day or so ago (04/03/2017) so I will post if any problems arose otherwise assume all was ok.

Good Luck!

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Rhinelander, WI 54501, USA
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Copyright 2011 evergoodfarm.com. All rights reserved.