Second Task: Walk-in Refrigerator
As a farmer, the harvest is the most exciting time of the year. All of your hard work finally pays off in the form of abundant fruits and vegetables as far as the eye can see. Your CSA members are giddy with anticipation to see what their shares will contain, and the farmer's market is fast approaching. However, if you want to be able to deliver the freshest produce you can, as well as being able to save some for yourself, then a walk-in refrigerator is a necessity.
Homemade walk-in refrigerators are not a new idea, and plans for them are abundant on the web. An awesome device called the "CoolBot" was created about 7 years ago by a farmer from New Paltz, NY, Ron Kholsa, that allows for a walk-in refrigerator to be built with a normal window AC unit as the cooling system. The CoolBot is a small robot that essentially runs the AC unit in overdrive, so that the cooling space is able to drop to temperatures much lower that the AC unit would normally allow, around 30-40 degrees F.
With these developments in mind, I set out researching plans and designs for homemade walk-in refrigerators on the web. We decided that a 6'x8' floor plan would be about the right size, with a ceiling height of 7'. Here is a very simple sketchup rendering of what we wanted the final product to look like from the outside. It is basically just a large box with a lot of insulation, and a window size hole that fits the AC unit. It is built on two 8' long pressure treated 2x6's, with a future plan of attaching wheels to the bottom in order to make it mobile. There is also metal roofing on top so that having it outside is also an option.
The first step in building the refrigerator was to construct the floor. The 8' pressure treated 2x6s were placed on the sides, with four 6' 2x4s used as cross joists spaced out evenly between the 2x6s. Note: remember that 2x4s and 2x6s are not actually 2"x4" and 2"x6" when doing preliminary calculations, and are actually 1.5"x3.5" and 1.5"x5.5" respectively. In retrospect, the next step would have been to add the insulation between the cross joists, then add the plywood flooring on top of this to create a level space to build the walls up from. However, we did not have the insulation at this point in the project, so the insulation was left until the skeleton of the body was completed.
Next, the walls were built up from the floor, with 7' 2x4s. It was difficult to connect the wall joists so that that they were level by myself, so I used corner braces to hold them in place while I made sure that they were straight up and down. The ceiling was constructed separately and then connected after the walls were built.
The next step was to cover the outside of the skeleton. We chose to use a relatively inexpensive half-inch plywood siding. This ended up working great because it comes in large 4'x8' sheets, and also allows for the refrigerator to be placed outside without worry, and without having to stain the wood. Regular half-inch plywood was added to the roof and then covered with metal roofing. The metal roofing was also relatively inexpensive and was attached to the plywood using gasket screws to make sure that there was no chance for leakage.
Next, 4 inches of polyurethane rigid foam board was added between the floor joists. Since we are trying to keep the space inside as cool as possible, and we know that hot air rises and cool air drops, it is important to have the most insulation in the floor of the refrigerator. The insulation was cut to size, but cracks and gaps are impossible to avoid, so spray foam was added to fill the gaps in the insulation before it was covered with a piece of half-inch plywood flooring.
I then cut the rest of the insulation to fit for the walls and ceiling. Instead of putting the insulation between the wall joists, they were fitted to go on the inside of them so that all of the wood was covered, and also so that there was a 4" space between the siding and the insulation. This is so that it would be easier to create an airtight seal, and also because the 4" gap will provide added insulation due to the fact that air is a pretty decent insulator. The rigid foam board insulation was connected to the 2x4s using screws and large fender washers.
We used a pre-hung insulated, steel door for it's sealing ability. We got the standard size of 31"x82", and framed the door accordingly. As you can see from the picture, there are a few slivers of light that can be seen through the closed door. This means that cold air will easily escape from the cooler, decreasing its efficiency. To combat this problem, I attached a piece of foam board to the inside of the door that was slightly larger than the door itself, and eliminated the small cracks in the door frame.
Next, I added a large amount of spray foam to every crack and hole to ensure that the refrigerator was tightly sealed for maximum efficiency. The AC unit was then added to the platform, and installed and sealed with insulation and spray foam.