As a Central Texas Prepper, I have solved my food storage problem affordably, as follows: On my property there was an existing 20 foot by 24 foot sheetrock walled tool shed. I gutted this building and installed slabs of 8 inch styrofoam panels against interior walls.
These blocks of foam were salvaged from floating docks on a local lake as most people were installing plastic floats under their docks. The styrofoam blocks were free for the taking..As the floats were used and had been in the water in some cases for years, they looked gross and smelled bad also. I found if you cut as little as 4 inches off the side of the float, you now have new looking and smelling styrofoam blocks. The foam blocks come in [usually] 4 foot by 12 foot dimensions and need to be sized for re-use. This was accomplished with a 20 inch chain saw, with a tube sawing guide extending past the chain bar,and cut around the perimeter. The entire block will not be cut thru at this point but if you pop rivet two regular carpenters hand saws together to make a 5 foot blade, the remaining styrofoam cuts easily. You now have a 4 foot by 12 foot by 8 inch slab of pure insulation. Cut and tightly fit these slabs against your interior walls. Use foam sealer to seal the joints and you have an air tight interior. Inside the interior foam slabs, I built a 2×4 framed wall and insulated it with fiberglass insulation. These walls were then sheetrocked and taped. The ceiling received the same treatment with cutoff chunks of Styrofoam placed on top of the slabs in the attic. The thicker the better. A sheetrock ceiling was put up after all seams were sealed with foam. A solid core door with a foam rubber gasket was installed to keep things airtight.
Next, a high efficiency 10,000 BTU window air conditioner with a power saver feature was installed to cool the interior. The whole thing works better than expected, keeping the interior of the storage building at 60 degrees or below, no matter the outside temperature. The window unit is shut off in the winter with the interior temperature staying around 55 degrees. The electricity consumed by the window unit is negligible.The exterior of the building was left worn and weathered looking even our closest friends have no idea about the contents of the tool shed. Some work required but this resulted in a cheap and effective storage facility. – Don in Texas