Tips on building your own walk-in box
Walk in Cooler
First thing you have to do is create a vapor barrier on the outside (warm side) of the insulation. This will keep the insulation material inside the wall dry. I don't mean the vapor barrier of fiberglass batting type. I mean plastic sheathing. Do not use any fiberglass batting type insulation. It will only act as a sponge for moisture.
Most people use the type that has foil an both sides. Or even better . Use pink or blue board urethane . Gray color which is usually made for structural construction which would be good for insulating the floors but is a little more costly. Most people install (2) 1 7/8" between the 2x4 wall studs and 1" over the inside to cover the studs of the walk-in walls. Followed by 1/2" plywood then white fiberglass paneling* or similar. I have seen fiberglass paneling alone, glued onto the insulation. You can't just paint the plywood, it has to be covered. I have learned from a customer that because of the fire hazard of insulation and fiberglass board that the wood will have to be covered with "sheet metal". Contact the proper inspector for advise. Make sure you have support for mounting lights , thermostats and etc. If the box is made in a way where you can not get above the ceiling then you will have to make sure there will be adequate support for mounting the evaporator coil. Get the mounting hole measurements before hand and add support material accordingly. Add extra for error in measuring. Center the evaporator 12-14" from back wall to surface of evaporator blowing at the entrance door is the normal install.
I recommend installing an insulated floor. A lot of people, use an existing concrete floor for the floor but you are not only allowing extra moisture but you are adding a huge heat load by trying to refrigerate the concrete and materials several feet below the floor which heat is coming up from the ground 24 hours a day 7 days a week 365 days a year for many years.
Insulation slows down heat from coming into the walk-in. When you have a floor that is not installed you are not slowing the heat from going into the walk-in.
I do not recommend building your own walk-in freezer. Too many factors are involved like to build two supporting walls and insulate the center between the two walls with at less 4" of high R factor insulation. This way there is a thermo-barrier all around the walk-in. I have seen owner built walk in freezers and there is always ice problems around the outside. If you make your own then purchase a new entrance door which will have condensate heaters already installed.
A concrete floor that is properly insulated should have insulation below the concrete floor. Also you must have an air vent for ventilation under the poured concrete slab. Otherwise moisture or frost will collect if it can not dry out. Also there should be a break in the concrete floor under the walls and door to break the thermo-barrier.
Some thoughts about how refrigeration works