Refrigeration efficiency improves by eliminating multiple small systems, like reach-in refrigerators and chest type freezers, and replacing them with coolers and freezers large enough for people to enter. This increases the ratio of volume to exterior surface area of the cooled storage spaces, which reduces the heat loss per cubic foot of space. It also allows heat to be removed from the building with remote condensing units. This makes air conditioning of the building outside of the walk-ins more efficient too.
As a refrigerated space gets larger and reaches the scale of a walk-in cooler or an industrial refrigerated space, the insulated box and the refrigeration system that cools it will not be shipped from a factory as a single discreet product. The insulated box will usually be custom-built on site from separate interlocking panels purchased from a manufacturer or sometimes may be constructed from common building materials.
In order to minimize the heat gain from the exterior walls and ceiling of the box, they should be as insulated as much as practically possible, especially in the case of a freezer. Insulation can be thought of as resistance to the flow of heat. The principal measure of insulation is “R-value”, and the larger the R-value of a building material, the more insulation it provides. One of the best insulating materials for a cooler is polyurethane foam, with an R-value of up to 6 per inch and a closed cell structure that does not absorb much water, which also aids in maintaining its insulating value. The 2014 Federal Energy Standards of U.S Department of Energy (DOE) require that refrigerated boxes have higher standards of insulation than has been the case previously. They specify that coolers of under 3000 square feet have walls that are R-25 and freezers have walls of at least R-32. These standards are met by commercially available panels that can be assembled on site to form boxes of various sizes and configurations. Each panel has textured aluminum, galvanized steel, or stainless steel skins sandwiching 4-6” foamed-in-place polyurethane. They are connected with cam-action locking devices to fit tightly together with little air infiltration resulting, and provide an energy-efficient envelope.
Walk -in freezers should have R-28 (or higher) insulated floor panels as part their construction, because the cold from the freezer would otherwise cause the ground underneath the slab to freeze and heave. However, walk-in coolers are typically just placed on concrete slabs without an insulated floor. A 15% improvement in energy efficiency could result from the addition of an insulated floor on top of the slab. Drawbacks to an insulated floor include the extra cost of sturdier floor panels to withstand being walked on as well as the necessity of having a ramp in front of the door.
Although commercial refrigeration panels are very efficient, they are also expensive, so one way to save on the upfront cost is to build the cooler on site with common construction materials. However, it is very difficult to match the R-value, airtightness, cleanability, and durability of commercial panels. Although it may be a cheaper investment initially, a “stick-built” cooler can use much more electricity to cool in the long run, unless it is constructed with generous rigid foam insulation and attention is paid to making it as tight as possible. It is important to never use open-cell foam, fiberglass, cellulose or other loose insulation in such a cooler, as it will inevitably become saturated with condensed water seeping in from outside the box, causing the R-value to plummet. This process is particularly a problem without a good vapor barrier installed entirely around the outside of the box. This may seem opposite to normal building construction methods in which a vapor barrier is located just inside the interior surface, but in fact, the vapor barrier in each case should be on the heated side of the wall or ceiling, not the cold side.
Whether the box is made of commercial panels or wood and insulation, it is usually the best idea to use commercial walk-through and reach-in doors for any openings. Even coolers with stick-built walls and ceiling will always have reach-in doors that have a large glass area for viewing the product, that are easy to open, that automatically close, that seal tightly, and that have electric heaters that prevent condensation from forming on the glass and the frames. The cost to operate the door heaters can be minimized by installing an electronic controller like Freeaire’s Cooler Controller so that electric heat is only used when absolutely necessary. Adequate air conditioning of the space outside the doors, that cools and dries the air in the store, will also help to reduce the need for door heat. Triple-paned glass and proper detailing of the frames and sashes to minimize cold conduction are available for walk-in coolers, for a steep premium, but that require no door heat at all.