Walk-in coolers and freezers generally are constructed of modular panels made of an insulating material and metal or fiberglass protective skins. The purpose of the skin is to protect the insulation, which is fragile and cannot be used independently. The insulation usually is a plastic foam material that is either injected or placed between the metal skins. The insulation material allows the walk-in to resist heat flow and hold cold temperatures.
Temperature. Insulation performance usually is rated by its R-value -- the higher the R-value, the more resistance it has to heat flow and the better it insulates. The operating temperature of the insulation can affect its performance. Some types perform better at lower mean temperatures while others perform better at higher temperatures. For example, when comparing polyurethane and extruded polystyrene at -10 to 50 oF (-23 to 10oC) mean temperatures, the R-value of polyurethane drops at lower mean temperatures, while the R-value of extruded polystyrene increases (figure 1).
Moisture. The largest factor that can affect the insulation's performance is moisture. Usually, insulation is expected to keep something warmer or colder on one side than on the other. This temperature differential (TD) often causes a dewpoint to form inside the insulation. Once the dewpoint is reached, moisture is trapped and reduces the R-value of the insulation. The more water resistance the insulation has, the better the insulation performs in high TD situations.
Because water lowers the R-value of insulation, extruded polystyrene retains its R-value better than polyurethane.
Another study concluded that polyurethane loses more than 75 percent of its R-value in five years; extruded polystyrene loses only 25 percent of its R-value in the same time period (figure 3). This data indicates that the performance of extruded polystyrene in actual walk-in conditions (when moisture is considered) is more than three times that of polyurethane.
More About R-ValueWhat about the R-values of 32 to 34 associated with some insulation? These are “fresh” R-values -- the R-value of the insulation when it is first removed from the mold, typically at a 75oF (24oC) mean temperature. To find the actual operating R-value of any insulation, you have to look at how it performs in real-life applications. For a walk-in freezer that operates at -10oF inside and 75oF outside, a 32.5oF mean temperature should be used. Additionally, because moisture and aging mostly occur within the first few years of the system's operation, a 60-month decreasing R-value should be considered.
In a five-year comparison of polyurethane and extruded polystyrene, the R-value of the 4" polyurethane dropped to 6, while the R-value of the 4" extruded polystyrene dropped to 15. Based on these results, extruded polystyrene insulation can provide energy savings over the life of the walk-in. For example, in a 10' x 12' x 7'6" freezer operating at a 32.5oF mean temperature with a refrigeration system with 90 percent efficiency and an electric cost of $0.09/kWh, extruded polystyrene insulation can save up to $5,139 in energy costs over polyurethane in the first five years (figure 4).