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What insulation to pick for your Walk-in Cooler or Freezer
Polyurethane (Urethane) vs. Extruded Polystyrene (XPS)
Walk-in Cooler and Freezer insulation
What’s Important? When buying a walk-in cooler or freezer, the initial cost of the walk-in is almost always the deciding factor. The cost to operate the walk-in is rarely considered. You may think all walk-ins perform about the same. This mistake can cost you dearly. You pay for the walk-in once, but if the insulation is inefficient, you will pay for that every month for the life of the walk-in. In the following, you will find an evaluation of two types of insulation that are being used in walk-in freezers foamed-in-place polyurethane & extruded polystyrene.
Walk-in coolers and freezers are generally constructed of modular panels made of insulating material and protective skins. The protective skins can be made of metal or fiberglass. Metal is preferred, and a durable galvanized metal with enamel painted surface is known as the best. The purpose of the skin is to protect the insulation, which is fragile and cannot be used independently. The insulation is usually plastic foam material that is either injected or bonded between the metal skins. The insulation material provides the walk-in with resistance to heat flow, which allows the walk-in to be refrigerated and hold cold temperatures.
Types of Insulation
In the walk-in cooler and freezer business, there are two common types of plastic foam insulation, polyurethane and extruded polystyrene (XPS). Extruded polystyrene is not to be confused with expanded polystyrene, which was also used in walk-in coolers years ago. Expanded polystyrene is white and has different structural and insulating properties. In this document, we will be discussing extruded polystyrene only. You would be wise to stay away from the Expanded EPS or white insulation because it will not meet the EISA guidelines unless it is well over 6” thick which will make the panels VERY heavy.
Polyurethane (Urethane) can be applied in two different ways in the construction of walk-in coolers or freezers. One method is to pour or inject the two part plastic foam between two sheets of metal in a mold and produce a walk-in panel. Another method, less frequently used, is to box pour a block of foam 4 feet x 20 feet, then cut the block to panel thickness to be laminated between two sheets of metal to form a walk-in panel.
Extruded Polystyrene ( XPS):
The newest & best insulation to be used in walk-in coolers and freezers is extruded polystyrene foam or XPS. It has been used very successfully for years in insulating areas of high moisture exposure and high R-value requirements, including low temp warehouses, underground concrete insulation and house sheathing. This insulation is manufactured by extruding polystyrene plastic through an extrusion mold that creates a foam panel with the thickness, width, and any length needed to produce walk-in panels. The insulation is planned to exact tolerances of width, thickness, and length. The finished foam is then bonded between two sheets of metal and run through an automated press to form the walk-in panels. What difference does it make? Insulation is NOT all the same. Each type of insulation brings with it strengths and weaknesses that must be evaluated for each individual application requirement. The two factors that can affect the insulation’s performance are temperature and moisture.
Insulation performance is usually rated in what is termed R-value. The higher the R-value, the more resistance to heat flow, therefore the better its insulating properties. Some insulation performs better the lower the mean temperature and some perform better at higher temperatures. Therefore, the operating temperature of the insulation can affect its performance. When looking at polyurethane and extruded polystyrene at -10º F to 50º F mean temperature, polyurethane has a lower R-value the lower the mean temperature and extruded polystyrene has a higher R-value the colder the application. Therefore, extruded polystyrene (XPS) performs better at lower temperatures and polyurethane R-value actually performs worse at lower temperatures.
The largest factor that can affect the insulation’s performance is moisture. Usually insulation is expected to keep something warmer or colder than the other side of the insulation. This temperature differential (TD) causes, in many cases, a dew point to form inside the insulation. Once the dew point is reached, moisture is trapped. As water has a much lower R-value than insulation, the water reduces the R-value of the insulation. The more water resistance the insulation has, the better the insulation performs in high TD situations. Since walk-in coolers and freezers are high TD applications, sometimes 110º F on the outside and -20º F on the inside, the higher the moisture resistance of the insulation, the better. When looking at the water vapor permeance of the different foams, it is found that polyurethane allows more water vapor permeance and extruded polystyrene the least. Since water lowers the R-value of insulation, extruded polystyrene retains its R-value better than polyurethane.
As you can see
In the preceding graph, polyurethane loses over 75% of it’s R-value in five years and extruded polystyrene loses only 25% of its R-value. Therefore, extruded polystyrene’s performance, in actual walk-in conditions when moisture is considered, is over three times as good as polyurethane.
Does polyurethane have the highest R-value?
Why are some companies claiming polyurethane has the highest R-value? When some companies quote R-values of 32 to 34 for polyurethane walk-ins, they are quoting what is called fresh R-values, the R-value you have when you first take the insulation out of the mold. The operating temperatures of the insulation are not being considered. They are quoting R-32 to R-34 at 75º F mean operating temperature. The 32.5º F mean temperature of a walk-in freezer and the effect of moisture penetration, as in real life applications, are not being considered. To find the actual operating R-value of any insulation, you have to look at how it performs in real life applications. Since a walk-in freezer usually operates at -10º F inside and 75º F outside, a 32.5º F mean temperature needs to be used. Since moisture and aging mostly occur up front, a 60 month decreasing R-value needs to be considered. In 5 years the R-value of 4″ polyurethane has dropped to an R-value of 6 and 4″ extruded polystyrene to an R-value of 15. The performance R-value for extruded polystyrene has become almost three times better than urethane.
What does this all mean to you?
Extruded polystyrene can provide thousands of dollars in energy savings during the life of the walk-in. Using as an example, a 10′ x 12′ x 7’6″ freezer operating at a 32.5º F mean temperature with a refrigeration system with 90% efficiency and an electric cost of $.09/kWh, an extruded polystyrene walk-in will save up to $5,139.00* in energy savings over a polyurethane walk-in in the first five years.
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