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What’s Up With R?

Posted in Recommended Reading by mmathews on 09/01 at 08:09 AM

We’ve all heard about it, but what is an R-value and where does it come from in walk-in coolers and freezers?

We all remember from physics classes that the R-value is a measure of thermal resistance (it’s OK if you don’t remember). Think of it as a value that indicates a walk-in’s resistance to heat flow. So if you have a walk-in, let’s call it walk-in A, that is rated at R-32, it’s going to be twice as resistant to heat flow as another walk-in, walk-in B, that is rated at R-16. It’s pretty straightforward, the higher the R-value the more resistance to heat flow. This means that “all things being equal” the thermal resistance of walk-in A is greater than the thermal resistance of walk-in B.

Why does this matter?
Well, for one, refrigeration needs to be sized to match the heat load of the walk-in. So if both these walk-ins were the same size and had the same number of door openings every hour, you would have a smaller refrigeration system on walk-in A. If you chose your refrigeration system accurately & used a system that was energy efficient, you would see a nice savings on your electric bill when you compared the efficiency of the two walk-ins. We will dive further into this topic, in detail, in future newsletter.

Remember the “all things being equal” remark I made earlier? Here’s the catch with that one. The R-value is calculated from another value called the K-factor. The K-factor is a value that indicates the insulating value of a material and is rated by its thermal conductivity. All foam manufacturers will give you the K-factor of their foam and it’s measured by a one-inch thickness. The smaller the K-factor, the lower the thermal conductivity; which means it will have greater thermal resistance.

For example: Material A has a K-factor of .25 while Material B has a K-factor of .125. If both are 4 inches thick, then:
Material A would have an R-Value of (4/.25) = 16, while
Material B would have an R-Value of (4/.125) = 32

What would you do if you wanted to use Material A but wanted to have a R-value of 32?
Essentially, you would double the thickness of the walls and ceilings, so instead of having 4 inch-thick walls and ceilings, they need to be 8 inches thick. That’s easily accomplished but is it something that you have accounted for in the design? Two questions that come to mind immediately are:

1. Will the larger walls fit in the space?
2. How will having a smaller inside dimension affect the shelving layout?

For foamed in place polyurethane insulation, there are essentially two companies that supply their foam systems to all the major walk-in manufacturers. The two are BASF & DOW Chemical. Theses systems are going to be CFC free and will have very similar K-factors which means that they’ll have very similar R-values.

So why all the hoop-la over R-values?
We inherently know that the greater the R-value the greater the thermal resistance, so by increasing the R-value, we are helping our customers keep their food at the proper temperature. In addition, the federal government requires walk-ins to have an R-value of 25 for coolers and 32 for freezers (Energy Independence & Security Act of 2007 (PDF)). Finally, and we’ll discuss this further in future editions of this newsletter, when you increase the R-value of a walk-in, in many cases you can begin decreasing the size of the refrigeration equipment, which can save both money & electricity.

So to summarize, if there is a 4-inch panel with a K-factor of .125, it will meet or exceed the Federal requirements for thermal resistance. However, simply specifying an R-value for a walk-in panel may not result in a 4-inch panel. To insure that you are getting exactly what you intend, it’s best to specify the K-factor value and the maximum thickness of the panels themselves.

Common K-factors for Different Materials
Cork or Mineral Wood K- .30
Glass Fiber or Polystyrene K- .26
Urethane (Sprayed) K- .16
Urethane (Foamed in Place) K- .12

In conclusion, it’s important to be aware that there are other factors that affect thermal resistance and thermal conductivity and it’s likely you’ll hear about these from time to time. These include the temperature of the walk-ins, temperature of the air around the walk-in, the insulation type and some argument over initial R-Value versus an R-Value at a later time (8, 15, 25 years). There is merit to some of these items and if you are interested in more information on these items, please contact me, Rick Jones, directly.

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