Ok - here's a question, maybe stimulate a little controversy but as many of you know - that's the way I like it.
-10 F freezer, 100,000+ square feet. Reasonably steady product loading 24 hours a day (say 90 tons +/- 25 tons in a given hour).
Two envelope systems: 5" metal skin panel with expanded polyurethane vs. thermal mass panel of 8" HW concrete interior, 6" extruded polystyrene, 2-1/2 HW concrete exterior.
Polyu conductivity .0142 btu/h-ft-F (LTTR basis). Metal skin wall U without external skin resistance (DOE2) = ..033. Polystyrene .0167, thermal mass wall U = .030.
Question - which one is better? That's a loaded question but from a strictly thermal performance measure - kWh per year to cool - is there more difference than the .030 vs. .033 U-value would imply?.
We are using eQuest (Doe2.2) and it show the thermal mass as `slightly' better. The manufacturer, based largely on testimonial evidence, insists it is MUCH better.
One issue that they raise is infiltration through the metal skin panels and the impact on in-situ R value. They site ORNL test results that apparently show this but I've not seen them. I know ORNL did some testing on apparent R of mass walls but it didn't seem conclusive to me that it would apply in this situation.
Anyone out there that can cite well documented research about the differences? Anyone have comments about DOE2's strengths and weaknesses in capturing the effects of the large wall thermal mass?
Mark E. Case, President
etc Group, Inc.
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