Surface temperature control is the second reason for insulation. In many cases it is the most important reason for the insulation. I noticed this phenomena first while insulating potato storages. We had various customers ask us to insulate the buildings anywhere from two to five inches of urethane. The buildings insulated with two inches would hold the temperatures of the potatoes properly, just as well as the buildings insulated with five inches. The difference came in the condensation. Potato storages are kept up at very high humidity levels. The buildings with the two inches of urethane would have far more condensation than those with the five inches.
An engineer from the Upjohn company explained this to me. He stated that thicker insulation is absolutely necessary to maintain higher interior surface temperatures. One and a half inches of urethane on the walls and ceiling of a potato storage would control the heat loss from the building, but it took a minimum of three inches of urethane to control the interior surface temperature. Four inches was even better. With five inches the difference is practically negligible. The only place where we have felt the need for five inches of urethane was insulating the roof or ceiling of a subzero freezer.