Effect of free convection on the insulating value of a horizontal air space, two large parallel horizontal metal plates are separated by a 2.5 cm air gap, with the air at an average temperature of 100°C. How much hotter may the lower plate be (than the upper plate) without causing the onset of the cellular free convection discussed in Example 11.5-27 How much may this temperature difference be increased if a very thin metal sheet is placed midway between the two plates?