R1 and R2 show the inside and outside radius of the pipe respectively. R3 shows the radius of the insulation. Typically when dealing with insulations, engineers must be concerned with linear heat loss or heat loss per unit length. This coefficient can of course increase with wind velocity if the pipe is outside. The thickness at which insulation begins to decrease heat loss is described as the critical thickness.
Critical thickness is a concern however in insulating wires.
Figure 3 shows the heat loss vs. insulation thickness for a typical insulation. It's easy to see why wire insulation is kept to a minimum as adding insulation would increase the heat transfer. Thinking About Insulation from All Sides Three major factors play an important role in determining insulation type and thickness. Each situation must be studied to determine how to meet each one of these criteria. First, we'll examine each aspect individually, then we'll see how to consider all three for an example.