It is a constant battle to first generate heat and then keep hold of it, heat likes nothing more than to escape. Need proof? try sitting butt naked on a cold rock and feel the rush of heat from you in to the rock below. Now, do that for to long and you are going to turn blue pretty quickly, what you need is a layer of material that will not allow the heat to escape.. a layer of insulation. The mythical R-value indicates an objects resistance to the transfer of heat, the better the resistance the higher the R-value.