Convective heat loss
As discussed in Chap. 3, convection is heat flow caused by air currents. Although convective heat loss in insulation is rare, it can occur when large temperature differences above and below the insulation create tiny air currents, called convection loops, within the insulation. Studies have shown that convective heat loss typically occurs with lighter-density loose-fill materials, such as fiberglass, at the very low attic temperatures possible in extremely cold climates. Depending on the attic temperature, the insulation’s measured R-value could decrease by as much as 50 percent. These convection loops can be minimized by installing material in accordance with the manufacturer’s winter design conditions, installing blown-in cellulose, or placing a fiberglass or mineral wool blanket on top of the loose-fill fiberglass2 (Fig. 7.2).