Fiber Glass

Maintaining R-value at below freezing temperatures is a problem for light, fluffy blown fiber glass, because of a phenomenon called convective heat loss. At about 32o F air begins to circulate within the insulation. These air flows carry heat through the insulation, reducing its effective R-value, often by a very significant amount. Studies at Oak Ridge National Laboratory showed loss of nearly 15% of R-value at 20o F. At 18o F below zero the insulation had only about 60% of its nominal R-value. At least one northern state (Minnesota) has required fiber glass producers to change coverage charts to reflect this cold temperature R-value loss. A layer of cellulose on top of the fiber glass has been found to be effective at controlling convective heat loss.