Potential Thermal Degradation of Attic Framing and Wood Sheathing

The process of thermal degradation of wood products was reviewed in the context of the molecular constituents of wood-cellulose, hemicellulose, and lignin. Test data on 2×4 lumber subjected to 66°C (150°F) for 48 months were reviewed, and the loss of strength as measured by modulus of rupture was reported to be 32%. The basics of heat transfer in attics were reviewed and test data on simulated attics in several locations were summarized. It was noted that thermal degradation is a cumulative process, dependent upon both temperature and duration of exposure. The effect of combined temperature and duration on potential thermal degradation of attic framing and sheathing can be minimized at the design stage by specifying light-colored shingles and roof details that provide for maximum attic ventilation. Through the use of a continuous soffit and ridgeline ventilation system, peak attic temperatures from solar radiation will be reduced and thus significant thermal degradation of attic framing and sheathing should be unlikely during the life of the roof structure.