Thermal and Hygrothermal Performance of Ventilated Attics With and Without Breathable Underlayments
The primary purpose of passive ventilation is to manage attic moisture and lower attic air temperature. This paper details results collected from a full year of field-testing at a facility constructed with 7 roof/attic configurations including both sealed and ventilated designs with breathable and non-breathable underlayments. The Natural Exposure Testing (NET) facility was built in Charleston, South Carolina to study the impact of a hot, humid climate on the hygrothermal performance of roof and attics. Two attics were configured identical except one was fitted with breathable and other with a non-breathable underlayment to compare effect of breathability on hygrothermal performance. Both attics were ventilated with identical 1:300 soffit-ridge ventilation system. A third attic with non-breathable underlayment and 1:150 ventilation was also constructed to evaluate the ventilation effect. Humidifiers introduced identical moisture loads in the attics to simulate moisture loads emanating from the conditioned space due to occupancy habits. The average moisture load was estimated from previously collected data by ORNL on various attics in Southern climates. Attics in the study were fitted with temperature, humidity and heat-flux sensors at identical locations. Results from the analysis of field data indicate that breathable membranes and ventilating attics in hot, humid climates provide effective temperature and moisture control.
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