Full Description

The rate of drying of construction moisture in excess of fiber saturation is investigated in timber-framed flat roof specimens under laboratory conditions. The methodology of factorial design was used to choose appropriate specimens and conditions. Four runs with specimens with low air leakage characteristics are described here. The specimens were placed in controlled-climate chambers and allowed to dry under steady-state conditions of temperature and relative humidity. Runs lasted from 49 to 92 days. The results show linear drying above fiber saturation with rates ranging from 0.6% to 2.0% moisture content per day and exponential drying below fiber saturation with time constants ranging from 9.7 days to 24.0 days. The exception was a very vapor tight specimen operating under cold external conditions. This case showed condensation and dried very slowly. Temperature gradients were found to be a significant driving force in the transfer of moisture inside the timber joists. While drying was taking place, significant vertical moisture gradients opposing the temperature gradient were observed in the timbers. Due to construction moisture, condensation occurred in specimens when it would not be predicted by the dew-point profile technique.

Units: SI

Citation: ASHRAE Transactions, 1987, vol. 93, pt. 2, Nashville, TN