• Daniel Haaland
    RDH Building Science Inc.
  • Christopher Marleau
    RDH Building Science Inc.

Current industry practices for the installation of in-slab ducts including building codes, standards, and manufacturer documentation are insufficient to fully mitigate the moisture related issues observed in the Lower Mainland of British Columbia. An analysis of interior moisture sources including clothes dryers confirmed the importance of maintaining adequate airflow rates and velocities to balance wetting and drying periods within the ducts.

In addition to an analysis of in-slab duct design, the primary purpose of this research report was a monitoring study of installed systems. RDH investigated three separate sites experiencing a range in the severity of moisture related issues. Significantly, the study found that even an undamaged system with airflow rates approaching the dryer manufacturer’s recommendations experienced periods of condensation within the in-slab duct during normal operation. The study also found that the severity of the moisture related issues increased with decreasing exhaust airflow. Remediation aimed at increasing airflow rates including duct cleaning was shown to temporarily reduce moisture build-up.

When in-slab ducts are used, architects and mechanical designers should consider layouts which reduce the effective duct length and should include the ducts in the mechanical commissioning process to ensure the installed airflow rates meet or exceed the design.

This report concludes with recommendations for remediation of in-slab ducts experiencing moisture issues including regular inspections and duct cleaning. However, given the relative difficulty of repairing in-slab duct systems and the inability to fully control the moisture risks, many designers or occupants may instead choose ductless equipment as a more permanent solution.

RDH Building Science