Ventilation Ducts

Select duct materials that have a flame spread no greater than 25

Select flexible ducts that are UL 181 listed or Air Diffusion Council approved

Select rigid ducts of 28 gauge or thicker

Select duct insulation with a flame spread/smoke development index of 25/50 or less when tested according to ASTM E84 or UL 723

Install ventilation ducts as short, straight, smooth and fully extended as possible considering the following: Vent termination location; Amount of space for duct run; Roof condition, type, and access (e.g., metal, shingle, bowstring, flat)

Choose duct diameter that is equal to or greater than the exhaust fan outlet

Slope duct downwards towards the termination when possible

Install flexible duct so the radius at the centerline of any turn is no less than one duct diameter

If combining ducts, combine them on the upstream side of fan using "Y"-fittings or collection boxes

Do not combine dryer, kitchen, or garage exhausts streams with any other exhaust stream

Fasten all ducts as follows:

Round metal-to-metal or metal-to-PVC with a minimum of three equally spaced screws

For other metal-to-metal or metal-to-PVC with welds, gaskets, mastics (adhesives), mastic-embedded-fabric systems, or tapes

Flexible duct-to-metal or flexible duct-to-PVC with tie bands using a tie band tensioning tool

PVC-to-PVC materials with approved PVC primer and cement

Fasten other specialized duct fittings in accordance with manufacturer specifications

Seal all duct connections with UL 181B or 181B-M listed materials (e.g., mastic, tape)

Insulate all ducts installed outside of the thermal boundary to a minimum of R-8

Insulate all ductwork exposed to the exterior of the building to a minimum of R-12

Support flexible and duct board ducts every 4' or less using a minimum of 1-1/2" wide material

Install support materials in a way that does not crimp ductwork or cause the interior dimensions of the ductwork to be less than specified

Support metal ducts every 10' or less using 1/2" or wider material, using 18 gauge or greater strapping or 12 gauge or greater galvanized wire