Duct Installation, Sealing, and Insulation
Duct Installation, Sealing, and Insulation tests field practices that determine leakage, condensation, noise, and system capacity. Questions cover support spacing, flex duct routing, mastic and tape systems, vapor retarders, insulation compression, and fire/smoke damper installation.
Installation rules
Support flex duct at intervals of about 4-5 ft and within about 2 ft of connections; support rigid duct per code and SMACNA practice. Fully extend flex duct, keep the inner liner smooth, and avoid sharp bends; a bend radius near one duct diameter is a practical minimum. Do not compress flex into a short run or let it sag. Seal all seams, joints, takeoffs, and penetrations with mastic plus mesh or a listed UL 181 tape; cloth duct tape is not a permanent seal. On cooling ducts, the vapor retarder must be continuous on the outside, with seams lapped and sealed.
Insulation and dampers
Insulation R-value must match the design and climate; compressing it lowers R-value. Fire dampers go in rated assemblies and must be correctly oriented, sleeved, and accessible. Duct leakage testing compares measured leakage at a set pressure with the program target.
Worked example
A duct blaster measures 80 cfm of leakage at 25 Pa on a system moving 1,200 cfm. Leakage is 80 ÷ 1,200 = 6.7%. If the target is 5%, the system fails. Before adding sealant, inspect the most common paths: trunk joints, takeoffs, plenum connections, and flex collar ties. Seal those and retest rather than increasing blower speed.
Traps
Do not seal only the outer vapor barrier and leave the inner liner leaking. Do not use cloth tape or rely on zip ties as a seal. Do not compress insulation or bury flex under a sharp bend. Do not block a fire damper with insulation or duct sealant.