Install Ductwork, Plenums & Transitions

Build and attach supply/return plenums and transitions, seal every joint with UL181 mastic (not tape alone), support runs so flex doesn't sag, insulate ducts in unconditioned space to R-8, size to the design CFM, and keep the return fully sealed (no panned building cavities).

Approx. 360 min

Before you start

•A load calc (Manual J) and duct design (Manual D) giving the design CFM per zone, total external static budget, and duct sizes — don't eyeball it.
•Equipment set and its supply/return opening sizes known so the plenums and transitions match the cabinet.
•Which runs pass through unconditioned space (attic, crawl, garage) identified for insulation requirements.

Tools & materials

Sheet-metal hand tools / duct fabrication tools, self-tapping screws · UL181-listed duct mastic + fiberglass mesh tape, UL181 foil tape · Duct insulation (R-8 wrap) and listed flex with proper support straps · Manometer (static pressure) and an airflow/CFM measurement method · Drill, sheet-metal screws, support strapping

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Build plenums & transitions

Build and attach the supply & return plenums and transitions
Fabricate/attach the supply and return plenums to the equipment with smooth transitions (avoid abrupt size changes and tight turns that spike static and noise). Fasten with screws so connections are mechanically secure before sealing.
Target: Plenums/transitions sized to the design; gradual transitions; mechanically fastened with screws

Seal every joint

Seal all joints with UL181 mastic — not tape alone
Brush UL181 duct mastic (with mesh tape over wider gaps) on every seam, joint, boot, and connection on both supply and return. Cloth 'duct tape' fails and dries out; tape alone is not an acceptable long-term seal. The goal is to keep the air you paid to condition inside the ducts.
Tools: UL181 duct mastic, Fiberglass mesh tape
Target: All joints sealed with UL181 mastic (+ mesh on gaps); target low total duct leakage per the spec/code

Support runs

Support runs at proper spacing so flex doesn't sag
Strap and support both rigid and flex duct at the manufacturer's spacing with wide straps that don't pinch the duct. Pull flex taut between supports — sagging, kinked, or compressed flex chokes airflow and adds static, the most common field cause of low CFM.
Target: Supports at the maker's max spacing (commonly ≤ 4–5 ft for flex); flex pulled taut, no sag/kinks/pinching

Insulate in unconditioned space

Insulate ducts that run through unconditioned space
Wrap supply and return duct in attics, crawlspaces, and garages to the required R-value (R-8 is typical for unconditioned space) with the vapor barrier intact and seams sealed, so the air doesn't gain/lose heat and the duct surface doesn't sweat and drip.
Target: Ducts in unconditioned space insulated to ~R-8 (per code/climate) with intact, sealed vapor barrier

Size & sealed return

Size duct to the design CFM
Confirm trunk and branch sizes match the design CFM and the friction-rate/velocity targets. Undersized duct drives up static, noise, and energy use and starves rooms; verify with airflow/static measurement at commissioning.
Tools: Manometer
Target: Duct sized to the Manual D design CFM at the design friction rate; measured static within the equipment's budget
Use a fully sealed/ducted return — no panned cavities
Return air must travel through sealed duct, not a building cavity (panned joist bays, wall stud cavities, or platform returns) which leak, pull in attic/garage/combustion-zone air, and can backdraft gas appliances. Duct the return properly all the way back to the equipment.
Target: Return is fully ducted and sealed; no building cavities used as return-air pathways
⚠ Panned/cavity returns can pull combustion gases (CO) or contaminants into the airstream and backdraft gas appliances.