Rooftop Unit Ductwork: Curb Adapters, Transitions, and Plenums

Rooftop packaged units (RTUs) are the workhorse of commercial HVAC. Retail stores, office buildings, schools, and warehouses depend on them for their simplicity: the entire heating, cooling, and fan system is housed in one outdoor unit, eliminating the mechanical room requirements of split systems. But connecting an RTU to the building's duct distribution system requires careful attention to curb adapter design, transition geometry, and weatherproofing that many contractors get wrong on their first RTU installation.

How RTU Ductwork Is Organized

A rooftop unit discharges supply air and collects return air through openings in its base. These openings align with a roof curb — a structural framed opening in the roof deck that supports the unit and provides the duct pathway into the building. Supply and return air pass vertically through the curb into a curb adapter inside the ceiling plenum, which transitions to the horizontal distribution ductwork below.

The critical interface is the curb adapter — the fitting that connects the RTU's curb openings to the building ductwork. Poorly designed curb adapters create significant static pressure loss, noise, and condensate problems. A well-designed curb adapter is a low-loss transition that delivers the RTU's full rated airflow into the building.

Curb Adapter Sizing

The curb adapter must match the RTU's supply and return opening dimensions exactly at the top (curb interface) and transition to the building ductwork dimensions at the bottom. Dimensional mismatch at the curb interface creates air leakage between the curb adapter and the unit — outdoor air entering the return side or conditioned air escaping the supply side.

To size the curb adapter correctly:

1. Obtain the RTU's certified dimensional drawing showing supply and return opening locations and dimensions. These vary by manufacturer and model — do not estimate.
2. Determine the building ductwork dimensions at the entry point below the curb. For ceiling plenum systems, this may be the plenum face area; for ducted systems, it is the trunk duct dimensions.
3. Design the curb adapter to transition from the RTU opening at the top to the building duct at the bottom, maintaining taper angles within 15 degrees per side for the supply transition and within acceptable limits for the return.

Supply Plenum vs Ducted Supply Distribution

RTU supply distribution in commercial buildings uses one of two approaches:

Ceiling plenum supply. The RTU supplies into the ceiling plenum directly, and ceiling diffusers draw from the plenum. This is the lowest-cost and most common approach. The curb adapter opens into the plenum with minimal transition. Disadvantages: the plenum must be properly sealed (no combustion equipment, no pest access), and supply air temperature is moderated by plenum conditions before reaching diffusers.

Ducted supply. The RTU supplies into a trunk duct system that distributes air to specific zones or rooms. This approach costs more to install but provides far better airflow control, is required for zoned systems, and allows duct leakage testing. The curb adapter transitions to the trunk duct entry, and the trunk duct system distributes from there.

Return Air Configuration

RTU return air enters the unit through the return opening in the curb base. The return side of the curb adapter collects air from either the ceiling plenum or a dedicated return duct. For plenum return systems, the curb adapter return simply opens to the plenum space — the return opening area must be at least 1.5 times the supply opening area to prevent excessive return velocity.

For ducted return systems, the curb adapter includes a defined return plenum section that connects to the return trunk. Return plenums for RTUs must accommodate the full system CFM, which can be substantial — a 20-ton RTU moves approximately 8,000 CFM. A return plenum undersized for this flow creates high velocity noise audible in the occupied space below.

Weatherproofing and Condensate Management

The curb-to-RTU interface is exposed to outdoor weather from above. The RTU base gaskets seal against the curb, but condensate that forms on the cold supply air plenum in cooling mode must be managed:

• The curb adapter interior should slope toward a drain pan or drip edge, not toward the building interior. Condensate running into the ceiling plenum creates mold and structural damage.
• All supply ductwork below the curb in unconditioned plenum spaces must be insulated — typically R-8 — to prevent additional condensation on the duct exterior.
• The curb-to-unit joint should be inspected annually. Gasket failure allows both water infiltration and conditioned air leakage — the two most common sources of unexplained energy waste in RTU-served buildings.

RTU Replacement Curb Adapters

When replacing an existing RTU with a new unit of different dimensions, the existing curb adapter almost certainly does not match the new unit's opening dimensions. A replacement curb adapter must be fabricated to interface the new unit with the existing curb. This is one of the most common custom fabrication requests — the adapter is unique to the specific combination of old curb dimensions and new unit opening dimensions, and no stock fitting covers it.

PMX Ductwork fabricates custom RTU curb adapters, supply and return plenum sections, and transitions in any dimension in galvanized or stainless steel. Provide the curb dimensions and unit opening dimensions and get instant pricing for precisely fabricated curb adapters and building-side transition pieces.

RTU Ductwork Checklist Before Installation

Before the RTU goes on the roof and the ductwork gets enclosed, verify: (1) the supply plenum is sized for the unit's CFM at no more than 700 FPM, (2) return air openings are sized to match supply — a common mistake is cutting return openings 30-40% smaller than supply, (3) the plenum knockout is aligned with the first elbow to avoid a direct impingement condition that causes noise and high pressure drop, (4) all duct joints are sealed with mastic (not just tape), and (5) any duct in an unconditioned ceiling plenum is insulated to R-8 minimum per energy code. Getting these details right before the ceiling is closed saves expensive callbacks later.