Parts 1 and 2 covered how air moves and the hardware that moves it. Part 3 is where it all comes together: how the duct is laid out in the building, how much air each room actually needs, and the energy-conservation practices — insulation, vapor barriers, and air-tight sealing — that separate a working duct system from an efficient one.
1. The Five Residential Duct Layouts
Every residential supply system you’ll encounter is a variation of one of five basic layouts. Each is suited to a particular building shape, foundation type, climate, and budget.
A. Perimeter Loop
A continuous loop of supply duct runs around the perimeter of the building (usually buried in or under a slab-on-grade), fed by short feeder ducts from a central plenum.
- Best for: Slab-on-grade homes in cold climates. The supply loop runs along exterior walls, where the heat is needed most, and pre-warms the slab itself.
- Floor registers sit directly above the loop along outside walls.
- Pros: Excellent cold-climate comfort, warm floors, uniform perimeter heat.
- Cons: Hard to clean, can’t be reconfigured, slab moisture/leakage risks. Less ideal for cooling-dominated climates.
B. Radial
Individual round duct runs radiate outward from a central plenum like spokes from a wheel, each terminating at a single register. No trunk line.
- Best for: Compact, single-story homes (often basement or crawlspace installations) where every register is roughly the same distance from the equipment.
- Pros: Simple, low installed cost, low pressure drop per run, balanced by sizing each radial.
- Cons: Doesn’t scale to large or sprawling floor plans — long radials get unwieldy.
C. Extended Plenum
A single rectangular trunk duct of constant cross-section runs the length of the building, with branch ducts tapped off along its length feeding individual registers.
- Best for: Rectangular homes up to ~24′ trunk length. The most common residential layout in North America.
- Pros: Easy to fabricate and install, predictable behavior, easy to add branches in retrofit.
- Cons: Air velocity drops the further you get from the plenum (since CFM decreases but area stays constant), so the last takeoffs see lower pressure available and tend to starve unless oversized.
D. Reducing Trunk (Reducing Extended Plenum)
A trunk duct that steps down in size after each major branch takeoff, maintaining a roughly constant air velocity (typically near 700 FPM at design CFM).
- Best for: Larger or longer homes where a constant-CSA trunk would either over-size the entire run or starve the far end.
- Pros: Best balance between material cost and performance. Each branch sees similar static pressure available, so balancing is easier and quieter. Treated as best practice by ACCA Manual D for medium-to-large residential.
- Cons: More fittings to fabricate; more carefully designed.
E. Furred-In Duct
Not really a topology but a construction style: ducts are built into chases, soffits, or dropped ceilings rather than installed in attics or crawlspaces.
- Best for: Two-story homes, retrofits, and any home where the duct must run through conditioned space without being visible.
- Pros: Ducts are inside the thermal envelope, so duct losses go to conditioned space (basically zero penalty). This is the energy gold standard.
- Cons: Reduces ceiling height; requires architectural coordination; harder to access for service.
2. Sizing Room CFM — Heating vs Cooling
Duct sizing without load calculations is guessing. The correct sequence is always:
- Manual J — calculate each room’s heating and cooling load.
- Manual S — select equipment to match the building’s total load.
- Manual T — select registers and grilles based on room CFM, throw, drop, and NC.
- Manual D — size ducts based on the equipment’s available external static pressure and the design CFM per branch.
The Cooling CFM Equation
Cooling distribution is sized on sensible load, because the equipment’s sensible capacity is what raises or lowers temperature:
CFM = Sensible Btuh ÷ (1.08 × ΔT)
Where 1.08 is the standard-air constant (lb/ft³ × 60 min/hr × specific heat) and ΔT is the temperature difference between supply air and room air. Typical residential cooling ΔT = 18–22°F at design.
Example: A bedroom with 4,800 Btuh sensible cooling load at a 20°F ΔT needs CFM = 4800 ÷ (1.08 × 20) = 222 CFM.
The Heating CFM Equation
Same equation form, applied to total heating output and the heating ΔT:
CFM = Total Btuh ÷ (1.08 × ΔT)
Furnace ΔT is typically 40–70°F — much higher than cooling ΔT. As a result, the CFM required for the heating load is normally a fraction of the CFM required for the cooling load in the same room. That’s why we size ducts on cooling CFM in mixed climates — cooling needs more air, and the ducts must accommodate the larger number.
Balancing Heating vs Cooling
Cooling-driven CFM is delivered through ducts sized for cooling. In heating mode the same ducts will deliver the same CFM at a higher ΔT — which usually works out fine because the heating load is proportionally smaller per room. Where heating and cooling loads are not proportional (e.g., a bathroom with big heat loss but tiny cooling load), use:
- Manual balancing dampers to throttle individual branches in one season.
- Zoning to control supply temperature and CFM per zone.
- Two-stage or variable-capacity equipment to handle wide load swings without short-cycling.
3. Energy Conservation in Duct Systems
Air can leak out of bad ducts at 30% or more. Heat can conduct through uninsulated ducts in unconditioned space at another 10–15%. Together that’s 40%+ of equipment capacity wasted before it ever reaches a register. The energy-conservation triad — insulation, vapor barrier, and air-tight sealing — addresses all three loss paths.
A. Duct Insulation
Insulation reduces the heat transfer between conditioned air inside the duct and the surrounding ambient air. The colder or hotter the ambient (attic, crawl, garage), the more insulation matters.
IECC 2021 minimum R-values (most jurisdictions follow this or a stricter local code):
- Supply ducts in attics: R-8 minimum.
- Supply ducts in other unconditioned spaces (crawl, garage, unconditioned basement): R-6 minimum.
- Return ducts in unconditioned spaces: R-6 minimum.
- Ducts located inside the thermal envelope: no insulation required (but it’s still wise where condensation is a risk).
Forms: external wrap (faced fiberglass blanket secured with staples and seamed with foil tape), pre-insulated flex duct (insulation is integral), or fiberglass duct board (insulation is the duct).
B. Vapor Barriers
Whenever a duct’s surface temperature can fall below the dew point of the surrounding air, moisture will condense on it and either drip or absorb into the insulation. The vapor barrier sits on the warm-in-summer side of the insulation (so on the outside of a cooling duct in a humid climate) to keep humid air from reaching the cold surface.
- All-service jacket (ASJ) or foil-scrim-kraft (FSK) facings on duct wrap serve as the vapor barrier.
- All seams, joints, and penetrations in the vapor jacket must be sealed with matching foil tape or mastic. A vapor barrier with gaps is no vapor barrier at all — humid air will find every gap.
- In cold climates with heating-dominated loads, the vapor barrier orientation is reversed — it goes on the room side. Climate-zone awareness matters.
- Insulation that has absorbed moisture loses most of its R-value and can support mold; replace it, don’t just dry it.
C. Duct Sealing
Air leakage is the largest single distribution loss in most U.S. homes. The IECC duct leakage limit is ≤4 CFM at 25 Pa per 100 ft² of conditioned floor area for ducts located outside the thermal envelope (some jurisdictions use 8 CFM25 for ducts entirely inside). Hitting that target requires deliberate sealing — not just “wrap some tape around it.”
Approved sealing materials:
- Duct mastic (water-based fibrous mastic, applied with a brush or glove, 1/16″ thick or to manufacturer spec). The gold standard — permanent, fills small gaps, paints over seams and screw heads.
- UL 181A-P tape for rigid sheet metal joints.
- UL 181B-FX tape for flex duct connections.
- Mastic + mesh tape for gaps wider than ~1/16″.
Never use cloth-back “duct tape” — it’s the worst-performing material for the purpose. Most adhesives fail within 1–3 years from heat and dust. Despite the name, it’s not approved for sealing ducts.
The Duct Blaster Test
A calibrated fan attached to the air handler (with all registers temporarily sealed) measures total duct leakage to outside in CFM at 25 Pa (CFM25). Procedure summary:
- Seal all supply and return registers with thin plastic.
- Attach the Duct Blaster fan at the air handler cabinet or return grille.
- Pressurize (or depressurize) the duct system to 25 Pa relative to the outside.
- Read the airflow the fan is moving to maintain that pressure — that’s the total leakage rate.
- For “to outside” leakage, run a separate test with the house under matching pressure so only ducts leaking outside the envelope count.
Result is compared to the code threshold. Failed systems are visually inspected (often with a smoke pencil at every joint) and re-sealed at the leakage points. New construction must pass this test before final inspection in most code-adopting jurisdictions.
4. The Field Workflow For An Energy-Conscious Duct Job
- Run Manual J on the home; do not skip rooms.
- Select equipment per Manual S — right-size, don’t oversize.
- Choose layout based on building shape, foundation, climate (see Section 1).
- Locate ducts inside the thermal envelope wherever possible. This is the single biggest design lever.
- Size ducts via Manual D using the equipment’s rated ESP available after the coil, filter, and accessories are subtracted.
- Use proper fittings: conical takeoffs, long-radius elbows, turning vanes in square elbows, gentle transitions.
- Pull flex tight, support every 4 ft, no kinks.
- Seal every joint with mastic — transverse, longitudinal, takeoffs, plenum corners, and where the cabinet meets the plenum.
- Insulate to or above IECC minimums. R-8 in attics is cheap insurance.
- Maintain a continuous vapor barrier on the warm-in-summer side. Tape every seam.
- Commission the system: total external static pressure, supply CFM (flow hood per register), ΔT across the coil, blower amp draw, and a Duct Blaster test if code-required.
- Balance with dampers until every room is within ±10% of design CFM.
5. Common Pitfalls — Design & Energy Edition
- Designing on rules of thumb instead of Manual J — usually leads to oversized equipment, oversized ducts, short cycling, poor humidity control.
- Putting ducts in vented attics in hot-humid climates without R-8 insulation and a continuous vapor barrier — results in massive condensation and capacity loss.
- Using cloth duct tape on seams — will fail within a year; system leaks worse than before.
- Skipping the return-air path — pressurizes the house, depressurizes the basement, drives infiltration, hurts every metric.
- Calling the system “balanced” without a flow hood — you don’t know what you don’t measure.
- Putting a small return on a large system to “keep it quiet” — cures the symptom and creates a chronic high-static disease.
Series Wrap
Three articles, one objective: give every HVAC tech a single source of truth on air distribution that’s honest about the physics, accurate to the curriculum, and useful on the truck. If you absorbed even half of this, you’re ahead of most installers in your market. Bookmark all three parts, share them with your apprentices, and use them as a checklist on your next install or service call.