6.1 Duct Layouts, Materials, Insulation, Duct Sealing, and Friction Loss

Air Distribution System Layouts

Air distribution systems are designed to deliver conditioned air from the heating and cooling equipment to the occupied spaces of a building and return air back to the equipment for reconditioning. The layout of the duct system directly impacts airflow performance, system static pressure, material costs, and noise levels. Texas HVAC contractors must understand four primary supply duct system configurations.

Extended Plenum System

An extended plenum system features a main supply plenum of constant cross-sectional size extending from the air handler or furnace outlet throughout the length of the building structure. Branch ducts connect along the sides of the extended main trunk to feed individual room registers.

• Advantages: Simple design, low fabrication labor cost, and easy installation between floor joists or ceiling spaces.
• Disadvantages: As air branches off down the line, air velocity inside the main plenum drops. This causes static pressure to build up near the closed capped end of the plenum while lowering static pressure near middle takeoffs. If not properly dampered, registers closest to the air handler and at the very end of the plenum can receive uneven airflow.

Reducing Trunk System

A reducing trunk system (also called a graduated trunk system) decreases the cross-sectional area of the main supply trunk as branch ducts draw air off the main run.

• Aerodynamic Mechanics: By reducing duct volume in proportion to the remaining CFM requirement, air velocity is maintained at a relatively constant level throughout the entire length of the trunk.
• Performance: Static pressure remains balanced across all branch takeoffs from the first takeoff to the last. While requiring higher metal fabrication and installation labor, reducing trunk systems represent the premier design for long residential and commercial supply runs.

Radial Duct System

A radial duct system utilizes a single central supply plenum with individual branch ducts running directly from the plenum to each supply register like spokes on a wheel.

• Application: Most commonly installed in compact single-story homes, slab-on-grade structures, or drop-ceiling utility areas.
• Characteristics: Because each room register has its own direct duct run of nearly equal length, pressure losses across runs are balanced and total duct friction is low. However, radial systems require large central plenum clearance and can use significant total linear footage of flexible or round metal ductwork.

Spider (Perimeter Radial) System

A spider system combines elements of trunk and radial designs. A large main supply trunk feeds one or more secondary sub-plenums or manifolds positioned in remote zones of the building. Individual branch ducts then radiate out from these secondary sub-plenums to room registers. This system is ideal for large, spread-out residential layouts with distinct architectural wings.

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Ductwork Construction Materials and Mechanical Codes

Selecting the appropriate duct material requires balancing aerodynamic efficiency, fire safety compliance, sound attenuation, and thermal performance under International Mechanical Code (IMC) Chapter 6 and SMACNA standards.

Galvanized Sheet Metal

Galvanized steel ductwork is the industry benchmark for durability and air distribution performance. Smooth sheet metal offers the lowest friction coefficient ($C = 0.0005$), maximizing airflow velocity while minimizing fan static pressure losses.

• Fire Safety: Sheet metal is non-combustible and complies fully with NFPA 90A and NFPA 90B standards for high-occupancy commercial and residential structures.
• Insulation Requirement: Because metal is an excellent thermal conductor, sheet metal installed in unconditioned spaces must be wrapped externally with foil-faced fiberglass duct wrap or lined internally with fiberglass duct liner to prevent heat loss/gain and surface condensation.

Fibrous Glass Ductboard (UL 181 Class 1)

Fibrous glass ductboard consists of rigid resin-bonded glass fiber boards faced on the exterior with a reinforced aluminum foil vapor barrier (FRK). Ductboard panels are grooved, folded, and stapled/mastic-sealed to fabricate rectangular duct sections.

• Integrated Properties: Combines duct structure, thermal insulation, acoustic lining, and vapor barrier into a single material.
• Operating Limits: Under UL 181 Class 1 testing standards, ductboard is limited to maximum air velocities of 2,400 FPM, maximum static pressures of 2.0 in. w.g. (positive or negative), and maximum internal air temperatures of 250°F.

Flexible Air Ducts (UL 181 Class 1 Flex)

Flexible ducting consists of a spiral wire helix encapsulated in an inner plastic film core, wrapped in fiberglass insulation, and encased in a tough poly outer vapor jacket.

• Installation Rules (IMC 603.6):
  • Full Extension: Flexible duct must be installed fully extended. Compressed or sagged flex duct dramatically increases internal friction loss—a 15% longitudinal compression can increase friction pressure drop by over 100%.
  • Maximum Sag: Sag between supports must not exceed 1/2 inch per foot of span.
  • Support Spacing: Flexible duct must be supported at intervals not exceeding 4 feet using minimum 1.5-inch-wide hanging straps to prevent sagging or crimping the inner core.
  • Bend Radius: Centerline bend radius at turns must be at least one duct diameter ($1D$). Sharp bends over joists or ceiling framing cause severe airflow restriction.

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Texas Insulation Requirements and IECC Energy Code Standards

Thermal performance of duct systems is regulated by the International Energy Conservation Code (IECC) as adopted across Texas municipalities (IRC Chapter 11 / IECC Section R403.3). Preventing thermal loss through duct walls is essential for system efficiency and humidity control in Texas climate zones (Zones 2 and 3).

Contractor Note: Installing R-6 ductwork in an unconditioned Texas attic where R-8 is required by energy code is a major code violation that will fail municipal building inspection.

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Duct Sealing Standards and Duct Leakage Testing (Duct Blaster)

Leaky ductwork in unconditioned attics draws hot, humid air into return runs and leaks conditioned cooling air into the attic, wasting energy and causing moisture damage.

Approved Sealing Materials (IMC 603.9 & IECC R403.3.3)

All joints, longitudinal seams, and transverse connections in duct systems must be mechanically fastened and sealed using approved materials:

• Rigid Ducts (Metal and Ductboard): Sealed with fiber-reinforced liquid mastic applied to a minimum wet thickness of 1/16 inch (often embedding glass fabric mesh) complying with UL 181A-M, or pressure-sensitive tape labeled UL 181A-P. Standard cloth-backed duct tape is strictly prohibited.
• Flexible Ducts: Secured to sheet metal collars using draw bands (nylon zip ties tightened with a tensioning tool or stainless steel worm-gear clamps) over mastic, and outer vapor barrier jacket sealed with tape labeled UL 181B-FX.

Duct Leakage Testing Procedures (Duct Blaster Test)

Duct leakage testing is required on all new residential HVAC installations where any portion of the duct system extends outside the conditioned space.

• Test Apparatus: A calibrated Duct Blaster fan is temporary mounted to a central return register while all supply and return grilles are sealed airtight with specialized masking film.
• Test Pressure: The system is pressurized or depressurized to a test static pressure of 25 Pascals (0.10 in. w.g.).
• Maximum Allowable Leakage Rates:
  • Post-Construction Total Leakage: Total duct leakage must not exceed 4 CFM25 per 100 sq ft of conditioned floor area served.
  • Rough-In Test (without Air Handler installed): Total duct leakage must not exceed 3 CFM25 per 100 sq ft of conditioned floor area served.

$\text{Max Allowed Duct Leakage (CFM}_{25}\text{)} = \frac{\text{Conditioned Floor Area (sq ft)}}{100} \times 4$

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Friction Loss and Equal Friction Sizing Fundamentals

Duct Sizing uses the Equal Friction Method, where ducts are sized so that the pressure loss per unit length remains constant throughout the supply and return system.

Friction Rate ($FR$) Sizing Target

The design friction rate represents the static pressure loss per 100 feet of equivalent duct length ($\text{in. w.g. / 100 ft}$).

• Standard residential duct sizing targets a friction rate of 0.10 in. w.g. per 100 ft on an ACCA Manual D duct slide rule (Ductulator).
• Quiet, high-efficiency low-resistance designs target 0.08 in. w.g. per 100 ft.

Recommended Maximum Air Velocity Limits (FPM)

To prevent air turbulence noise and excessive pressure drop, air velocities inside ducts must not exceed recommended limits: