IMC 603.4 · SMACNA · ACCA Manuals J/S/D

How duct construction is chosen — IMC 603.4, SMACNA & Manual J/S/D

Pressure classes, gauge and reinforcement, leakage classes and the leakage formula, and the load → equipment → duct design sequence, with worked examples and a link to the free ICC viewer.

Need the code text? Read IMC Section 603 in the free ICC code viewer →

How to pick duct construction from the SMACNA standard, step by step

Step	What you do	Watch out for
1	Get the duct pressure class from the drawings or spec (positive or negative, in inches of water gauge)	If it isn’t shown, send an RFI — the standard has a default, but guessing is how you end up re-hanging duct
2	Find the rectangular (or round) construction table for that pressure class	Positive and negative pressure use different tables for rectangular duct
3	Enter with the largest duct dimension and choose a reinforcement spacing	Gauge and reinforcement trade off: heavier metal can mean farther-apart reinforcement
4	Read the minimum sheet gauge and the reinforcement class for that combination	Joint types count as reinforcement only if the table says they qualify
5	Apply the sealing requirement (seal class) the spec or energy code calls for	Energy codes generally require sealing all joints and seams regardless of pressure
6	Check hangers and supports from the hanger section, not the construction tables	Hanger size/spacing is a separate lookup

Our own summary, not the code or standard text. Read IMC Section 603 in the free ICC viewer (codes.iccsafe.org); SMACNA and ACCA manuals are sold by their publishers. Verify against the edition adopted by your AHJ and the project specs.

Which rule answers the question?

Question	Where to look	Notes
What must commercial metal duct be built to?	IMC 603.4 → SMACNA HVAC Duct Construction Standards — Metal and Flexible	The IMC adopts the SMACNA standard by reference
Thinner metal inside a single dwelling unit?	IMC Table 603.4 (dwelling-unit duct thickness)	Limited to small, low-pressure residential ducts
How many screws at a round joint?	IMC 603.4.1	Equally spaced so the joint can’t hinge
Does the duct need a leakage test?	Energy code (IECC C403 / R403, ASHRAE 90.1) and the spec	Usually tied to higher pressure classes or a spec percentage
What leakage is allowed in the test?	SMACNA HVAC Air Duct Leakage Test Manual + the spec’s leakage class	Leakage class × test pressure formula (below)
What size is each duct run?	ACCA Manual D (residential) or ASHRAE/SMACNA design methods (commercial)	Sizing is design — construction tables don’t size ducts

ACCA residential design sequence: J → S → D

Manual	What it answers	What it hands to the next step
Manual J	Room-by-room heating and cooling loads	Block load for equipment; room loads for airflow split
Manual S	Which equipment actually meets the load at design conditions	Blower airflow and its external static pressure rating
Manual D	Duct sizes that deliver each room’s airflow within the blower’s pressure budget	Duct sizes, fittings, and register airflow for install and balance
Manual T	Supply outlet and return grille selection	Throw, noise, and face velocity checks

Manual D comes after equipment selection because it needs the selected blower’s available pressure. Consult ACCA’s manuals for the procedures.

Worked examples — leakage allowance and friction rate (illustrative inputs)

Input / step	Value	Source
Spec leakage class (CL)	4 cfm/100 ft² at 1 in. w.g.	Assumed project spec
Test pressure (P)	2 in. w.g.	Duct pressure class
F = CL × P^0.65 = 4 × 2^0.65	≈ 6.3 cfm per 100 ft²	SMACNA leakage relationship
Section surface area tested	1,500 ft²	Takeoff of the test section
Allowed leakage for the section	≈ 94 cfm	6.3 × 15
Manual D: available static pressure (ASP) after coil, filter, grilles	0.20 in. w.c.	Assumed, from blower data
Total effective length (longest run + fitting equivalents)	250 ft	Assumed layout
Design friction rate = ASP × 100 ÷ TEL	0.08 in. w.c. per 100 ft	Manual D method

Illustrative inputs, not requirements. Your spec sets the leakage class and test pressure; your equipment data sets the pressure budget.

Common field mistakes

Mistake	What to do instead
Building everything to the lightest pressure class	Duct near the fan and on the discharge of VAV systems is often a higher class — read the drawings
Treating a sealed duct as automatically tight	Seal class and leakage class are different things; the test is what proves it
Sizing ducts off a rule-of-thumb cfm-per-ton	Run Manual J, pick equipment with Manual S, then size with Manual D
Using the full blower static rating as the duct budget	Subtract coil, filter, and grille losses first — what’s left is the ASP for the ducts
Skipping the leakage test until ceilings are closed	Test sections before insulation and ceiling close-in so you can still fix them

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From pressure class to a tight, balanced system

Read IMC Section 603 in the free ICC code viewer. The code section is short because it hands the heavy lifting to SMACNA’s HVAC Duct Construction Standards, which SMACNA sells. ACCA sells Manuals J, S, D, and T. This page explains how the pieces fit; it does not reproduce any of them. Edition referenced: 2021/2024 IMC — verify against the edition adopted by your AHJ and your spec.

Pressure class drives everything. SMACNA groups duct by the static pressure it will see, from low-pressure residential-style runs up through high-pressure mains, and separately for positive and negative pressure. For rectangular duct, the construction tables then let you trade sheet thickness against reinforcement spacing — heavier metal with fewer reinforcements, or lighter metal with more. Round duct is inherently stiffer and follows its own tables.

Leakage classes are about the test, not the build. A leakage class (CL) is the allowable leakage per 100 ft² of duct surface at 1 in. w.g. At any other test pressure, the allowance is CL times the pressure to the 0.65 power. So a CL 4 spec tested at 2 in. w.g. allows about 6.3 cfm per 100 ft²; a 1,500 ft² test section can leak about 94 cfm and pass.

Residential design sequence. Manual J first (room-by-room loads), Manual S second (equipment that meets those loads), Manual D third (duct sizes). D comes last because its friction rate depends on the selected blower: take the blower’s external static rating, subtract the coil, filter, and grille losses to get the available static pressure, multiply by 100, and divide by the total effective length of the longest run. With 0.20 in. w.c. available and 250 ft of effective length, the design friction rate is 0.08 in. w.c. per 100 ft.

Related references: the IMC/ASHRAE 62.1 ventilation rates guide and the IECC insulation and climate zone guide. Field PM tracks duct leakage test reports, submittals, and punch lists by area, so the TAB and test paperwork is ready at turnover.

FAQ

What does IMC 603.4 require for metal duct?+

It requires metallic ducts to be built to the SMACNA HVAC Duct Construction Standards — Metal and Flexible, with a separate thickness table allowed for ducts inside a single dwelling unit. Read the section free in the ICC viewer at codes.iccsafe.org.

How do I find the right duct gauge?+

Start with the pressure class on the drawings, open the SMACNA construction table for that class, enter with the duct’s largest dimension and your chosen reinforcement spacing, and read the minimum gauge and reinforcement. The table is in the SMACNA standard; we don’t reproduce it.

What is a duct leakage class?+

It is the allowable leakage in cfm per 100 ft² of duct surface at 1 in. w.g. Allowed leakage at the actual test pressure scales with pressure raised to the 0.65 power, so a leakage class of 4 tested at 2 in. w.g. allows about 6.3 cfm per 100 ft².

What order do Manual J, S, and D go in?+

J, then S, then D. Manual J gives the loads, Manual S picks equipment that meets them, and Manual D sizes the ducts to fit the selected blower’s available static pressure. Manual T then picks the registers and grilles.

Does residential duct need a leakage test?+

Recent IECC editions require total duct leakage testing for most new homes, with exceptions such as ducts entirely inside the conditioned envelope in some editions. Check the edition your AHJ adopted.

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