19.2 Seams, Locks, Taping, Mastics & Liners
Key Takeaways
- A Pittsburgh lock is a formed pocket-and-insert longitudinal seam that is hammered or machine-closed; it is the default rectangular metal long seam up through typical 2- and 3-inch pressure classes.
- Button-punch snap lock is faster and has no closing hammer blow, but it is a lower-pressure residential/light-commercial lock; it is not a substitute for a Pittsburgh or standing seam on a 2-inch-class over-2-foot main.
- Standing seams and standing-S / TDC-TDF joints add a stiffening rib or flange; they show up on the over-2-foot and higher-pressure SMACNA rows, not as decoration.
- Seal Class A seals transverse joints, longitudinal seams, and duct-wall penetrations; Seal Class B seals joints and seams; Seal Class C seals joints only. SMACNA historically maps 4 in. w.g. and up to A, 3 in. to B, and 2 in. and under to C, but FBC Energy Conservation 2023 still wants ducts sealed, and many Florida specs require Class A on everything.
- Interior liner is adhesive plus mechanical fasteners (weld pins), metal nosing on leading edges in high-velocity airstreams, and a net-free-area penalty of twice the liner thickness on each internal dimension; fibrous-glass board closures are UL 181A-P, 181A-M, or 181A-H — cloth duct tape is not a listed closure.
19.2 Seams, Locks, Taping, Mastics & Liners
The same Trade Area B 9 percent that asked you to pick a gauge now asks you to tape seams, build standing seams, snap lock, and other locks and seams, and to fabricate interior-lined sheet metal. Open SMACNA HVAC Duct Construction Standards, 4th Edition (2020) for metal locks and seal class, and SMACNA Fibrous Glass Duct Construction Standards, 8th Edition (2021) for duct-board closures. FBC Mechanical Chapter 6 and FBC Energy Conservation R403.3 / C403 still want the finished duct sealed.
Quick Answer: Pittsburgh is the pocket-and-insert longitudinal lock you close with a hammer or machine. Snap lock (button-punch) snaps together for low-pressure residential work. Standing seams and TDC/TDF add a rib or flange for wide, high-pressure duct. Seal Class A seals joints, seams, and penetrations. Interior liner is adhesive plus pins plus metal nosing — it is not a roll of insulation stuffed in a trunk. Fibrous-glass board uses UL 181A tape or mastic, not cloth duct tape.
Pittsburgh lock
A Pittsburgh lock (SMACNA longitudinal type used on most rectangular coil-line duct) is a female pocket formed on one edge and a male flange formed on the mating edge. The male side is inserted into the pocket and the pocket is closed — hammer, screwdriver, or a Pittsburgh-closing rolls/machine. A well-closed Pittsburgh is a strong, relatively tight long seam through ordinary 2- and 3-inch HVAC classes when the gauge matches the table. Leaving the pocket open “because mastic will fill it” is not a closed Pittsburgh. The pocket sits on a corner; you do not run a Pittsburgh down the middle of a 36-inch panel and call it a standing seam.
Shop sequence on a coil line: form the two longs, form the two shorts (or wrap a single wrapper), snap or Pittsburgh the longs, then add the transverse joint profile (S-slip/drive, standing S, or TDC/TDF). On the exam, “Pittsburgh” means that pocket lock, not “any shop seam.”
Snap lock
Button-punch snap lock (and grooved snap lock) forms two edges so they snap together. There is no closing hammer blow. It is faster on residential and light-commercial 26-gauge duct. It is also easier to open under handling, roof traffic, or higher static. SMACNA and shop ratings typically keep snap lock on lower pressure classes — think 2 in. w.g. and under on modest widths, not a 4-inch-class 40-inch main. Substituting snap lock for a specified Pittsburgh on a 2-inch-class over-2-foot trunk is a failed submittal, not a labor savings.
Worked comparison. A 14×8 bedroom run at ½ in. w.g.: snap lock longs, S-slip and drive, Seal Class A mastic at the drives — legal residential construction. A 30×12 2-inch-class supply: Pittsburgh or standing seam longs, standing S or TDC, reinforcement per the table. Snap-locking that 30-inch wrapper because the button-punch machine is free is the Trade B trap.
Standing seams and other locks
A standing seam turns both edges up, engages them, and leaves a standing rib. That rib is both a lock and a stiffener. It appears on larger rectangular duct and on some high-pressure work. Do not confuse it with a Pittsburgh, which lies flat in the corner.
Transverse joints (the ends you connect in the field) are a different SMACNA family:
| Joint / seam | Where it lives | Exam trap |
|---|---|---|
| Pittsburgh | Longitudinal, rectangular corners | Leaving the pocket unclosed |
| Snap lock / button punch | Longitudinal, low-pressure | Using it on a 2-inch-class wide main |
| Grooved (Acme) lock | Longitudinal, some pipe and light duct | Treating it as a grease-duct weld |
| Standing seam | Longitudinal rib that stiffens | Calling any standing S-slip a “Pittsburgh” |
| S-slip and drive (T-1 / T-5) | Transverse, light gauge | Drive on the wide side with no standing reinforcement when the table wants a standing joint |
| Standing S, bar slip | Transverse, more stiffness | Using a flat drive as the only reinforcement on a 36-inch panel |
| TDC / TDF (T-25a / T-25b) | Transverse four-bolt flange, gasketed | Skipping the gasket and calling it sealed |
| Companion angle / welded flange | High pressure, industrial, grease | Screws through a grease-duct wall |
| Spiral lockseam / pipe snap lock | Round longitudinal | Counting the spiral as the three-screw transverse joint |
Welded seams belong on high-pressure industrial duct and on Type I grease duct (continuous liquid-tight external weld, FBC 506.3.2). Screws, rivets, and Pittsburgh locks shall not penetrate a grease-duct wall for supports.
Taping, mastics, and seal class
SMACNA seal class tells you what you seal; leakage class (cfm per 100 ft² of duct surface at a test pressure) tells you how tight the assembled system must test.
| Seal class | What you seal | Historical SMACNA pairing |
|---|---|---|
| A | Transverse joints, longitudinal seams, and duct-wall penetrations (spin-ins, damper rods, hanger rods that pierce the wall) | 4 in. w.g. and higher |
| B | Joints and seams | 3 in. w.g. |
| C | Transverse joints only | 2 in. w.g. and under |
Those historical pairings are the SMACNA default if the spec is silent. They are not a Florida energy-code holiday. FBC Energy Conservation 2023 requires ducts and air handlers to be sealed; many Division 23 specs simply say Seal Class A on every pressure class. On the exam, if the question states a pressure class and asks the SMACNA default, use the table above. If the question states an energy-code or “all ducts sealed” spec, seal joints, seams, and penetrations.
Mastic (water-based or solvent, listed for the use) is the primary sealant on metal joints. Brush or trowel it onto the assembled joint so it bridges the gap; a thin smear that you cannot see after paint is not a seal. Gaskets belong in TDC/TDF and companion flanges and must be compressed by the bolts. Listed foil tapes can close accessible metal joints when the listing and SMACNA/spec allow; SMACNA has long warned that ordinary pressure-sensitive tape is a poor primary sealant on metal, especially where the joint can move.
Fibrous glass duct board (8th Edition 2021) is a different closure system. All closures shall meet UL 181 or UL 181A. Printed markings:
- 181A-P — pressure-sensitive tape
- 181A-M — mastic
- 181A-H — heat-activated (heat-seal) tape
Flexible duct uses UL 181B (181B-FX tape, 181B-M mastic), plus a mechanical fastener (clamp or strap) — tape alone is not the listed flex connection. Cloth “duct tape” is not 181A or 181B. Using it on Florida attic duct is a failed inspection and a failed exam item.
Worked seal example. A 2-inch-class 24×12 trunk with Pittsburgh longs, TDC joints, and two spin-in taps: Seal Class A means mastic or gasket at every TDC, close and seal the Pittsburgh if the spec/class requires seams (Class A and B), and seal the spin-in collars where they pierce the wall. Sealing only the TDC (Class C) leaves the spin-ins leaking into a 130°F Florida attic.
Interior-lined sheet metal
Interior liner is acoustical/thermal insulation inside the metal. Typical HVAC liner is 1 inch (sometimes 2 inch), 1.5 pcf or 3 pcf, mat-faced so the airstream does not erode fibers. Installation, per SMACNA and the liner manufacturer:
- Adhesive on the metal, covering the specified percentage of the surface (often essentially full coverage except at fastener washers).
- Mechanical fasteners — welded pins or impact-welded nails with washers — typically starting about 4 inches from edges and corners and then on a grid (commonly on the order of 16 inches on center; follow the liner and SMACNA pin-spacing chart for the velocity).
- Metal nosing (channel or zee) on leading edges facing the fan, at fan discharges, and wherever velocity would lift the facing. High-velocity systems (on the order of 4,000 fpm and up, and always at the fan blast area) do not get a raw liner edge.
- Butt joints in the liner tight; no gaps that dump fibers into the coil.
Net free area: 1-inch liner on all four sides of a 20×12 duct leaves 18×10 of air path. Size the metal up, or you just stole the Chapter 17 cfm. A designer who says “20×12 lined” usually means 20×12 inside metal with liner eating the area unless the schedule says “clear inside.”
Do not line Type I grease duct, dishwasher-steam Type II that the listing forbids, outdoor duct without a weather jacket, or a duct that will run wet (untreated outdoor air without a drain). NFPA 90A (2015) still wants liner with a flame-spread index not over 25 and smoke-developed not over 50. Lined duct is not fibrous-glass board; board is the 8th Edition 2021 book and has its own tie-rod and closure details.
Florida HVAC scenario
A Class A shop in Broward fabricates a 3-inch-class 40×20 supply off a large air handler (over 25 tons — Class A statutory territory) and a 1-inch-class 20×12 return. The 40×20 gets Pittsburgh longs and TDC with gasket, Seal Class A (joints, seams, penetrations), 1-inch liner with pins and a metal nosing at the fan discharge, and the metal is oversized so the net inside matches the Ductulator size. The 20×12 return is snap-lock, S-slip and drive, unlined, Seal Class A because the energy spec said so even though SMACNA’s silent default at 1 inch is Class C. The helper wants to close the fibrous-glass outside wrap joints with cloth tape leftover from a flex job. Wrong book: exterior wrap is a jacket, not 181A board closure, and cloth tape is still not 181A-P. Class B candidates still need this theory; the Class B outline does not drop Trade B, it drops the 25–100 ton install cluster.
Traps: (1) Snap lock on a specified Pittsburgh. (2) Seal Class C on a spec that said A. (3) Liner with adhesive and no pins. (4) Forgetting the 2× liner-thickness area penalty. (5) 181B flex tape on 181A duct board, or cloth tape on either.
Which description correctly distinguishes a Pittsburgh lock from a button-punch snap lock on rectangular HVAC duct?
A specification calls for SMACNA Seal Class A on a 2 in. w.g. supply trunk with Pittsburgh longitudinal seams, TDC transverse joints, and spin-in taps. What must be sealed?
Which statement about interior-lined sheet metal and fibrous-glass closures is accurate for the Florida exam?