Cleanroom HVAC Ducting (GI) — Precision Airflow Integrity for Controlled Environments
The ductwork is the circulatory system of the cleanroom HVAC — the network through which treated, filtered air travels from the AHU to every HEPA housing and diffuser in the ceiling. A standard ventilation duct that leaks 5–10% of its air volume is adequate for comfort HVAC. In a cleanroom, even a 1% air leakage through an unsealed joint allows unconditioned, unfiltered air to bypass the filtration train and potentially collapse the pressure differential between grades. Metrolabs GI ducting is precision-fabricated and pressure-tested to SMACNA Class A — the highest cleanroom standard — delivering every cubic metre of filtered air to its intended destination.
Why Standard Ventilation Ducting Cannot Be Used in a Cleanroom
Standard HVAC ductwork is designed for comfort ventilation — where a 5–10% air leakage at joints is acceptable because the only consequence is slightly higher energy bills. In a cleanroom, the consequences of duct leakage are fundamentally different and potentially catastrophic for product quality and regulatory compliance.
Leakage Collapses the Pressure Cascade
Every Pascal of pressure differential between cleanroom grades is maintained by delivering precisely calculated volumes of conditioned air through the ductwork. A leaking joint downstream of the fan delivers less air to the cleanroom than intended — reducing supply volume, dropping differential pressure between zones, and potentially reversing the pressure gradient that prevents contamination from flowing from lower grades to higher grades.
Internal Fibres = Immediate ISO Class Failure
Standard ductwork insulated on the interior with fibreglass or mineral wool releases microfibres into the airstream continuously. Each fibre is a particle that contributes to the ISO class particle count. A single 25mm thickness of interior fibreglass insulation sheds millions of fibres per hour into the supply air — rendering even H14 HEPA filtration meaningless if fibres are shed downstream of the terminal filter. Metrolabs uses external closed-cell insulation only — zero fibrous material contacts the air stream at any point.
Oily or Contaminated Internal Surfaces
Standard GI sheets are often delivered with a light oil coating from the rolling mill — used to prevent oxidation during transport. If this oil is not removed before duct installation, it off-gasses volatile organic compounds into the supply air and traps particles on the duct internal surface. Metrolabs degreases and inspects all duct sections before assembly and caps every open end during transport to site to prevent construction dust ingress.
Vibration Generates Particles
Standard ducting that is not correctly stiffened “oil-cans” — flexing back and forth under the pressure variations of the air handling system. This repeated flexing causes the zinc coating to micro-crack and shed particles into the airstream, and causes the sealant at joints to fatigue and crack, opening leakage pathways. Heavy-gauge G-120 GI with TDF flanged joints eliminates oil-canning and provides a mechanically rigid duct system that does not vibrate under cleanroom HVAC operating conditions.
Six Engineering Features That Define Cleanroom-Grade GI Ducting
Metrolabs GI ducting for cleanrooms incorporates six specific engineering requirements that differentiate it from standard ventilation ductwork:
G-90 / G-120 Galvanized Steel — Heavy Zinc Coating
Metrolabs specifies premium G-90 (275g/m² total zinc) or G-120 (365g/m² total zinc) galvanized steel sheets — significantly heavier zinc deposits than standard G-60 commercial duct material. The heavier zinc coating provides superior corrosion resistance in the moisture-laden conditioned air environment and structural rigidity that prevents oil-canning under HVAC pressure. G-120 is specified for pharmaceutical facilities with aggressive disinfectant fogging cycles and for sections running through high-humidity zones.
Smooth Internal Bore — Pittsburgh Seam Construction
The longitudinal seam of every Metrolabs cleanroom duct section is formed using the Pittsburgh seam — a mechanically locked fold that creates a smooth, flush internal surface with no projecting burrs, sharp edges, or voids that could trap particles or microbial contamination. The internal surface is inspected after fabrication for any residual swarf, weld spatter, or surface contamination before sections are capped and dispatched. A smooth internal bore also reduces friction losses and HVAC fan energy requirements compared to spiral wound duct with visible internal seams.
Transverse Duct Flange (TDF) Joints
Each duct section is joined to the next using Transverse Duct Flanges (TDF) — roll-formed steel flanges that interlock and are secured with corner clips and drive cleats. An anti-fungal, non-toxic, VHP-compatible gasket is seated in the TDF profile before assembly, creating a continuous compressed gasket seal around the full perimeter of each joint. This system achieves SMACNA Class A air leakage performance (<1% of design airflow). Alternative gasketed slip couplings are used for short connecting sections and flexible connections to AHU fans.
External Insulation Only — No Internal Fibres
All thermal insulation on Metrolabs cleanroom ductwork is applied externally — closed-cell nitrile rubber (NBR) or cross-linked polyethylene (XPE) foam, 25mm or 32mm thickness. Closed-cell insulation does not absorb moisture, does not support microbial growth, and releases zero fibres into the airstream. No fibreglass, no mineral wool, and no internal duct lining of any kind is used — eliminating the particle and fibre contamination risk that interior insulation creates in every standard comfort HVAC installation.
VHP-Compatible Sealants & Anti-Fungal Gaskets
For pharmaceutical facilities that use room-wide Vaporized Hydrogen Peroxide (VHP) sterilisation cycles, standard duct sealants and gaskets are incompatible — VHP at concentrations of 200–1000 ppm rapidly degrades standard EPDM and PVC-based gaskets, causing them to swell, crack, and lose sealing performance within 12–24 months. Metrolabs specifies VHP-resistant silicone gaskets and sealants throughout — tested to maintain their sealing performance after 500+ VHP cycles. Anti-fungal formulation prevents biofilm growth on gasket surfaces in humid duct environments.
Airtight Access Doors & Tapered Transitions
Strategically located airtight access doors (hinged panels with gasketed perimeter seals and quarter-turn fasteners) provide inspection and cleaning access without compromising duct integrity — critical for periodic PAO smoke testing of HEPA housings and for duct internal cleaning validation during HVAC qualification. Tapered transitions between duct cross-sections are formed with smooth curves rather than abrupt angles, reducing turbulence, noise, and the particle generation associated with high-velocity flow separation at sharp duct transitions.
Metrolabs GI Ducting — Technical Standards & Performance
Every Metrolabs cleanroom duct project is designed, fabricated, and installed to the following technical parameters:
| Feature | Metrolabs Standard | Cleanroom Benefit |
|---|---|---|
| MaterialGI sheet specification | G-90 / G-120 GI | Maximum zinc coat — corrosion & rigidity |
| Fabrication MethodManufacturing process | CNC factory / Pittsburgh seam | Precision fit — minimal site debris |
| Joint SystemSection connection | TDF flanged — gasketed | SMACNA Class A — hermetic seal |
| SealantJoint sealant formulation | Anti-fungal, non-toxic | VHP-compatible — no off-gassing |
| Leakage StandardAir tightness class | SMACNA Class A / DW 144 | Zero pressure loss to leakage |
| Internal SurfaceBore condition | Smooth, degreased, capped | Zero particle shedding or off-gas |
| Insulation TypeThermal treatment | Nitrile / XPE (external) | No internal fibres — no condensation |
| Access DoorsMaintenance provision | Gasketed, airtight, gasket-sealed | Validation access — integrity maintained |
External Insulation — Three Closed-Cell Options
All insulation on Metrolabs cleanroom ductwork is applied externally and uses closed-cell foam materials — preventing both condensation and internal fibre contamination:
Nitrile Rubber (NBR) Sheet — Standard Pharmaceutical
Closed-cell nitrile rubber (Armaflex-type) is the standard external insulation for pharmaceutical cleanroom ductwork. Self-adhesive application with butt-jointed seams sealed with compatible adhesive — creating a continuous vapour barrier that prevents condensation on cold duct sections passing through warm spaces. Resistant to most cleaning agents and disinfectants used in pharmaceutical environments. Specified in 25mm thickness for supply ductwork and 32mm for chilled water pipework sections.
Cross-Linked Polyethylene (XPE) Foam
XPE closed-cell foam provides excellent thermal insulation properties with a lower thermal conductivity than NBR (λ≈0.033–0.036 W/mK vs NBR λ≈0.036–0.040 W/mK), making it the preferred choice where minimising condensation risk is the primary concern — particularly for chilled ductwork in high-humidity production areas. Available in faced aluminium foil versions for additional vapour barrier performance in high-dewpoint environments.
PIR / PUR Board — Cold Room & Cleanroom Roof Ductwork
For ductwork sections running through plant rooms above cold room cleanrooms or in outdoor covered areas, Metrolabs specifies polyisocyanurate (PIR) or polyurethane (PUR) rigid board insulation — pre-cut and adhered to the duct exterior with glass cloth reinforced aluminium foil facing. PIR board provides Class B fire performance, significantly higher compressive strength than foam rolls, and lower λ-value (≈0.022 W/mK) for maximum thermal performance in challenging environmental conditions.
Understanding SMACNA Duct Leakage Classification
The SMACNA (Sheet Metal and Air Conditioning Contractors’ National Association) standard classifies ductwork by maximum permissible leakage. Cleanrooms require Class A — the most stringent classification:
| Class | Max Leakage (CL) | Application |
|---|---|---|
| Class A | CL = 6 (lowest) | ✓ Metrolabs Standard — Cleanrooms, pharmaceutical, aseptic |
| Class B | CL = 12 | High-importance commercial — hospitals, data centres |
| Class C | CL = 24 | Standard commercial buildings — offices, retail |
| Class D | CL = 48 | Industrial ventilation — warehouses, manufacturing |
Metrolabs GI Ducting in Every Regulated Cleanroom Sector
Pharmaceutical GMP Suites
SMACNA Class A GI ducting maintaining air delivery from AHU to terminal HEPA for Grade A through D — VHP-compatible gaskets and sealants throughout.
Aseptic & Sterile Manufacturing
G-120 heavy-gauge GI ductwork for high-ACH aseptic filling lines — zero leakage ensures laminar H14 HEPA supply is not diluted by unfiltered air ingress.
Biotech & ATMP Facilities
Factory-fabricated GI duct systems for gene therapy and ATMP GMP cleanrooms — SMACNA Class A tested and documented for ISO 14644 and EU Annex 1 compliance.
Hospital HVAC Systems
GI ducting for operating theatre positive pressure supply and isolation room negative pressure exhaust — TDF flanged for leak-free integrity in critical healthcare environments.
BSL-2 & BSL-3 Labs
Negative pressure exhaust ductwork in G-120 GI with HEPA-filtered discharge — SMACNA Class A leak-tested to ensure no containment breach through duct wall or joints.
Food Processing Cleanrooms
GI supply ducting for FSSAI and HACCP high-care food production — anti-fungal gaskets, degreased internal bore, external NBR insulation, no internal fibrous material.
Electronics Cleanrooms
Precision GI ductwork for ISO Class 4–7 semiconductor and electronics cleanrooms — smooth bore, rigid construction, zero particle generation from oil-canning or vibration.
Research & NABL Labs
GI duct systems for NABL-accredited controlled environment research facilities — SMACNA pressure-tested and documented for accreditation submission records.
Clean Installation Protocol — From Factory to Commissioned Duct System
Long-term cleanroom HVAC performance begins not with commissioning — but with how the ductwork is handled from the moment it leaves the factory to the moment it is pressurised and tested on site:
Capped & Sealed — Every Section from Factory
Every duct section leaves the Metrolabs factory with both open ends capped with polythene and taped — preventing construction dust, insects, and moisture ingress during transport and site storage. Caps are removed only at the moment each section is connected, and the inside of the section is visually inspected before joining. This clean-site protocol is the most important single factor in ensuring a contamination-free commissioned duct system.
Factory Fabrication — Minimal Site Cutting
Metrolabs duct sections are CNC-fabricated to the approved ductwork layout drawing dimensions — meaning the vast majority of sections arrive on site ready to connect without modification. Site cutting is kept to an absolute minimum to reduce the metal swarf and debris that falls inside duct sections during site fabrication. Where site cutting is unavoidable, the cut section is immediately cleared of swarf and re-capped before installation.
SMACNA Class A Pressure Test — Documented
After the complete ductwork system is installed and all joints sealed, Metrolabs conducts a SMACNA Class A pressure test — pressurising the system and measuring total leakage with calibrated equipment. The test result, compared against the SMACNA Class A allowable leakage limit for the installed duct surface area, is documented and included in the HVAC IQ record at project handover.
VHP Compatibility — Verified Before Installation
All sealants, gaskets, and insulation materials used in pharmaceutical duct systems are verified for VHP compatibility before procurement. Material data sheets confirming VHP resistance are included in the IQ documentation package — providing the QA team with documented evidence that the ductwork will maintain its integrity through repeated room-wide VHP sterilization cycles.
Metrolabs GI Ducting vs. Standard Comfort Ductwork
Clean-Site Duct Installation — Four Stages from Factory to Commissioned System
Metrolabs follows a strict clean-site protocol for every cleanroom duct installation — from factory fabrication through SMACNA pressure test and IQ documentation:
Design & Factory Fabrication
Ductwork layout designed to approved cleanroom plan. All sections CNC-fabricated to exact dimensions. Pittsburgh seam formed. Internal bore inspected. Both ends capped with polythene and taped before dispatch. Sections marked for installation position.
Clean-Site Delivery & Storage
Duct sections stored under cover with caps intact until installation. Caps removed only at moment of connection. Internal inspection before joining confirms bore is clean and free of debris. No on-site cutting or modification unless unavoidable.
Installation, Sealing & Insulation
Sections hung and supported to approved layout. TDF flanges connected with corner clips and drive cleats. Anti-fungal VHP-compatible sealant applied. Gasket compressed. Airtight access doors fitted at specified positions. External insulation applied after joint sealing.
SMACNA Pressure Test & IQ
Complete ductwork system pressurised to specified test pressure. Leakage measured against SMACNA Class A limit for installed duct surface area. Test report issued. Material data sheets (GI grade, sealant, gasket, insulation) compiled into HVAC IQ documentation.
Cleanroom GI Ducting Required by Every Major Regulatory Framework
WHO & EU GMP
WHO GMP and EU Annex 1 require that HVAC systems maintain the design pressure differentials between cleanroom grades at all times. This is only achievable if the ductwork delivering air to each zone is airtight — SMACNA Class A leak-tested GI ducting is the engineering standard that ensures this requirement is met.
WHO GMP · EU Annex 1SMACNA & DW/144
SMACNA (US) and DW/144 (UK) are the internationally recognised ductwork leakage classification standards. Class A (SMACNA) / Class C (DW/144) are the minimum specifications for pharmaceutical cleanroom ductwork — Metrolabs designs, installs, and pressure-tests all cleanroom ductwork to these standards with documented test results.
SMACNA Class A · DW/144ISO 14644 & GMP Validation
ISO 14644-4 (cleanroom design) requires that air supply systems deliver the specified air change rates and pressure differentials — which is only verifiable if the duct system is leak-tested. The SMACNA pressure test record is a standard component of ISO 14644-4 HVAC design documentation and GMP Installation Qualification.
ISO 14644-4NABL & FSSAI
NABL laboratory accreditation and FSSAI food facility licensing require documented HVAC systems that demonstrably deliver controlled air conditions. Metrolabs GI ducting with SMACNA pressure test documentation provides the evidence of system integrity needed for both accreditation bodies.
NABL · FSSAI · HACCPReady to Commission SMACNA Class A Cleanroom Ductwork?
Contact Metrolabs for a free consultation. Our HVAC team will assess your cleanroom duct layout requirements, ISO class, pressure test standard, and VHP compatibility needs — and design, fabricate, install, and pressure-test a complete GI duct system with SMACNA Class A documentation at handover.
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