ASHRAE 170 Ductwork Requirements: Materials, Lining, and Sealing for Healthcare Facilities

Key Takeaways

• ANSI/ASHRAE/ASHE Standard 170 sets minimum ventilation design requirements for hospitals, outpatient facilities, and nursing facilities, including air changes, pressure relationships, filtration, temperature, and humidity for each space type.
• The current edition is Standard 170 (2025 edition), which the 2026 FGI documents reference; many projects are still governed by earlier editions, so the authority having jurisdiction decides which one applies.
• Standard 170 prohibits duct lining downstream of the final filter bank, which pushes healthcare supply systems toward hard, cleanable metal duct surfaces.
• Galvanized steel is the usual default, stainless steel serves wet, corrosive, and laboratory exhaust, and aluminum or 300 series stainless serves nonferrous zones such as MRI suites.
• Duct sealing under IMC Section 603.9 and SMACNA seal classes protects the room pressure relationships Standard 170 requires.

What ASHRAE 170 Covers and Why Ductwork Matters to It

ASHRAE 170, formally ANSI/ASHRAE/ASHE Standard 170, Ventilation of Health Care Facilities, sets minimum ventilation system design requirements for environmental control in hospitals, outpatient facilities, and nursing facilities. It covers temperature, humidity, odor control, asepsis, and ventilation rates for more than 80 room types, and it addresses chemical, physical, and biological contaminants.

The standard is jointly developed by ASHRAE and the American Society for Health Care Engineering (ASHE), in coordination with the Facility Guidelines Institute (FGI). ASHRAE describes it as a set of minimum design requirements written for adoption by code enforcing agencies, not as a design guide.

Most coverage of Standard 170 focuses on the air side: air changes, pressure, and filters. Ductwork is where those numbers are either delivered or lost. A leaky duct cannot hold an operating room positive. A lined duct downstream of the final filter can harbor the contaminants the filter just removed. An exhaust duct in the wrong material can corrode in a wet or chemical exhaust stream.

This guide reads Standard 170 from the sheet metal side. It explains what the standard requires of each space, then translates those requirements into duct material, lining, sealing, and ordering decisions.

If you work across other critical environments, the same thinking applies to data halls. Our guide to HVAC ductwork requirements for data centers covers that vertical.

Which Edition Applies: ASHRAE 170 2021, 2025, and the FGI Guidelines

The edition of Standard 170 that governs a project is the one adopted by the authority having jurisdiction, which is not always the newest. Publication of a new edition does not make it law; adoption through FGI documents, state rules, or other codes does.

Standard 170 is revised on a four year cycle aligned with FGI. The 2021 edition incorporated 17 addenda to the 2017 edition and is referenced by the 2022 FGI Guidelines. The 2025 edition is now published and is referenced by the 2026 FGI documents.

Standard 170 also becomes enforceable through NFPA 99 and the International Mechanical Code, and accreditation bodies apply related expectations. Existing spaces are commonly held to the edition in force when they were built or last substantially renovated.

If a specification references an edition you have not worked with before, ask the engineer which sections affect ductwork. Changes between editions often land in exhaust discharge, filtration, and space types rather than in duct construction itself.

Air Changes and Pressure Relationships by Room Type

Standard 170 assigns each healthcare space a minimum total air change rate, a minimum outdoor air change rate, a pressure relationship to adjacent spaces, and in many cases a requirement that all room air be exhausted outdoors. These values drive duct sizing and determine which rooms need dedicated exhaust duct.

The table below summarizes selected spaces from the 2021 edition as amended. Always design to the edition and tables your project has adopted.

Pressure differentials of at least 0.01 inch of water column apply to operating rooms, Class 3 imaging rooms, and protective environment rooms (positive), and to airborne infection isolation and bronchoscopy rooms (negative).

Higher air change rates also mean larger supply duct. An operating room at 20 air changes needs far more supply air than a patient room of similar size at 4, so main and branch duct serving surgical suites is sized accordingly.

Filtration Requirements and What They Mean for Duct Layout

Standard 170 sets minimum filter efficiency by space type, and the location of the final filter determines which downstream duct must stay clean and unlined. Operating rooms, Cesarean delivery rooms, and Class 3 imaging rooms require MERV 16 filtration under the 2021 edition, up from earlier requirements.

For orthopedic, transplant, neurosurgery, and dedicated burn unit procedures, Standard 170 requires HEPA filters located in the air terminal device. Protective environment rooms and burn intensive care units also call for HEPA filtration.

Filter placement has direct duct consequences. Everything downstream of the final filter bank is clean air path. That stretch of duct must stay sealed so unfiltered air from ceiling plenums cannot be drawn in, and it must have interior surfaces that do not shed fibers or hold moisture.

In practice, that means designers and fabricators pay closest attention to supply duct between the air handler's final filters and the room diffusers. Joints are sealed to a high standard, access doors are placed so coils and dampers can be inspected, and lining is kept out of the airstream.

Supply diffusers in operating rooms, delivery rooms, trauma rooms, protective environments, and critical care spaces must also be cleaned before initial use. That cleaning starts with a clean installation: sealed duct openings during construction and clean metal surfaces at delivery.

Smooth round spiral duct is a common choice for long clean supply runs because it has fewer longitudinal seams to seal than rectangular duct of the same capacity.

Duct Lining, Insulation, and Interior Surfaces in Clinical Air Paths

Standard 170 prohibits duct lining downstream of the final filter bank in healthcare HVAC systems. The concern is that fibrous liner can trap dirt and moisture that support microbial growth, and can degrade and release fibers into the airstream over time.

The standard does allow lining with an impervious cover in specific components such as terminal units and sound attenuators. It also restricts duct liner for a defined distance downstream of humidifiers, where moisture carryover is likely.

The result is that most healthcare supply duct is bare metal inside, with insulation applied to the outside. External insulation on cooling duct needs a continuous vapor retarder. The International Mechanical Code (Section 604.11) calls for a vapor retarder of 0.05 perm or less on externally insulated cooling ducts, with joints sealed.

Many institutional design standards go further. Some university and hospital owner specifications ban internally lined duct and fibrous glass ductboard entirely, and allow double wall duct where acoustic control is needed.

For the fabricator, this means delivering duct with clean, smooth interior surfaces, sealed seams, and no liner unless the drawings call for a specific impervious or double wall product. For background on the default material, see galvanized steel ducts explained.

Where sound control is required without lining, designers often rely on duct layout, sound attenuators with impervious fill covers, and lower air velocities. These choices are made in design, so the fabricator’s role is to build exactly what the drawings show, without substituting lined sections.

Galvanized, Stainless, or Aluminum: Choosing Duct Material by Space

Healthcare ductwork material follows the air it carries: galvanized steel for most supply, return, and general exhaust; stainless steel for wet, corrosive, and laboratory exhaust; aluminum or 300 series stainless where nonferrous material is needed near MRI equipment.

Owner design standards confirm these patterns. One major university mechanical specification, for example, makes galvanized steel the default and requires stainless steel for laboratory exhaust, dishwasher exhaust, and other high humidity areas.

Gauge is a separate decision from material. Pressure class, duct size, and reinforcement set the gauge, as explained in the HVAC duct gauge selection guide. For a side by side comparison of the three metals, see our galvanized vs aluminum vs stainless steel material guide.

Mixing materials inside one building is normal. A single hospital wing can have galvanized supply duct, stainless dishwasher and laboratory exhaust, and aluminum duct in an imaging suite. Clear material callouts on the drawings keep each system from being built in the wrong metal.

Stainless Steel Ductwork for Exhaust, Wet, and Laboratory Spaces

Stainless steel ductwork is the standard choice in healthcare facilities wherever exhaust air is wet, corrosive, or chemically aggressive. It resists the moisture and cleaning chemicals that shorten the life of galvanized duct in those locations.

Typical stainless applications in hospitals include laboratory and fume hood exhaust, dishwasher and kitchen exhaust, and other high humidity exhaust runs. Owner specifications commonly call for Type 304 in dishwasher and wet exhaust and Type 316 in laboratory and process exhaust, with welded seams and joints.

Construction details matter as much as alloy. Wet exhaust duct is often sloped and drained so condensate does not pool. Welded or gasketed joints prevent leaks of contaminated air into occupied space. Finish requirements may differ between exposed and concealed sections.

Cost is the main trade off. Stainless steel costs more than galvanized per pound and takes more fabrication time, especially when welded. Limiting stainless to the runs that need it keeps budgets realistic. Our breakdown of what drives stainless steel spiral duct pricing explains the cost factors.

Duct2Go fabricates stainless steel ductwork for healthcare, food service, and laboratory environments through its custom fabrication service, built from your drawings in round or rectangular profiles. For rectangular runs, the stainless steel rectangular ductwork guide covers when that profile is the right fit.

Exposed stainless duct in clinical areas also benefits from its smooth, cleanable surface, which is one reason some owners specify it where duct runs are visible and cleaned regularly.

Aluminum Ductwork in MRI Suites and Other Nonferrous Zones

MRI suites need nonferrous ductwork near the magnet, which makes aluminum or 300 series stainless steel the usual choices. Ferrous metal in a strong magnetic field can be pulled toward the magnet and can distort image quality.

Engineering guidance for MRI rooms states that all ductwork, hangers, and supports inside the RF shield must be nonferrous. HVAC penetrations through the RF shield pass through RF filters or waveguides so outside radio frequencies cannot reach the scanner.

Air distribution guidance for MRI rooms recommends keeping ductwork and outlets outside the 50 gauss line where possible. Between the 50 gauss and 5 gauss lines, it recommends ducting and outlets made of nonferrous material such as aluminum or 300 grade stainless steel.

The MRI equipment vendor and the shielding contractor define the gauss lines and penetration details for each project. The duct fabricator's job is to supply the correct nonferrous material, fasteners, and supports for the zones shown on the drawings.

Outside MRI suites, aluminum also suits damp locations where its corrosion resistance and lighter weight help, though stainless is preferred for washdown and chemical exposure. Duct2Go builds aluminum ductwork to order through its custom fabrication service.

Exhaust Ductwork for Isolation Rooms and Critical Spaces

Airborne infection isolation rooms must be negative to adjacent spaces with all room air exhausted outdoors, so their exhaust duct must be tight, clearly routed, and protected from cross contamination with supply air. The same applies to bronchoscopy rooms and other spaces marked for full exhaust.

The 2021 edition of Standard 170 expanded the conditions under which AII room exhaust may discharge into a general exhaust system. The 2025 edition clarified stack discharge heights and separation distances between exhaust outlets and outdoor air intakes for complex situations. Both changes affect how exhaust duct is routed to the roof.

Leakage on the exhaust side matters as much as on the supply side. A leaking negative pressure duct inside a ceiling plenum draws plenum air in rather than pulling it from the room, which reduces the effective exhaust from the isolation room itself. CDC guidance on environmental infection control also notes that poorly maintained exhaust duct can reduce negative pressure and air changes over time.

From a fabrication standpoint, isolation and critical exhaust duct calls for sealed seams and joints, accessible test and balance points, and clear labeling. Round spiral duct and factory duct fittings reduce the number of field joints that need sealing.

Duct Sealing, Leakage Class, and Pressure Testing for Healthcare Projects

Healthcare duct systems must be sealed tightly enough to hold the pressure relationships Standard 170 requires. The International Mechanical Code (Section 603.9) requires duct joints, seams, and connections to be fastened and sealed with welds, gaskets, mastics, tapes, or other approved closures, installed per SMACNA construction standards.

SMACNA defines seal classes that specifications reference. Seal Class A covers transverse joints, longitudinal seams, and duct wall penetrations; Class B covers transverse joints and longitudinal seams; Class C covers transverse joints only. Institutional specifications, such as university design standards, often require Seal Class A on supply, return, and exhaust.

Specifications also assign pressure classes and leakage classes by system. As one example, a university specification sets laboratory and process exhaust at a 4 inch pressure class with leakage class 3, and requires welded seams and joints on laboratory, kitchen, and dishwasher exhaust.

Pressure testing verifies the result. Project specifications often require leakage testing of a portion or all of the duct before insulation and ceilings close it in. Testing is far easier when the duct arrives with factory sealed seams and consistent joint geometry.

Our plain English explainer on SMACNA compliance explains what SMACNA construction standards mean in practice.

Factory sealing also shortens field work. Duct that arrives with sealed longitudinal seams leaves installers to seal transverse joints and connections, which reduces labor in ceilings that may be crowded with medical gas, electrical, and plumbing services.

Codes Beyond ASHRAE 170: IMC, NFPA 99, FGI, and the AHJ

Healthcare ductwork answers to several documents at once. Standard 170 sets ventilation requirements, the FGI Guidelines set facility design requirements and reference Standard 170, NFPA 99 sets health care facility safety requirements, and the International Mechanical Code governs duct construction, sealing, insulation, and fire penetrations.

The IMC carries several duct requirements that apply in hospitals as in any building. Section 603.9 covers sealing, Section 603.10 points metal duct supports to SMACNA standards, Section 604.3 limits flame spread and smoke development of duct coverings and linings, and Section 607 covers duct penetrations of fire rated assemblies.

The authority having jurisdiction ties these together. State health departments and building officials decide which editions of FGI, Standard 170, NFPA 99, and the mechanical code apply to a project. Accrediting organizations then hold facilities to the ventilation conditions those documents define.

Owner design standards add another layer. Hospital systems and universities often publish their own mechanical specifications that are stricter than code, especially for duct lining, seal class, and exhaust materials. Those owner standards appear in the project specification the fabricator works from.

For industrial and process exhaust outside healthcare, our guide to the best materials for industrial air duct installation covers related material choices.

For renovations in occupied hospitals, Standard 170 also addresses ventilation during construction, which the 2025 edition reorganized. Temporary containment and duct protection during construction protect patients while new duct is installed.

Ordering Custom Healthcare Ductwork: What to Send Your Fabricator

The fastest way to get accurate healthcare ductwork is to send the fabricator complete, dimensioned information: shop drawings or plans, material and gauge by system, seal class, and any owner specification requirements. Clear inputs prevent the material and lining errors that are hardest to fix in a clinical building.

• Drawings or dimensioned plans. PDF shop drawings or plans with dimensions for each run, fitting, and transition.
• Material and gauge by system. Galvanized, stainless (with type), or aluminum for each supply, return, and exhaust system.
• Seal class and joint type. Seal class, welded or gasketed joints, and any leakage class targets.
• Lining requirements. Confirmation of unlined duct, or the specific double wall or impervious product called for.
• Special zones. Nonferrous requirements for MRI areas, and any sloped and drained wet exhaust runs.

Duct2Go fabricates complete duct system packages from PDF shop drawings or dimensioned plans, in galvanized steel, aluminum, or stainless steel, and ships nationwide from Chandler, Arizona. Quantity takeoffs are not part of the service, so send the drawings or a measured list. Our guide on how to measure for custom ductwork and what to include in a quote request shows exactly what to send.

Ventilation ductwork for healthcare facilities succeeds when the duct delivers what ASHRAE 170 designs for: clean air paths, tight seals, and the right metal in the right room. When your drawings are ready, Duct2Go's custom fabrication team can build the stainless, aluminum, and galvanized duct your project specifies, alongside standard rectangular and round duct.