British Columbia
CHOOSING LINED OR DOUBLE-WALL DUCT AND SIZING IT RIGHT
Lined duct is the usual pick for quieter rooms. Double-wall keeps the insulation behind metal, for air that may touch only metal or runs where lining could get wet.
Use double-wall duct when the air may touch only metal, or moves faster than lining is rated for. It also suits runs where lining could get wet. For most quiet runs to offices and classrooms, lined duct does the job with less metal and weight. Either way, the insulation takes up room, so mark each size on the order as the airway inside or the metal outside. This guide covers lining types and thicknesses, where lining isn't used, and what to write.
Three ways of insulating a duct do three different jobs. The first question is whether you're fighting noise, heat, or both.
| Option | What it is | Commonly used for | What it does for noise |
|---|---|---|---|
| Lined duct | A layer of sound-absorbing lining glued and pinned to the inside of the metal | Supply and return duct to occupied rooms where fan noise carries | Soaks up fan noise travelling along the duct, the higher-pitched part most |
| Double-wall duct | Insulation sealed between an inner metal wall and an outer metal wall | Hospitals, labs, food plants, and runs in unheated space or outdoors | Soaks up noise when the inner wall is perforated with small holes. Check the maker's data for a solid one |
| Wrapped duct | Insulation with a vapour retarder wrapped around the outside | Keeping heat in or out, and stopping condensation on the duct | Very little, because the insulation sits outside the air |
Where the insulation sits in each kind of duct
Cross-sections, not to scale.
Lined duct
Lining glued and pinned inside one metal wall. The air touches the lining.
Double-wall duct
Insulation between two metal walls. The air touches only the inner wall, perforated or solid.
Wrapped duct
Insulation and a vapour retarder around the outside. The air touches bare metal.
Wrap and lining can go on the same run when a job needs both quiet and condensation control. The order just has to say which one, or both.
On lined duct, the sizes on the drawings usually mean the space the air flows through, inside the lining. That one detail decides how big the metal has to be.
| Airway on the drawing | Lining thickness | Metal size it's built to |
|---|---|---|
| 24 x 12 in. | ½ in. | 25 x 13 in. |
| 24 x 12 in. | 1 in. | 26 x 14 in. |
| 24 x 12 in. | 2 in. | 28 x 16 in. |
Airway size and metal size on one lined duct
A 24 x 12 in. airway with 1 in. lining. The metal grows by twice the lining thickness each way.
1 in. of lining on every side, between the metal and the air
If the metal were built to 24 x 12 in. instead, the air would get a 22 x 10 in. opening. Write "airway" beside a lined size on the order.
Double-wall works the same way, and makers size it by the inner wall. Write "airway" beside lined sizes and "inner" beside double-wall sizes, and nobody has to guess.
Most lining is one of two materials. Fibreglass absorbs more sound, and foam handles water and cleaning better. NRC, the noise reduction coefficient, is the share of sound a surface soaks up, from 0 for none to about 1 for nearly all.
Fibreglass lining is a glass fibre mat with a coated face toward the air. One maker lists NRC 0.70 at 1 in., and it's rated for air up to 30.5 m/s (6,000 ft/min). If it ever gets soaked, it has to come out. Commonly used for supply and return ducts to occupied rooms.
Closed-cell foam lining is a rubber-like foam that doesn't soak up water and wipes clean. One maker lists NRC 0.40 at 1 in., less than fibreglass absorbs. Commonly used where moisture or cleaning matters more than the most sound absorption.
The National Building Code, which the BC Building Code is based on, limits foamed plastic in duct systems. If your spec names a foam lining, the engineer and the building official confirm it's accepted before it goes on the order.
The engineer picks the thickness from the duct size, the kind of noise, and how much quieter the room has to be. Thicker lining absorbs more, and every extra half inch makes the metal bigger.
| Thickness | Sound absorbed (NRC) | Insulating value | Added to each dimension of the metal |
|---|---|---|---|
| ½ in. (13 mm) | 0.50 | RSI 0.37 (R-2.1) | 1 in. |
| 1 in. (25 mm) | 0.70 | RSI 0.74 (R-4.2) | 2 in. |
| 1½ in. (38 mm) | 0.80 | RSI 1.06 (R-6.0) | 3 in. |
| 2 in. (51 mm) | 0.95 | RSI 1.41 (R-8.0) | 4 in. |
These figures are one maker's published data for a common fibreglass lining. Check the data sheet for the product your spec names. Lining works far better on a hiss than on a low rumble. The same maker's 1 in. lining soaks up about 0.85 of the sound at 1000 Hz, but only 0.18 at 125 Hz.
Lining stays out of runs that carry grease or very hot air, or that could get wet. It also stops at fire separations and in hospital rooms with stricter rules, where bare metal or double-wall goes instead.
If the duct carries grease, fumes, or dust, it stays unlined. Lining would trap what the air is meant to carry away. Kitchen grease duct also follows NFPA 96, with its own rules for the metal and the joints.
If the lining could get wet, it can grow mould and has to be replaced. The liner standard keeps fibreglass away from cooling coils and humidifiers unless it's protected. Ask the engineer what goes there.
If the air runs above 120 °C, the building code calls for a lining that can't burn. Lining also stops where the run passes through a fire separation, and it never goes inside a fire damper sleeve. Mark fire separations and fire dampers on the order.
The liner standard advises against lining for operating rooms, delivery and recovery rooms, nurseries, isolation rooms, and intensive care. CSA Z317.2 sets the HVAC rules for Canadian health care buildings, and the engineer's spec says which runs can be lined.
Double-wall duct puts the insulation between two metal walls, and the air only ever touches metal. Each of the three layers does its own job.
The outer wall is solid metal and does the work a single-wall duct would. If the run has a pressure class, it applies to this wall, as covered in duct gauge and pressure class.
The insulation fills the space between the walls, often behind a thin fabric that keeps fibres out of the air. Say which thickness the drawings call for. It sets the outer size.
A perforated inner wall is punched with small holes that let sound reach the insulation, and a solid one keeps the air sealed from it. Makers often perforate straight lengths and leave fittings solid, so name the one you need on each piece.
Double-wall uses more metal and weighs more than lined duct. It pays off where the air may touch only metal, moves fast, or could wet a lining. If none of these fits a run, lining is usually enough there.
| Situation on the job | How lined duct handles it | How double-wall handles it |
|---|---|---|
| Air that may touch only metal, in hospitals, labs, or food plants | Not suited, because the lining faces the air | Suited, because only metal faces the air |
| Air faster than 30.5 m/s (6,000 ft/min) | Not suited, because common fibreglass lining isn't rated that fast | Suited, because the inner wall holds the insulation in place |
| Runs in unheated space or outdoors | Possible, if the engineer says what the outside needs | A common choice for these runs |
| Large duct where the ceiling can only carry so much weight | Lighter, with one metal wall | Heavier, with two walls and the insulation |
| Ordinary supply and return to offices and classrooms | The usual choice | Works, but with more metal than the job needs |
Lining is held in place by more than glue, and the extra parts depend on how fast the air moves. Tell us the air speed and where the lined section starts, and each of these can be built in.
The liner standard calls for adhesive over at least 90% of the metal, and some specs ask for full coverage. If your spec names a coverage or an adhesive, send that line with the order.
Metal pins are fixed through the lining at set spacing, closer together where the air goes by faster. They can't squash it by more than 3 mm, because a crushed spot absorbs less sound.
Nosing is a metal cap over the upstream edge of the lining. The liner standard calls for it where the air passes 20.3 m/s (4,000 ft/min), at the fan outlet, and where lining starts after a bare section.
Cut edges of fibreglass lining that face the air without nosing get a coating. It keeps the air from picking at loose fibres. If the spec asks for a particular coating, list it.
Each of these orders names lining or double-wall but leaves out the detail that sets the build. The right-hand column shows the line that settles it.
| What the order says | What goes wrong | What to write instead |
|---|---|---|
| "24 x 12, lined" | Nobody knows if 24 x 12 is the airway or the metal | "24 x 12 airway, 1 in. lining" |
| "Lined" | No type or thickness, so the order comes back with a question | "1 in. fibreglass lining, product as named in the spec" |
| "Insulated" | It could mean lining inside or wrap outside | "Lining inside" or "wrap outside", or both |
| "Lined" on the run right after a humidifier | Damp lining can grow mould and has to be replaced | A question to the engineer about lining near the humidifier, answered before ordering |
| "Lined" on a kitchen exhaust run | Grease and lining don't mix, and grease duct follows NFPA 96 | "Kitchen exhaust unlined, listed separately" |
| "Double-wall round, 20 in." | The 20 in. could be the inner or the outer wall | "Double-wall round, 20 in. inner, 1 in. insulation, perforated inner wall" |
The sizes in this table are examples, not from a job. The rest of a fitting order, from the bend to the joints, is covered in how to specify a duct fitting.
Every lined or double-wall order is checked for its sizes before cutting, whether it's one plenum or a full run of sound-insulated ductwork.
Airway sizes confirmed first. We check whether each size on your list is the airway or the metal. If the drawings don't say, we ask.
Lining checked against the spec. We match the type and thickness to the spec, and flag any run where lining shouldn't go.
Priced as a quote. Lined and double-wall duct are priced from your drawings, while bare duct and standard fittings are priced on WebDuct.
Built and lined in Pitt Meadows. We cut and form the metal, then fit the lining to your spec names. The lining itself is bought in.
Double-wall supplied to the drawings. We supply double-wall with the inner size, insulation, and inner wall your drawings call for.
If we fit the wrong lining or make the airway too small, we fix or remake that section at no charge. When to order on WebDuct and when to ask for a quote is laid out in how to order ductwork online.
Lining is for noise, and wrap is for heat and condensation. Double-wall is for air that may touch only metal, fast air, and runs where lining could get wet.
If the only worry on a run is condensation, wrap on the outside does that job without making the metal bigger. We'll say so before quoting lining, even if it means a smaller order.
Every line ticked here is one less thing we'd have to ask about the lining. The checklist for the rest of the job is in What Duct Duct Goose Needs to Quote Custom Metalwork.
More decisions like these sit in HVAC Ductwork and Fittings.
A finished spec and a sketch marked "quiet room here" are both fine places to start. We'll read what you have, ask about the gaps, and come back with a price once each run is defined.
Bare ductwork and standard fittings can be priced and ordered on WebDuct, the online pricing and ordering portal, once your company account is set up. Lined and double-wall duct, grease duct, kitchen stainless, curbs, installs, and other custom work start with a request or a call.
At Duct Duct Goose, we specialize in sheet metal fabrication and stainless steel services for commercial and industrial applications, including HVAC systems, ducts, and custom metalwork. Please note that we do not offer services for automotive-related projects, such as vehicle parts, repairs, or modifications.
