Duct insulation field guide: wrap vs liner for HVAC crews

What duct insulation does, and the two ways to do it

Duct insulation is the thermal blanket on a duct that keeps the air inside close to the temperature it left the air handler at, and on a cold supply duct it also keeps the metal warm enough not to sweat. Bare sheet metal in a hot attic gives up cool air to the space all the way to the register. The insulation is what stops that bleed.

There are two ways to insulate a duct, and they are not interchangeable. External wrap is a fiberglass blanket applied to the outside of the duct, with a foil vapor jacket facing the room. Internal liner is a fiberglass or other fibrous material bonded to the inside of the duct, where the air actually runs. Wrap is the common one and it does the thermal and condensation job. Liner does that job too, but the reason you reach for liner is sound, because the lined surface absorbs fan and air noise that bare metal would carry straight to the room.

The duct type underneath drives part of the choice. Bare galvanized sheet metal gets wrapped or lined; fiberglass ductboard is insulation in the wall already; flex duct ships pre-insulated. The ductwork-types guide covers those materials. This guide is about adding insulation to the metal duct and choosing between wrapping it and lining it.

Why do you insulate ductwork?

Three reasons drive duct insulation, and they do not all apply to every run. The first is energy. Conditioned air loses heat to a hot attic in summer and gains it in winter, so a long uninsulated supply run through unconditioned space arrives at the register warmer or cooler than it should, and the equipment runs longer to make up the difference. The duct is moving air you paid to condition through a space you did not.

The second is condensation, and it is the one that does real damage. A cold supply duct sitting in warm, humid air sweats the same way a glass of ice water does. That water drips onto ceilings, soaks insulation, rusts the metal, and feeds mold. On a chilled-water or low-temperature supply duct in a humid climate, condensation control is not optional, and it is the reason the vapor barrier matters as much as the R-value.

The third is noise, and this one belongs to liner specifically. The fibrous surface inside a lined duct absorbs the fan rumble and air rush that bare metal reflects, so the air arrives at the room quieter. Wrap does nothing for noise, because it is on the wrong side of the metal. If sound is the problem, the insulation has to be inside.

External duct wrap

External wrap is a fiberglass blanket, commonly 1-1/2 in to 3 in thick, with a foil-scrim-kraft (FSK) facing laminated to one side. You roll it onto the outside of the bare sheet metal duct, foil out, and the metal duct keeps its full inside dimension because nothing went into the airstream. This is the default way to insulate metal duct, and on most jobs it is what the spec means by duct insulation.

The out-of-package R-value runs with the thickness and density. A 1-1/2 in blanket at common density lands somewhere around R-4.5 to R-5 out of the package, and you go thicker to hit a higher number. Treat any printed R-value as the manufacturer's rating at full loft, and confirm the product and thickness against the project specification, because the installed R is what counts and it is lower if the blanket is squeezed.

Wrap has two clear advantages over liner. It adds nothing to the airflow path, so the duct is sized normally, and it keeps the fibrous material out of the airstream, so there is no question about fibers or cleaning. What it does not do is quiet the duct. The trade leans on wrap for the thermal and condensation job and reaches for liner only when noise forces it.

The vapor barrier has to be on the warm side and sealed

The FSK foil facing on duct wrap is the vapor barrier, and where it goes is not a style choice. It faces out, toward the room, on the warm humid side of the assembly. That is the rule that keeps the duct from sweating, and getting it backward or leaving it open is the most common wrap failure there is.

Here is the mechanism. On a cold supply duct, the cold surface is the duct metal and the warm moist air is in the room. If the vapor barrier is on the outside and sealed, room moisture never reaches the cold metal, so nothing condenses. Leave the seams open, miss the staples with tape, or tear the foil, and humid air drives through the fiberglass to the cold metal and condenses inside the insulation, against the duct, where you cannot see it. The blanket gets wet, loses R-value, holds water against the steel, and grows mold in the dark. The duct sweats on the inside of its own insulation.

So the install rule is blunt: every seam butted and taped with matching foil tape, every stapled tab taped over, every penetration and tear patched. The vapor barrier is only as good as its weakest open seam. A wrap that is perfect everywhere except one unsealed lap will sweat at that lap and ruin the run around it.

Do not over-compress the wrap

Fiberglass insulates because of the air trapped between the fibers, so when you crush the blanket you crush out the air and the R-value drops with it. A 1-1/2 in wrap pulled drum-tight around a duct and cinched down at the corners is not a 1-1/2 in wrap anymore where it is squeezed, and the rated R never makes it onto the duct.

The field move is to wrap firmly enough that the blanket stays put and the seams close, but not so tight that the loft is gone. Manufacturers publish a stretch-out width that accounts for the duct perimeter plus the corners plus the stapling tab, so the blanket lands at roughly its full thickness instead of stretched thin. Cut to the stretch-out, not to the bare perimeter.

Corners and hangers are where loft disappears quietly. The blanket bites down hard over a sharp corner and at every hanger strap, so the insulation is thinnest exactly at the edges and supports. On a cold duct those compressed spots are also the cold spots, which is where condensation starts if the vapor barrier ever lets moisture in. Pad the hangers and ease the corners so the wrap keeps its thickness where the duct is coldest.

Internal duct liner

Duct liner is a fibrous insulation, usually fiberglass, bonded to the inside surface of the sheet metal duct so the air runs across the liner instead of bare metal. It does the same thermal job as wrap, since the insulation is still between the air and the room, but the reason it goes inside is acoustic. The exposed fibrous face absorbs sound, and bare metal does not.

Liner is held two ways at once, and both are required on anything but the lowest-velocity duct. It is bonded to the metal with adhesive over the full contact area, and it is pinned with mechanical fasteners, commonly weld pins or stick pins with washers, on a grid spacing the manufacturer sets. The adhesive alone does not survive the airstream over time, and the pins are what keep the liner from lifting and tearing loose when the fan is running and the glue has aged.

The leading edges matter more than the field. Every edge that faces into the airflow, at the upstream end of a section and at every joint, gets coated or mechanically secured so the moving air cannot catch the cut face and peel it back. A liner that erodes starts at an unprotected leading edge. ASTM C1071 is the material standard for fibrous glass duct liner, and the product is tested for service in a moving airstream, but that rating assumes the liner was installed and edged the way the standard expects.

What does duct liner do for noise?

Sound control is the number one reason to line a duct. The fibrous face turns the air-rush and fan noise into a tiny amount of heat as the sound waves work into the fibers, so the duct stops acting like a speaking tube and the room gets quieter. This is the job wrap cannot do, because wrap is on the outside of the reflecting metal.

The high-value place to line is right off the air handler. The fan is the loudest source, and the first runs of supply and return duct carry that noise straight into the building, so lining the first several feet out of the unit and the return back to it kills the most noise for the least liner. Plenums, the discharge, and any duct that runs over a quiet space like a conference room or a bedroom are the other candidates.

Sound absorption climbs with the lined surface area the air passes, which is the perimeter and the length, not the cross-section. A longer lined run and a duct shape with more wetted perimeter attenuate more. That is why a stretch of lined rectangular duct near the unit often does more for quiet than a short fat section, and why acoustic specialists talk about insertion loss per foot of lined duct rather than a single R-value.

Liner steals area and adds friction

Liner takes up room the air used to have. A 1 in liner on all four sides of a rectangular duct removes 2 in from each inside dimension, so a 20 in by 12 in duct flows like an 18 in by 10 in duct once it is lined. The free area drops, the velocity climbs, and the friction goes up because the fibrous surface is rougher than bare galvanized.

The fix is to size the duct for the liner, not in spite of it. The inside dimension after the liner is the dimension that carries the air, so you order the duct larger by twice the liner thickness on each lined side and call the lined-clear opening the design size. Skip that and the system is undersized the day it is built, the static climbs, and the registers run short on air no matter how the fan is set. Duct sizing by static pressure and friction is the design topic that drives this, and it is covered in the ductwork-types guide.

The penalty is real but bounded. On a system designed around the lined dimension from the start, the larger duct costs more metal and the rougher surface costs a little static, and you accept both to get the quiet. The expensive version is lining a duct that was sized as if it were bare, then chasing low airflow through balancing that cannot fix a duct that is simply too small inside.

Is duct liner bad for indoor air quality?

This is the real debate around liner, and the honest answer is that it depends on the liner, the velocity, and whether anything wet ever gets into the duct. The concern is that an exposed fibrous surface inside the airstream can hold dust, that the dust plus any moisture can feed mold, and that at high velocity the surface can erode and shed fibers into the air. Those failure modes are real when a liner gets wet or is run faster than it was rated for.

The manufacturers and the testing push back, and they have a point. Fibrous liner is tested under UL 181 for erosion at service velocity, and quality liner holds together at the velocities it is rated for. The bigger lever is keeping the duct dry, because a liner that never gets wet does not grow anything. The trouble starts when a coil floods, a roof leaks onto a duct, or humid air condenses inside, and now there is a wet fibrous surface that is hard to clean and easy for mold.

Two practical answers come out of this. Use coated or encapsulated liner, with the air-side fibers bonded under a tougher facing, where cleanability and fiber erosion are a concern, such as hospitals, labs, and other IAQ-sensitive spaces, or skip liner entirely there and quiet the duct another way. And keep liner out of any duct that is likely to get wet. A liner you cannot keep dry and cannot clean is a liability, and that is the case where wrap on the outside is the better call even though it does nothing for noise.

What is the difference between duct wrap and duct liner?

Duct wrap goes on the outside of the duct and duct liner goes on the inside, and that one difference drives everything else. Wrap does the thermal and condensation job with no airflow penalty and no fiber in the airstream, but it does nothing for noise. Liner does the thermal job and quiets the duct, but it steals airflow area, adds friction, and puts a fibrous surface in the air that raises IAQ and cleaning questions.

The decision usually comes down to one question: do you need the duct quiet? If sound is not the issue, wrap is simpler, cheaper to keep clean, and carries no airflow tax, so wrap is the default. If sound is the issue, especially on the first runs off the air handler, liner earns its downsides, and you size the duct up to pay for the lost area. Many jobs use both: liner near the unit for sound, wrap on the rest of the run through unconditioned space for the thermal and condensation job.

Neither one fixes a leaky duct, and that is the trap. Both insulate; neither seals. The table below is the quick decision, and the leakage guide covers why sealing comes first.

What R-value does duct insulation need?

The required R-value is set by where the duct runs and which energy code the jurisdiction adopted, not by a single universal number. As a working range, supply and return duct in unconditioned space commonly runs R-6 to R-8, and duct in attics, on roofs, or otherwise exposed in cold climate zones is pushed to R-8 and up to R-12 in the coldest zones. Duct inside conditioned space needs little or none. Confirm the exact number against the adopted code edition.

Both ASHRAE 90.1 and the IECC drive this on commercial work, and they tie the R-value to climate zone and location. The pattern is that exterior and unconditioned-space duct in warm zones lands around R-6 to R-8, and cold zones step up toward R-12, with attics, crawl spaces, parking garages, and rooftop duct counted as exterior. Residential work under the IECC follows the same idea, commonly R-8 for duct in an attic and R-6 elsewhere outside conditioned space.

The number on the drawing is the installed R, not the package R, so the insulation has to land at thickness without being crushed to count. Pull the required R from the energy code the project is built to and the project specification, confirm the climate zone, and verify the product reaches that R at its installed thickness. The code edition and local amendments control, and these values move between cycles.

Insulating by location

Where the duct runs decides how much insulation it needs and whether it needs a vapor barrier at all. The energy code thinks in terms of which side of the building envelope the duct is on, and so should you when you spec the insulation.

Duct in conditioned space, inside the insulated envelope, loses its conditioned air back into a space you are conditioning anyway, so the energy case is weak and the code asks for little. Duct in unconditioned space, an attic, a crawl, a mechanical mezzanine, a garage, is on the wrong side of the envelope, so it carries the most insulation and, on cold supply duct, the sealed vapor barrier to stop sweating. Outdoor and rooftop duct is the worst case: full code R-value plus a weatherproof jacket over the insulation.

The mistake is treating the whole system the same. A run that starts inside the envelope and crosses into an attic needs the heavy insulation only where it leaves conditioned space, and the cold supply duct in that hot humid attic needs the vapor barrier sealed tight while the same duct downstream inside a conditioned chase may not. Insulate to the location each section actually sits in.

Condensation control on cold duct

A cold supply duct in warm humid air will sweat unless the insulation and its vapor barrier are continuous and sealed. This is the failure that rots ceilings and grows mold, and it is entirely preventable with the right detail. The colder the supply air and the more humid the space, the less margin you have and the more the vapor barrier has to be perfect.

Two things control it together. Enough insulation keeps the outer surface above the dew point of the room air so the surface itself does not condense, and an unbroken vapor barrier on the warm side keeps room moisture from reaching the cold metal inside the insulation. Either one alone is not enough. Thin insulation sweats on the outside; a broken vapor barrier sweats on the inside. You need the R-value and the sealed barrier both.

The hard spots are the gaps. Hangers, supports, register boots, takeoffs, and the seams between blanket sections are where the barrier breaks and a cold bridge forms, and that is exactly where the drip starts. On chilled-water and low-temperature supply duct in humid climates, full coverage with no bare metal and every seam sealed is the standard, not an upgrade. Leave a cold fitting bare and it sweats on its own, no matter how good the rest of the run is.

Rigid board insulation

Some applications call for rigid fiberglass or mineral board instead of a flexible blanket. The board comes in stiff panels with a facing, gets fitted to the duct and adhered or pinned, and gives a firmer, more durable surface than wrap where the insulation is exposed to traffic, weather, or a finish jacket. It costs more in labor than rolling a blanket, which is why it shows up where the blanket would not hold up.

Rigid board is common on large rectangular plenums, on equipment casings, and as the substrate under a metal or PVC jacket on outdoor and rooftop duct. It holds a clean shape, takes a jacket cleanly, and resists the dents and crushing that thin a blanket out. Where the spec wants a hard, finished, durable insulated surface, board is the answer.

The vapor-barrier rule does not change for board. On a cold duct the facing or jacket is still the vapor barrier, it still goes on the warm side, and the joints between boards still have to be sealed. A rigid board with open joints sweats inside exactly like a wrap with open seams.

Outdoor and rooftop duct

Outdoor duct needs the code R-value plus a weatherproof jacket over the insulation, because bare fiberglass and a foil facing do not survive sun, rain, and foot traffic. The jacket, commonly an aluminum or stainless metal jacket or a PVC jacket, sheds water, takes UV, and protects the insulation from the weather and from anyone walking on the duct during service.

The detail that matters is shedding water. The jacket laps like roofing, high piece over low piece, with the seams oriented so rain runs off instead of into the laps, and it is sealed and banded so wind-driven rain cannot get behind it. Water that gets under an outdoor jacket has nowhere to go, soaks the insulation, kills the R-value, and rots the duct from the outside in while everything looks fine from the walkway.

On a cold supply duct outdoors the vapor barrier still lives on the warm side under the jacket, and the jacket is the weather barrier on top of it. Two layers, two jobs: the vapor barrier stops interior moisture from reaching the cold metal, and the jacket stops exterior weather from reaching the insulation. Outdoor and high-pressure duct is also where the energy code tends to require leakage testing, which the leakage guide covers.

Fire, smoke, and why kitchen grease duct is different

Duct insulation and liner have to meet a flame-spread and smoke-developed rating, and the common benchmark is a flame-spread index not over 25 and a smoke-developed index not over 50, tested to the methods behind UL 181 and the listings the code references. Fibrous glass duct products are typically classified to that 25 and 50 limit, but treat it as a rating the listed product carries, and confirm the product listing and the adopted code rather than assuming.

UL 181 also covers the erosion and service-velocity side for liner, which is the part that keeps the fibrous face from shedding into the airstream. So the same standard family is doing two jobs: limiting how the material burns and smokes, and limiting how it erodes in the air. Spec a listed product and the rating comes with it.

Kitchen grease exhaust duct is a separate animal, and the wrap and liner in this guide do not apply to it. A commercial kitchen hood duct that carries grease-laden vapor is protected with a listed fire-rated grease duct wrap, a thick high-temperature blanket assembly tested as a system to contain a grease fire and reduce clearance to combustibles. That is a fire-containment product to its own listing and the mechanical and fire codes, not thermal insulation. Do not insulate a grease duct with ordinary FSK duct wrap.

Do you seal the duct before insulating it?

Yes, and the order is not negotiable. Seal the duct first, then insulate it, because the leakage is usually the bigger loss and insulation hides the leaks you did not seal. A duct that leaks 15 percent of its air into an attic loses far more than the same duct losing a little heat through an insulated wall, and once the wrap is on you cannot find or fix the leak without tearing the insulation back off.

The sequence on the job is seal, test if the spec requires it, then insulate. Seal the seams and joints with mastic or listed tape to the seal class, verify the leakage if the energy code calls for a test on that duct, and only then wrap or confirm the liner. Insulation over an unsealed joint just buries the problem and traps the leaking air against the metal where, on a cold duct, it can condense.

This is the spec-versus-reality gap that costs the most. The drawing shows insulation and assumes the duct is tight, but the field truth is that an unsealed duct wastes more air through its holes than a perfectly insulated wall ever saves. The leakage guide covers the seal classes, the sealants, and the test. Get the duct tight before you cover it.

Installation that holds up

Wrap and liner fail in different ways, and both failures trace back to the install. For wrap, the workmanship is about the vapor barrier and the loft. Butt the blanket seams without gapping and without crushing, tape every seam and stapled tab with matching foil tape, keep the facing continuous around corners and over hangers, and patch every tear and penetration. Secure the blanket so it cannot sag or slide, but do not cinch it so tight that the loft is gone. A wrap is only as good as its sealed seams and its kept thickness.

For liner, the workmanship is about adhesion, fasteners, and edges. Coat the full contact area with adhesive, set the mechanical fasteners on the manufacturer's grid so the liner cannot lift, and protect every leading edge facing the airflow with coating or a metal nosing so the air cannot catch and peel it. Cut the liner so the coated air side faces the airstream and the joints butt clean. The first thing that erodes on a bad liner job is an unprotected upstream edge.

On both, the duct underneath was sealed first. Insulation is the last layer over a duct that is already tight, sized, and supported, not a fix for any of those.

Maintenance and inspection

Insulated duct gets checked for two different failures depending on which method is on it. On wrapped duct, you are looking for damage to the vapor barrier and gaps in coverage. Torn foil, open seams, crushed spots over hangers, and missing tape at penetrations all let moisture in or let R-value out, and on a cold duct any one of them shows up as a wet spot, a stain, or a rust streak below the gap. Find the wet spot and you find the broken seam above it.

On lined duct, you are looking at the condition of the liner itself. A liner that has eroded shows a roughened or shedding face, usually starting at a leading edge, and a liner that got wet shows discoloration, odor, or visible mold. A musty smell at the registers on a lined system is a liner inspection, not an air-freshener problem. If the liner is wet or shedding, it does not get glued back; the affected section gets cut out and replaced, and you find why it got wet.

Either way, the inspector and the service tech look at the same things the install controlled: is the vapor barrier intact and sealed, and is the liner dry and bonded. The wet stain and the musty smell are the two tells, and they point at opposite methods failing the same way, with water where it should not be.

Commercial, data center, and high-pressure duct

On large commercial and data center jobs the insulation rules tighten, because the duct is bigger, the pressures are higher, and the energy and humidity stakes are larger. The R-values lean toward the high end the code allows for the climate zone, rigid board and jacketed assemblies show up where blanket would not survive, and the vapor-barrier and sealing details get inspected harder because a sweating chilled-air duct over a server room or a clean space is a real problem.

Liner placement on these systems is deliberate. Acoustic liner goes where the spec wants the air handler and the first runs quieted, and IAQ-sensitive spaces drive coated liner or no liner at all. The high-pressure duct that the energy code makes you leakage-test gets sealed and tested before any insulation goes on, so the test is on the bare sealed duct and the insulation is the last layer. The methods are the same as a small job; the tolerances and the documentation are tighter.

What to document

Insulation that nobody recorded is insulation nobody can verify at inspection or defend after a callback. Capture the method used on each run, where it was used, the product and its rated R, the installed thickness, and the detail that makes it work, the sealed vapor barrier on wrap and the secured edges on liner. If the run was sealed and leakage-tested before insulating, record that the test came first.

The table is the quick version of the record. Tie each run to its method, its R, and the one detail an inspector will check, so the next person can confirm the duct was insulated to the location it sits in and the code it was built to.

Common mistakes

• Putting the vapor barrier on the wrong side or leaving seams unsealed, so the cold duct sweats inside its own insulation.
• Over-compressing the wrap at corners and hangers, so the installed R never reaches the rated R where the duct is coldest.
• Lining a duct in a hospital, lab, or other IAQ-sensitive space without using coated liner or choosing another way to quiet it.
• Sizing the duct as if it were bare and then lining it, so the lined-clear opening is undersized and the registers run short on air.
• Insulating before sealing and testing the duct, so the leaks are buried and the bigger loss is hidden.
• Leaving a cold supply fitting or boot bare in a humid space, so it sweats on its own no matter how good the rest of the run is.
• Using the wrong R for the location and climate zone, or treating a package R-value as the installed R.
• Insulating a kitchen grease exhaust duct with ordinary FSK duct wrap instead of a listed fire-rated grease duct assembly.

Field checklist

Standards and references

The energy codes set the R-value. ASHRAE 90.1 and the IECC both require duct insulation by climate zone and location, with unconditioned-space and exterior duct carrying the most and the cold zones pushed toward R-12, while attics, crawl spaces, garages, and rooftop duct count as exterior. The exact R for a given zone and location shifts between code editions, so confirm the adopted edition and any local amendments before you spec a number.

SMACNA covers the duct construction and the sealing that comes before insulation, and the leakage testing the energy code may require on high-pressure and outdoor duct. NFPA 90A is the air-handling standard that mandates the flame-spread and smoke-developed limits, commonly the 25 and 50 figures, and UL 181 and the listings the code references carry those ratings on the products along with the erosion and service-velocity rating for liner. ASTM C1071 is the material standard for fibrous glass duct liner. NAIMA and the insulation manufacturers publish the install details, the stretch-out widths, and the rated R-values for specific products.

Cite the standard that actually governs the point and let the project specification and the manufacturer's instructions control the rest. The R-values and the flame-spread limits here are common targets and listed ratings, not a substitute for the adopted code edition, the product listing, and the contract documents, which are what an inspector holds you to.

Units and terms

Duct insulation carries a handful of terms and units that read differently across a drawing, a product sheet, and the energy code, so the same idea shows up under more than one name.

R-value is the thermal resistance of the insulation, higher is more insulating, and the number that counts is the installed R at full thickness, not the package R. FSK is the foil-scrim-kraft facing that serves as the vapor jacket on wrap. Density is the weight per cubic foot of the fiberglass, which tracks with the R per inch. Flame-spread and smoke-developed indices are the burn ratings, commonly held to 25 and 50. Insertion loss is the acoustic measure of how much noise a length of lined duct removes.

R-value (installed)

Thermal resistance of the insulation at its installed, uncrushed thickness, the value the code checks

FSK facing

Foil-scrim-kraft laminate on duct wrap that serves as the vapor barrier, installed to the warm side

Vapor barrier

The facing or jacket that keeps room moisture from reaching the cold duct metal, sealed at every seam

Duct liner

Fibrous insulation bonded inside the duct for sound absorption and insulation, in the airstream

Lined-clear opening

The inside duct dimension after the liner, the dimension that carries the air for sizing

Flame-spread / smoke-developed

Burn-behavior ratings for the material, commonly held to 25 and 50 under the referenced listings

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