Ductwork Insulation: Interior vs Exterior, Types & R-Values

Uninsulated ductwork in an unconditioned attic can lose 10-30% of the energy the HVAC system puts into the air before it reaches the living space. In cooling season, cold supply ducts in a hot attic also sweat, producing condensation that drips onto ceiling drywall, breeds mold, and damages insulation below. Proper duct insulation prevents both problems, and the energy code requires it in most scenarios.

Why Insulate Ductwork?

There are three reasons to insulate ducts, and they apply differently depending on where the ductwork is located:

• Thermal loss prevention. Supply duct carrying 55 F air through a 130 F attic gains heat rapidly. The air arriving at the register might be 65-70 F instead of 55 F, which means the system runs longer to satisfy the thermostat. Insulation slows this heat transfer.
• Condensation control. When the surface temperature of a duct drops below the dew point of the surrounding air, moisture condenses on the outside. This is common on cold supply ducts in humid climates. Insulation keeps the exterior surface above the dew point.
• Noise attenuation. Insulation - especially internal liner - absorbs airborne sound and reduces both breakout noise (sound passing through duct walls) and velocity noise from turbulence.

Types of Duct Insulation

There are three main insulation approaches for sheet metal ductwork. Each has distinct trade-offs in performance, cost, and impact on duct sizing.

1. Fiberglass Duct Liner (Internal)

Fiberglass liner is bonded to the inside surface of the duct with adhesive and secured with mechanical fasteners (weld pins or stick clips). The liner face exposed to the airstream is coated with a durable mat facing (typically acrylic-coated fiberglass) that resists erosion up to 6,000 FPM.

• Typical thickness: 1/2", 1", 1-1/2", or 2"
• R-value per inch: Approximately R-4.2 per inch (R-4.2 for 1", R-6.3 for 1.5", R-8.4 for 2")
• Pros: Excellent sound attenuation, no external space required, protects metal from condensation on the air side
• Cons: Reduces interior duct cross-section (a 12" x 12" duct with 1" liner is effectively 10" x 10"), can harbor dust and microbial growth if not maintained, must be accounted for in duct sizing

When using internal liner, you must size the sheet metal duct larger to maintain the required air passage. A duct designed for 12" x 12" airway with 1" liner needs to be fabricated at 14" x 14" - add twice the liner thickness to each dimension. Keep this in mind when ordering straight duct and fittings.

2. Fiberglass Duct Wrap (External)

Duct wrap is a flexible fiberglass blanket with a factory-applied vapor barrier facing (typically FSK - foil-scrim-kraft). It wraps around the exterior of the duct and is secured with outward-clinch staples, tape, or mechanical fasteners.

• Typical thickness: 1.5", 2", or 3"
• R-value: R-4.2 for 1.5", R-6.0 for 2", R-8.0 for 3"
• Pros: Does not reduce airway size, easier to install than liner, vapor barrier controls condensation
• Cons: Takes up space around the duct (can be tight in joist bays), vulnerable to physical damage if not protected, compressed insulation loses R-value

The vapor barrier must face outward (toward the warm side in cooling-dominated climates, or toward the unconditioned space). All seams in the vapor barrier must be sealed with matching FSK tape. Any breach in the vapor barrier allows moisture to reach the duct surface and condense.

3. Rigid Fiberglass Board (External)

Rigid board insulation is used primarily on large rectangular duct and plenums. It provides high R-value in a defined thickness and maintains its shape under compression better than wrap. However, it is more labor-intensive to cut and fit around elbows, tees, and other fittings.

• Typical thickness: 1", 1.5", or 2"
• R-value per inch: R-4.3 per inch
• Pros: Rigid structure does not compress, clean appearance, available with FSK or ASJ facing
• Cons: Expensive, hard to field-fit on fittings, seams require careful vapor sealing

R-Value Requirements by Climate Zone

The International Energy Conservation Code (IECC) specifies minimum duct insulation R-values based on climate zone and whether the duct is in conditioned or unconditioned space. The 2021 IECC requirements for ducts in unconditioned spaces:

Ducts in conditioned spaces (within the building envelope) generally do not require insulation under IECC, since the temperature difference between the duct surface and the surrounding air is small. However, local codes may still require it for condensation control or noise reduction.

ASHRAE 90.1 (the commercial energy code) has similar but not identical requirements. For commercial projects, always check the specific standard adopted by the authority having jurisdiction (AHJ).

Vapor Barriers: When and Where

A vapor barrier is essential on any duct insulation in a cooling climate where the duct surface temperature will drop below the ambient dew point. Without a vapor barrier, moisture migrates through the insulation, reaches the cold duct surface, and condenses. Over time, the insulation becomes waterlogged and loses its R-value entirely.

Key rules for vapor barriers:

• The vapor barrier goes on the warm side of the insulation - the outside in cooling season, the inside in heating-only systems (rare).
• For duct wrap, the FSK facing serves as the vapor barrier. All longitudinal and circumferential seams must be sealed with FSK tape.
• For internal liner, a separate exterior vapor barrier is needed if the duct is in a high-humidity unconditioned space.
• Penetrations (hangers, supports, sensors) must be sealed to maintain vapor barrier continuity.

Insulating Fittings

Straight runs are simple to insulate. The challenge is fittings - elbows, tees, transitions, reducers, and offsets all have irregular shapes that make wrapping difficult. Approaches include:

• Duct wrap: Cut triangular darts in the wrap to conform to curves. Overlap and seal all cuts with FSK tape.
• Internal liner: Factory-lined fittings are available from some manufacturers. Otherwise, field-lining fittings is labor-intensive and prone to gaps at corners.
• Spray-on insulation: Spray-applied fiberglass or cellulose can cover irregular fittings uniformly but requires specialized equipment and adds cost.

Do not leave fittings uninsulated. An uninsulated elbow or tee in an attic creates a thermal bridge and a condensation point that can drip onto ceiling drywall.

Conditioned Space vs. Unconditioned Space

The best strategy for avoiding duct insulation complications is to keep the ductwork inside the conditioned envelope. Ductwork in a conditioned basement or within insulated floor trusses does not need insulation (or needs minimal insulation) because the temperature difference between the duct and surroundings is small.

If the design requires ductwork in unconditioned space, insulate every inch - including fittings, return boots, and plenums. Seal all joints before insulating (insulation does not stop air leaks). And support the insulation properly - sagging wrap compresses at the bottom of the duct, reducing R-value where gravity pulls it thin.

Worked Example: Heat Gain by R-Value

To see what each R-value actually buys, run the numbers on one trunk. Take 30 feet of 12" x 8" supply duct carrying 600 CFM of 55 F air through a 130 F attic. The duct perimeter is 40 inches (3.33 feet), so 30 feet of duct has 100 square feet of surface, and the temperature difference across the duct wall is 75 F.

Two formulas do the work:

• Heat gain (Btu/h) = surface area x temperature difference / R-value
• Air temperature rise (F) = heat gain / (1.08 x CFM)

The bare duct picks up 7,500 Btu/h, more than half a ton of cooling lost over one 30-foot run, and the air arrives near 67 F instead of 55 F. R-8 wrap cuts that gain to 938 Btu/h, a reduction of about 87%. Going from R-6 to R-8 saves another 312 Btu/h on this run. This is a first-pass estimate: it holds the temperature difference constant along the run and ignores air leakage, so use it to compare options, not to predict a register temperature to the degree.

Get the Right Duct, Then Insulate It Right

Insulation is only as effective as the ductwork underneath it. Poorly fabricated joints leak air regardless of how well they are wrapped, and undersized ducts run at higher velocities that increase both noise and thermal transfer. Start with properly sized, precision-fabricated ductwork from PMX Ductwork, then insulate to code.

Frequently Asked Questions

What R-value is required for ductwork insulation?

Supply ducts in unconditioned space need R-8 in every climate zone under the 2021 IECC, and return ducts need R-6 in zones 1 through 3 and R-8 in zones 4 through 8. Ducts inside the conditioned envelope generally need no insulation under the IECC, although local codes may still require it for condensation control or noise reduction.

Is it better to insulate ductwork on the interior or the exterior?

Exterior duct wrap is the better choice for most jobs because it does not reduce the airway and its FSK facing serves as the vapor barrier. Interior liner is the choice when noise control matters, since it absorbs sound, but the sheet metal has to be fabricated larger: add twice the liner thickness to each dimension to keep the same airway.

Do ducts inside conditioned space need insulation?

No, ducts inside the conditioned envelope generally do not need insulation under the IECC, because the temperature difference between the duct and the surrounding air is small. A conditioned basement or an insulated floor truss space both qualify. Local codes may still call for insulation to control condensation on cold supply ducts or to reduce noise.

What materials are used to insulate ductwork?

The three common ductwork insulation materials are fiberglass duct liner, fiberglass duct wrap and rigid fiberglass board. Liner goes inside the duct at about R-4.2 per inch. Wrap goes outside and runs R-4.2 at 1.5 inches, R-6 at 2 inches and R-8 at 3 inches. Rigid board, about R-4.3 per inch, is used mainly on large rectangular duct and plenums.