AIR DISTRIBUTION SYSTEM

INSTALLATION AND SEALING

Proper Duct Installation Increases Efficiency

BUILDING TECHNOLOGIES PROGRAM

OFFICE OF ENERGY EFFICIENCY AND RENEWABLE ENERGY • U.S. DEPARTMENT OF ENERGY

Buildings for the

21st Century

Buildings that are more

energy efficient, comfortable,

and affordable... that’s the

goal of DOE’s Building

Technologies Program.

To accelerate the development

and wide application of energy

efficiency measures, the

Building Technologies Program:

• Conducts R&D on technologies

and concepts for energy

efficiency, working closely with

the building industry and with

manufacturers of materials,

equipment, and appliances

• Promotes energy/money

saving opportunities to both

builders and buyers of homes

and commercial buildings

• Works with state and local

regulatory groups to improve

building codes, appliance

standards, and guidelines for

efficient energy use

Technology Fact Sheet

INTRODUCTION

Central heating and cooling

systems use an air distribution or

duct system to circulate heated

and/or cooled air to all the

conditioned rooms in a house.

Even when properly designed,

duct systems must be installed

correctly to be efficient, maintain

uniform temperatures throughout

the house, operate quietly, and

not adversely impact comfort or

indoor air quality.

WHY DUCT INSTALLATION AND

SEALING ARE IMPORTANT

The efficiency of air distribution systems has

been found to be 60-75% or less in many

houses because of insufficient and/or poorly

installed duct insulation and leaks in duct

systems. Properly designed and installed duct

systems can have efficiencies of 80% or more

for little or no additional cost, potentially

saving a homeowner $50-200 or more per year

in heating and cooling costs. Moreover,

efficient duct system installations can reduce

equipment size, further saving money for new

or replacement equipment.

Duct systems that leak and/or do not distribute

air properly throughout the house may make

some rooms too hot and others too cold. Leaky

and unbalanced duct systems force conditioned

air outside and unconditioned air into the

house. This increases heating and cooling

costs and may also draw humidity, dust, mold

spores, and other contaminants into a home

from the attic, crawlspace, or garage and radon

gas from the soil. In extreme cases, poorly

installed duct systems can induce backdrafting–

spillage of flue gases from combustion

appliances (e.g., furnace, water heater, fireplace)

into the living space–primarily when atmospheric

or natural-draft flues are used rather than

powered combustion systems.

Duct systems that are undersized for the

heating and cooling equipment, have been

pinched to fit around structural framing during

installation, or have been installed with

numerous bends and turns may lead to low air

flow rates and high air velocities. Low air flow

rates cause the heating and cooling equipment

to operate inefficiently. High air velocities

increase noise.

Unfortunately, researchers have found these

types of duct problems repeatedly in new and

existing homes because care was not taken

initially in installing the air distribution system.

BENEFITS OF INSULATING AND SEALING DUCTS
DUCT INSTALLATION STANDARDS

The installation guidance presented in this fact sheet should

be followed insofar as it does not conflict with applicable

building codes and the following standards:

Sheet Metal and Air Conditioning Contractors’ National

Association, Inc. (SMACNA) Residential Comfort System

Installation Standards Manual (1998, seventh edition) and

Fibrous Glass Duct Construction Standards (1992, sixth

edition). SMACNA’s HVAC Duct Construction Standards—Metal

and Flexible (1995, second edition), although primarily for

commercial and industrial projects, details good construction

and installation practices that could improve residential

applications.

Air Diffusion Council (ADC) Flexible Duct Performance &

Installation Standards (1996, third edition).

North American Insulation Manufacturers Association

(NAIMA) Residential Fibrous Glass Duct Construction

Standards and A Guide to Insulated Air Duct Systems.

Underwriters Laboratory (UL) Standard for Factory-Made Air

Ducts and Air Connectors (1996, ninth edition).

GENERAL DUCT INSTALLATION GUIDELINES

Air distribution ducts are commonly constructed from sheet

metal, rigid fiberglass duct board, or flexible duct. General duct

installation recommendations specific to these different

materials are outlined below. In addition, the air handler,

plenums, and duct boots form parts of the air distribution

system. General installation guidelines for these components

are also provided.

Duct systems should be installed inside the conditioned

(living) and semi-conditioned spaces of the house to the

greatest extent possible (but not in exterior wall cavities). Any

air leakage in ducts located inside the house will not

significantly affect the energy efficiency of the heating and

cooling system because the conditioned air will remain inside

the house. Also, ducts located inside the house need minimal

insulation (in hot and humid climates), if any at all.

Duct systems should be hard-ducted as much as possible.

Duct system installations that use the house structure or

building framing (e.g., building cavities, closets, raised-floor air

handler plenums, platform returns, wall stud spaces, panned

floor joists) as supply or return ducts can be relatively

DUCT INSTALLATION PRINCIPLES

The objectives of a properly designed and installed duct

system are occupant comfort, proper air distribution,

economical heating and cooling system operation, and

economical duct installation. Such a duct system is one that

• Provides conditioned air to meet all room heating and cooling

loads.

• Ensures that the pressure drop across the air handler is within

manufacturer and design specifications.

• Provides proper air flow.

• Prevents air from entering the house or duct system from

polluted zones.

• Maintains a neutral pressure in the house by having balanced

air flows between the supply and return systems.

• Minimizes duct air temperature gains or losses between the air

handler and supply outlets, and between the return register

and air handler.

During the design of the duct system, the layout and locations

of the ducts are identified, duct materials are chosen, the sizes

of the ducts are calculated, insulation levels are identified, and

registers are selected. Duct installation ensures that the duct

design is achieved in practice by addressing three critical

elements:

• Mechanical integrity—the system will remain as built for the

life of the house, without developing leaks, obstructions, or

insulation failure.

• Freedom from leaks—all the air moved by the air handler will

be drawn from and delivered to the intended conditioned

spaces.

• Proper insulation—conditioned air does not exchange heat

with unconditioned spaces.
inexpensive to install. However, they should be avoided

because they are difficult to seal and cannot always be

insulated. In addition, because they tend to be rough and have

many twists and turns, it is difficult to build them so as to

ensure good air distribution. Even return plenums built under a

stairway or in a closet, for example, should be avoided if a

completely ducted system is possible.

SHEET METAL DUCTS

Sheet metal is the most common duct material and can be used

on most all supply and return duct applications (for plenums,

trunks, branches, and runouts). Sheet metal ducts have a

smooth interior surface that offers the least resistance to air

flow. They must be carefully and completely sealed during

construction/installation, using approved tapes or preferably

mastic, because each connection, joint, and seam has potential

leakage. Screws should be used to mechanically fasten all

joints. When located in an unconditioned space, sheet metal

ducts must be insulated with either an interior duct liner or

exterior insulation.

BOOT AND DUCT CONNECTION HIGHLIGHTS DUCT BOARD

FIBERGLASS DUCT BOARD DUCTS

Fiberglass duct board is insulated and sealed as part of its

construction. It is usually used to form rectangular supply and

return trunks, branches, and plenums, although it can be used

for runouts as well. Fiberglass duct board provides excellent

sound attenuation, but its longevity is highly dependent on its

closure and fastening systems. Connections should be

mechanically fastened using shiplap or V-groove joints and

stapling and sealed with approved pressure-sensitive tapes

and mastic.

FLEXIBLE NONMETALLIC DUCTS

Flexible nonmetallic duct (or flex duct) consists of a duct inner

liner supported on the inside by a helix wire coil and covered

by blanket insulation with a flexible vapor-barrier jacket on the

outside. Flexible duct is often used for runouts, with metal

collars used to connect the flexible duct to supply plenums,

trunks, and branches constructed from sheet metal or duct

board. Flexible duct is also commonly used as a return duct.

Flexible duct is factory-insulated and has fewer duct

connections and joints. However, flexible duct is easily torn,

crushed, pinched, or damaged during installation. It has the

highest resistance to air flow. Consequently, if used, it must be

properly installed.
AIR DISTRIBUTION SYSTEM INSTALLATION AND SEALING

Flexible nonmetallic ducts should not have an air-permeable

core. Flexible ducts must be stretched to their full length and

cut to fit (not compressed) using the minimum length required

to make the connection. Flexible duct that is not fully stretched

has high air-flow resistance and tends to sag between

supports. Connections and joints must be mechanically

fastened using drawstraps and sealed using mastic. Bends in

flexible ducts should not exceed 90 degrees and should have a

radius greater than one duct diameter.

AI R HANDLER

Because system pressures are highest at the air handler, holes,

cracks, and other openings at the air handler will cause more

air leakage than elsewhere in the system. Air handlers located

in the attic, garage, or crawlspace, or outside in singlepackaged heat pumps or gas-packs, warrant special attention.

Use mastic, cork rope or tape, or caulk to permanently seal

unused holes, seams, wire penetrations, and refrigerant and

condensate line penetrations in the air handler cabinet. Use

approved pressure-sensitive tapes to seal access panels.

Connections between the air handler

cabinet and the main supply and

return plenums or ducts should be

sealed with mastic or approved

pressure-sensitive tapes. It is

sometimes easier to remove service

panels and seal these areas from the

inside using mastic.

PLENUMS

Fiberglass duct board is often used

to build supply and return plenums.

Mastics or approved tapes may be

used to assemble butt joints on

fiberglass duct board components

such as plenum boxes. Although it is

not recommended, closets, spaces

under stairs, and other such areas of

the house are often used as return

plenums. Because these plenums are

located inside the house, the need to

FLEX DUCT SEALING TIPS

seal them is often overlooked. Unless they are well sealed,

these plenums will frequently allow unconditioned air to be

drawn into the system from the attic, crawl space, or outside.

Be sure to seal all such plenums completely—all seams, gaps,

and penetrations through the ceiling, floor, and walls. Seal the

air handler unit to the shelf of a closet return system.

DUCT BOOTS AND ELBOWS

Seal all joints between ducts, duct boots, and/or elbows with

mastic or approved tapes. Seal any leaks in the boots or elbows

with mastic, approved tape, or elastomeric caulk. Extend duct

boots at the supply and return registers through the wall,

ceiling, or floor material and make sure duct boots are sealed to

the ceilings, walls, or floors. Sometimes boots may be most

easily sealed from the inside, after installation; however, the

sealing material must be installed so that it does not interfere

with installation or removal of the register.
AIR DISTRIBUTION SYSTEM INSTALLATION AND SEALING

SEALING MATERIALS

The two primary materials used to seal ducts during installation

are mastic and tapes, although other sealants may also be used

in special applications. Use all mastics, tapes, and other sealants

according to manufacturer instructions. Clean and dry joints,

seams, and openings of oil, grease, and dirt before applying

sealants, especially when using tapes and non-mastic sealants.

Duct insulation does NOT stop leakage and is NOT a sealing

material; in fact, dirty insulation is a telltale sign of air leakage.

• Mastics—Mastics that meet UL Standards 181A or 181B may be

used to seal duct joints. Use mastics labeled UL 181A-M for

fiberglass ducts, UL 181B-M mastics for flexible ducts, and either

for rigid metal ducts and components. Water-based mastics are

preferable to petroleum-based mastics because they have shorter

curing times, easier cleanup, and more “forgiving” application

characteristics. Mastic must not be diluted. Mastic should be

applied liberally over the entire joint between and over mated

surfaces. When using over holes or gaps that are larger than 1/8

to 1/4 inch, apply a thick layer of mastic followed by at least one

layer of fiberglass mesh tape, topped by a layer of mastic that fills

the scrim pattern completely and covers the mesh.

• Tapes—Heat-activated and pressure-sensitive tapes that meet

UL Standards 181A or 181B and are marked for use in the

intended application can be used to construct ducts and, in

some cases, seal ducts and other distribution system

components. Heat-activated tape should be labeled UL

181A-H and used only on rigid fiberglass ducts. Use pressuresensitive tapes labeled UL 181A-P for rigid fiberglass ducts,

181B-FX for flexible ducts, and either for rigid metal ducts and

components. Pressure-sensitive tape used on rigid fiberglass

ducts should be rubbed firmly until the pattern of the facing

reinforcement shows through the surface of the tape. When

taping traverse joints, wrap the tape three times.

Pressure-sensitive metallic tape with non-butyl adhesive is useful

to temporarily seal air handler access panels that may need to be

opened in the future. When sealing access panels, a card should

be taped to the access panel that says “Please Replace Any Tape

That is Removed” as a reminder to future repair personnel.

Cloth-backed rubber-adhesive (duct) tapes should not be used

because they deteriorate with time and exposure to heat.

Pressure-sensitive metallic tape with butyl adhesive (not

meeting UL Standard 181) can be used to seal metal-to-metal

connections. Cork tape can be used to seal gaps where

refrigerant lines penetrate the air handler unit cabinet.

INSTALLATION MATERIALS

F ASTENERS

Mechanical fasteners—screws, staples, and draw bands—

should be used to secure all joints between sections of duct.

Materials intended to seal against air leaks—such as tapes,

mastics, and other sealants—should not be used to hold

sections of duct together except where mechanical fasteners

are not feasible. Ducts should also be well supported so that

joints are not pulled apart or the duct distorted (pinched) so

that its air flow is reduced.

• Screws—On transverse joints in round sheet metal ducts that

use sleeves or swedge or crimped fittings, use at least three

metal screws equally spaced around the joint to ensure that

the joint cannot become separated. For vertical connections

made on flexible duct, install at least three screws below the

drawband used to secure the inner lining to prevent slippage.

Duct boots must be mechanically secured to the building

using screws so that the boots do not rely on the ducts for

support.

• Staples and Tape—Ducts constructed from fiberglass duct

board should be fastened together using clinching staples on

approximately 2-inch centers and approved pressure-sensitive

tape. Where staples cannot be used, joints should be held

together using approved pressure-sensitive tapes. The tape

should be placed over the seam and 8-inch-long (minimum)

crosstabs taped on each side of the duct with a distance of

12 inches or less between crosstabs.

• Drawbands—When joining flexible ducts to each other or to

other types of duct, the flexible duct must be fitted over a

beaded sleeve or collar and attached with drawbands

(UV-resistant nylon duct ties or preferably metal worm-drive

hose clamps, both with a temperature rating of 165°F)—one

drawband to secure the inner lining and a second drawband to

attach the outer insulation jacket.

• Support Straps—Round sheet metal ducts suspended in the

air should be supported by hangers at least every 10 feet.

Flexible ducts suspended in the air should be supported at

least every 4 feet by straps that are at least 11⁄2 to 13⁄4 inches

wide, and they should not sag more than 1⁄2 inch for each foot

of distance between the supports. Straps used on flexible

ducts should not constrict the inner diameter of the duct or

cut the outer jacket.
For more information, contact:

Energy Efficiency and Renewable

Energy Clearinghouse (EREC)

1-800-DOE-3732

www.eere.energy.gov

Or visit the Building Technologies

Program Web site at

www.buildings.gov

Or refer to the Duct Installation and

Sealing Standards, Specification of

Energy-Efficient Installation

Consortium for Energy Efficiency

617-589-3949

www.cee1.org/resid/rsac/hvac.php3

Written and prepared for the

U.S. Department of Energy by:

Southface Energy Institute

404-872-3549

www.southface.org

U.S. Department of Energy’s

Oak Ridge National Laboratory

Buildings Technology Center

865-574-5206

www.ornl.gov/btc

SMACNA’s manuals may be obtained

from 4201 Lafayette Center Drive,

Chantilly, Virginia 20151-1209

703-803-2980, www.smacna.org

ADC’s standards can be obtained from

1000 East Woodfield Rd, Ste. 102,

Schaumburg, Illinois 60173

847-706-6750, www.flexibleduct.org

NAIMA standards and guidelines can

be obtained from Canal Center Plaza,

Ste. 310, Alexandria, Virginia 22314

703-684-0084, www.naima.org

UL’s standards may be obtained from

www.ul.com

The International Energy Conservation

Code can be obtained from the

International Code Council

703-931-4533, www.intlcode.org

NOTICE: Neither the United States

government nor any agency thereof,

nor any of their employees, makes any

warranty, express or implied, or

assumes any legal liability or

responsibility for the accuracy,

completeness, or usefulness of any

information, apparatus, product, or

process disclosed. The views and

opinions of authors expressed herein

do not necessarily state or reflect those

of the United States government or any

agency thereof.

AIR DISTRIBUTION SYSTEM INSTALLATION AND SEALING

Printed with a renewable-source ink on paper containing at least

50% wastepaper, including 20% postconsumer waste.

March 2003 DOE/GO102003-0783

DUCT LEAKAGE TESTING

Once ducts have been installed and sealed, the

only way to really know how leaky they are is

to conduct a duct airtightness test. Such tests

can be performed using a special fan called a

blower door or, better, with a special duct

blower. Often, energy efficiency incentive

programs, such as the DOE/EPA Energy Star®

Program, require duct leakage testing (usually

performed in less than an hour) to confirm the

airtightness of the ducts. A typical requirement

is that duct leakage measured when the ducts

are pressurized to 25 Pascals should not

exceed 5% of the system air flow rate.

DUCT INSULATION

Insulate all portions of duct systems located

outside of the conditioned space (including

boots and plenums), typically using flexible or

rigid fiberglass insulation. Metal ducts located

in the conditioned space may be insulated to

prevent condensation. Adhere to the following

guidelines when selecting and installing the

insulation.

• Select the insulation levels for the duct

system in accordance with the 2000

International Energy Conservation Code. Use

higher duct insulation levels in ducts located

outside the conditioned space than those

specified by this Code, especially when

variable-speed air handling equipment is

being used. Lower air flows provided by

variable-speed heating and cooling systems

to improve operating efficiency increase the

resident time of air within the air distribution

system, which in turn increases thermal

losses in the winter and thermal gains in the

summer. Attic insulation placed over ducts

helps where it is possible.

• Insulation must be continuous, especially at

plenums, boots, elbows, and connections

(i.e., no gaps or voids in the insulation).

• Wraps of spirally-wrapped unfaced

insulation should overlap at least 2 inches.

• Permanently secure insulation to the duct

with draw bands, non-corrosive wire,

rust-resistant staples or nails, or

pressure-sensitive tape wrapped at least

three times around the circumference.

Parallel-wrapped faced insulation is usually

secured with pressure-sensitive tape. Secure

unfaced parallel-wrapped insulation at least

every 18 inches along its length using nontaping methods. Attachments should not

compress the insulation more than 20%

along straight duct lengths and no more

than 50% in bends.

• When flexible ducts are joined, the insulation

jackets must overlap at least 2 inches. The

overlap must be secured and sealed with a

draw band or with three staggered wraps of

pressure-sensitive tape.

• Install a continuous vapor barrier outside the

insulation (e.g., foil-faced fiberglass

insulation sealed with pressure-sensitive

metallic tape) on metal ducts that are used

for space cooling, except in dry climates

where condensation on the duct surface will

not occur. Tears in the insulation facing

(especially the outer lining of flexible duct)

must be avoided to prevent moisture

accumulation in the insulation, which

significantly reduces its effectiveness.

• At duct terminations (e.g., boots, collars), the

insulation jackets of flexible ducts should be

pulled over the insulation and secured and

sealed to the fitting with a draw band, mastic

and fiberglass mesh tape, or three wraps of

pressure-sensitive tape.