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LBNL - 45423
Stopping Duct Quacks: Longevity of
Residential Duct Sealants
Max H. Sherman, Iain S. Walker and Darryl J. Dickerhoff
Energy Performance of Buildings Group
Environmental Energy Technologies Division
Lawrence Berkeley National Laboratory
Berkeley, Ca 94720
This report was prepared as a result of work sponsored by the California Energy Commission (Commission),
through a contract with the Regents of the University of California, California Institute for Energy Efficiency
(CIEE). It does not necessarily represent the views of the Commission, its employees,
the State of California, The Regents, or CIEE. The Commission, the Regents, the State of California, CIEE,
their employees, contractors, and subcontractors, make no warranty, express of implied, and assume no legal
liability for the information in this report; nor does any party represent that the use of this information will not
infringe upon privately owned rights. This report has not been approved or disapproved by the Commission or
CIEE, nor has the Commission or CIEE passed upon the accuracy or adequacy of the information in this report.
Duct leakage has been identified as a major source of energy loss in residential
buildings. Most duct leakage occurs at the connections to registers, plenums or branches in
the duct system. At each of these connections a method of sealing the duct system is
required. Typical sealing methods include tapes or mastics applied around the joints in the
system. Field examinations of duct systems have shown that these seals tend to fail over
time periods ranging from days to years. We have used several test methods over the last
few years to evaluate the longevity of duct sealants when subjected to temperatures and
pressures representative of those found in the field. Traditional cloth duct tapes have been
found to significantly under-perform other sealants and have been banned from receiving
duct tightness credits in California's energy code (California Energy Commission 1998). Our
accelerated testing apparatus has been redesigned since its first usage for improved
performance. The methodology is currently under consideration by the American Society for
Testing and Materials (ASTM) as a potential new test method. This report will summarize
the set of measurements to date, review the status of the test apparatus and test method, and
summarize the applications of these results to codes and standards.
In the U.S. forced air systems are the dominant method of heating and cooling
residential buildings (Energy Information Administration (EIA) 1997). The air distribution
systems require some sort of seal between duct sections, at branches and at plenum and
register connections. Without these seals, duct systems would be extremely leaky and
inefficient. Field studies (Jump et al. 1996; Cummings et al. 1990; Downey and Proctor
1994; Modera and Wilcox 1995) have shown that existing residential systems typically have
30-40% of the total air flow leaking in and out of the duct system. Because these ducts are
often outside conditioned space, this leakage corresponds to a similar amount of energy (30-
40%) being lost from the duct system instead of going to heating or cooling the conditioned
space. In addition, a system with more supply leakage than return leakage causes a greater
penalty than just the amount of air lost. Increased infiltration from outside replaces supply
air and must be conditioned. There are also comfort, humidity and indoor air quality
problems associated with return leaks drawing air from outside or unconditioned spaces
within the structure (e.g., damp crawlspaces). Note that field studies (Walker at al. 1998)
have shown that ducts located within the thermal envelope (e.g., in joist spaces between
floors or interior partitions) can still have significant leakage to outside because these spaces
are not air sealed.
Residential duct systems in the U.S. are normally field designed and assembled.
There are many joints, often of dissimilar materials (e.g., plastic flex duct to sheet metal
collar). The mechanical connection of the duct system components does not usually provide
an air seal. High pressure differences in the vicinity of the air handler and associated
plenum, mean even small holes have potentially large leakage flows. Therefore, standard
practice (Sheet Metal and Air Conditioning Contractors' National Association (SMACNA)
1985) calls for all joints in the duct system to be air sealed in addition to being mechanically
fastened. However, field studies have shown that many systems are poorly sealed.
Each sealant choice has different advantages or disadvantages, but a reasonably careful
job of application, can produce a good initial seal for any of them. While any sealant method
can produce a good initial seal, it is not clear that all last equally well. The length of time a
duct seal can last is important given that houses are said to be designed to last 30 years and
flex duct systems are often rated at 15 year life. Ideally, duct seals should last at least as long
as the rest of the duct system, but are often observed to fail in a few years (Walker et al.
1998). Poor installation of sealants (e.g., on dusty or oily surfaces prevalent during
construction) can be a contributing factor (that will not be addressed here), but it appears that
physical properties of some of the sealants themselves may result in poor seal longevity.
While some duct sealant technologies are rated (e.g. by Underwriters Laboratory 1993,
1994, 1995) on their manufactured properties, none of these ratings addresses the in-service
lifetime. Selection of sealants that do not fail within the lifetime of the duct system requires
the existence of relative ratings for sealant longevity. The purpose of this study was to
develop such a rating method.
The duct sealing methods examined in this study can be split into the following classes:
• "Duct Tape" has a vinyl or polyethylene backing with fiber reinforcement and has a
rubber-based adhesive. It comes in wide variety of grades with different tensile
strengths. The composition and material of the backing has some variation, with some
tapes having a distinctive backing that has the appearance of cloth rather than vinyl or
polyethylene. The classic duct tape is silver/gray, but is available in many colors.
• "Clear UL181B Tape" has a thin polyester backing (typically clear) and an acrylic
adhesive. Clear UL 181B tape is often used on factory-assembled duct systems, and is
becoming more common in field assembled systems.
• "Foil Tape" has metal foil backing and like clear UL 181B tape has an acrylic adhesive.
Foil tapes are often used on rigid duct systems (e.g. duct board). Foil tapes with rubber-
based adhesives exist but have not yet been tested.
• "Butyl Tape" typically has foil backing as well, but uses a thick (0.38 to 1.3 mm) butyl
adhesive to allow it to conform to more irregular shapes.
• "Mastic" is a wet adhesive available in different consistencies (usually applied with a
brush) that fills gaps and dries to a semi-rigid solid. Mastics may also be used together
with reinforcing fibers or mesh tape.
• "Aerosol Sealant" is a sticky vinyl polymer that is applied to the leaks internally, by
blowing aerosolized sealant through the duct system. This sealant system was developed
by LBNL, and is discussed in more detail in Carrie and Modera 1995.
Two separate experiments were used to examine the longevity of these duct sealants:
1. Baking tests. Samples were placed in an oven and held at a steady temperature (about
65°C (150°F)) with no air flow through the test sections.
2. Aging tests. This was a more sophisticated experiment that alternately blew heated
(95°C (203°F)) and cooled (-5°C (23°F)) air through the test sections and also cycled the
pressure difference across the leaks.
This paper will present a summary of these test procedures and their results. Additional
information about thermal distribution systems and duct sealing can be found at the
following web page: http://ducts.lbl.gov.
Evaluating sealant longevity performance
The longevity measurements in this study focussed on the properties of the sealants
as opposed to installation issues. Therefore considerable effort was made to ensure good
initial seals, by following good practice and manufacturers instructions carefully. This is
particularly important for sheet metal that often has an oily residue (left over from the
manufacturing process) that impairs a good initial seal and would presumably impair
longevity performance. The ducts were thoroughly cleaned before applying the sealants.
The exception was that no cleaning was required for mastic and aerosol sealants. For the
tests in this report, the application of the sealant was meticulous and all the sample
connections were measured to ensure a good seal before beginning any of the tests.
In a field application, it is not practical to take this level of care during the installation
of the duct system. Access to the ducts may be limited and ducts may be or become dirty
before the sealant is applied. Because tapes are particularly sensitive to these issues, some
taped seals may not perform well because of their installation rather than any intrinsic fault
of the tape itself. Non-tape sealants can often be more tolerant of dirt and/or able to reach all
the leaks. The longevity tests discussed in this paper did not address these installation issues.
Existing UL 181 standards (Underwriters Laboratory 1993, 1994, 1995) concentrate
on evaluating safety, tensile strength, and initial adhesion. They have not been developed to
measure the ability of sealants to maintain the seal when subjected to the environmental
conditions normally experienced by ductwork. The three longevity test methods developed
for this study specifically focus on evaluating the longevity of the sealant. The longevity
tests stress a standardized joint configuration with different environmental conditions. The
testing includes visual observation of seal degradation and measurement of sample leakage.
It should also be noted that this paper does not attempt to correlate how long the sealants last
in the tests to how long they would last in a real house. This is because the range of
operating conditions varies enormously between installations in individual houses.
The longevity tests were designed to use conditions of temperature, pressure and
airflow that would be experienced by typical duct system installations. The testing is
accelerated compared to real installations by having the ducts at a continuously high
temperature in the baking test; and rapidly changing from hot to cold conditions in the aging
test. For the baking test, the temperatures are at a sustained high level (65°C (150°F)) that
would periodically be experienced by ducts in a hot attic (Carlson et al. 1992 and Walker et
al. 1999) or by ducts close to the supply plenum of a furnace (The Uniform Mechanical Code
(ICBO 1994) Canadian Natural Gas Installation Code (CGA 1995) give the same limit of
250°F (121°C)). For the aging test the high and low temperature and pressure limits are
individually typical of real duct systems, but it is unlikely that a duct system would
experience these rapid hot to cold and cold to hot transitions. The cycle time of ten minutes
was limited by the need to warm up and cool down the test sample.
For the leakage measurements of individual sealants, a standard pressure of 25 Pa
was chosen because this is a typical pressure that would exist in the branches of a residential
duct system. It is between the high pressures at a plenum (on the order of 100 Pa) and the
low pressures at registers (on the order of 5 Pa). In addition, existing leakage measurements
for duct systems installed in houses also use this reference pressure (California Energy
Commission (CEC) 1998, American Society of Heating Refrigeration and Air-Conditioning
Engineers (ASHRAE) 1999). In all the longevity tests, temperatures are kept below 93°C
(200°F) because some of the tested tapes had this as an upper limit temperature rating. The
aging apparatus has between 100 and 200 Pa of pressure across the sample joints (which is
higher than the pressures measured in most residential duct systems) but it acts to accelerate
any failure by putting a bigger mechanical stress on the seal than it would experience in a
real installation. More details about the test methodology can be found in previously
published reports (Walker et al. 1998; Walker et al. 1997; Walker and Sherman 2000) and
will not be repeated here.
Selection of sealants to be tested
The sealants tested in our apparatus were those tapes and sealants which are either
commonly used or are being considered for use in various duct sealing programs (e.g., within
utility sponsored energy efficient homes). Any tape that had a maximum temperature rating
below 60°C (140°F) was excluded. Not only would it be expected to fail quickly in the
longevity tests because of their higher temperatures, but any duct tape with such a poor
temperature rating should not be used, because either hot attics or normal heating systems
would expose ducts to such temperatures. In preparation for testing, major tape and sealant
manufacturers were contacted to ensure that a wide range of available products were tested
and to determine which ones have been certified by UL. Duct tapes are discussed separately
from the other sealants because duct tape is the most popular method of sealing ducts in the
U.S. and comes in the most grades and types. In addition, the test results showed that duct
tapes performed differently from all the other sealants.
The aerosol sealant was developed by Lawrence Berkeley National Laboratory as an
alternative duct sealing method. Two samples were prepared: one each for the baking and
aging tests. Mastic is available in several varieties (but an order of magnitude less variety
than tapes) some of which include added fibers for increased mechanical strength. The
mastic product tested here did not include these reinforcing fibers and was one with a UL
rating (only a few mastic products carry the UL rating). Only a few mastics are currently UL
181B-M (Underwriters Laboratory 1995) approved although many are UL 181A
(Underwriters Laboratory 1993). Clear UL 181B tape is produced by several manufacturers,
however, at the time these tests were performed only a single type was available.
Manufacturers of clear tapes have recently changed the tapes to have perforations to allow
for easier application and are producing the tapes in a range of colors. Three samples were
tested: one for baking and two for aging. The second aging sample was tested because part
way through the test program this product obtained a UL rating and it was important to
observe if the tape had been changed in any way that affected longevity (The aging test
results indicate that longevity was not changed). Butyl tapes are available with different
thickness adhesive and in several tape widths. As with the other tape products, 50 mm (2
inches) wide tape was used because this is the most common width used in field installations.
A single type of butyl tape was used in these tests that had a 0.38 mm (15 mil) thick adhesive
layer with a metal foil backing. Three different foil tapes were tested. The tapes were from
different manufacturers and had different foil thickness and formulations and all had acrylic
adhesive. Figure 1 shows pictures of four of the first set of samples that were tested on the
Figure 1. Four samples connections for the aging test. Clockwise from top left: clear
UL 181B tape, aerosol sealant, mastic and 181B-FX duct tape
There is a wide range of duct tape products available that claim to be suitable for duct
sealing, but there is often little in their specifications or product literature to differentiate
them. While there is general agreement that there are several grades of duct tape it is not
clear what that means. For example one major manufacturer lists 16 different duct tapes (not
including color variation) and 8 foil tapes. Some of these tapes have their product codes
printed on the tape, some on the cores, and some do not have any product number on them.
Some are listed as “Code Approved” (e.g., by codes from Building Officials and Code
Administrators International or U.S. Department of Housing and Urban Development).
There was nothing exceptional in the product specifications to separate the approved from
non-approved tapes. Catalogues call the different tape grades Economy, Utility, General
Purpose, Contractor, Industrial, Professional, Premium and even Nuclear! They are all listed
as being used on HVAC ducts. Several companies have recently produced UL 181B-FX
(Underwriters Laboratory 1995) tapes that were not listed in product catalogs when this
study was performed.
Longevity Test Results
When the aging experiments were started it was expected that it would take weeks to
begin to see degradation in performance. Surprisingly, some of the sealants failed in a matter
of days. Most of the failure modes to date have been what might be termed catastrophic
rather than gradual. In other words, the seal does not gradually become poorer with time,
rather the seal remains tight until rapid failure occurs. This is in some ways fortunate
because determining an exact numerical failure criterion is somewhat arbitrary.
Nevertheless, the failure criterion was selected based on the results of preliminary testing
such that a good seal is adequately differentiated from a failed seal. Failure was determined
by comparing the leakage of the sample to the flow through the holes in the sample before
any sealant was applied. The criterion was that a seal has failed when it lets more than 10%
of unsealed flow pass through. Analysis of the test results showed that the passing or failing
of a sample is not strongly dependent on this failure criterion. i.e., sealants did not fail a
little bit (e.g. at 20% of unsealed leakage) and then stop. Most samples were tested past this
10% failure criterion and showed continual degradation. Over 30 different samples have
been tested by baking and aging. We also made visual evaluations of the sealants, e.g., some
samples had visible catastrophic failure when the tape fell off.
Figures 2a and 2b show how leakage of some samples changed with the length of
time that the samples were in the test apparatus. The initial high leakage number (about 17
m3/hour (10 cfm) @ 25 Pa) is the leakage of the sample connection before the sealant was
applied. All of the rubber backed tapes showed visible signs of failure within about 3 days of
the start of the test. Visible signs include shrinkage of the vinyl or polyethylene backing and
wrinkling and delamination of the vinyl or polyethylene backing and the reinforcing mesh
from the adhesive. The measured leakage for the duct tapes shown in Table 1 showed that
samples had about 10% to 20% of the unsealed leakage after two weeks. The “Premium”
tape failed completely (it fell off the test section), but the other tapes had just started to
delaminate at this time. This complete failure was due to separation of the backing from the
adhesive (some of the adhesive was left behind on the sheet metal). A second sample of the
Premium Grade tape was tested to see if this was a repeatable failure; it lasted about 7 days
before complete failure (note that this second sample is not shown in the figures). The foil
backed tapes, the clear tape, the aerosol and the mastic show no visible or measurable signs
of degradation after these two weeks of testing.
0 5 10 15
Leakage [m^3/hour] at 25 Pa
UL723 General Purpose Cloth Tape
UL181B-FX Cloth Tape
Clear UL 181B Tape
10% Failure Criterion
Figure 2a. Changing test sample leakage at 25 Pa, from the aging apparatus
0 5 10 15
Leakage [m^3/hour] at 25 Pa
UL181 Foil Tape
Butyl Backed F oil Tape
Premium Cloth Tape
10% Failure C riterion
Figure 2b. Changing test sample leakage at 25 Pa from the aging apparatus
Table 1 summarizes the test results for the 18 failed duct tape samples. Most of the
duct tape samples failed within in a week in the aging test. The aging and baking test results
indicate that there is no clear advantage for the UL 181B-FX listed tapes; although they last
longer (on average) than the non-UL tapes they still fail prematurely, compared to the other
sealants. Although only duct tapes were observed to fail, four duct tape samples did not reach
the 10% leakage failure criterion over the three month test period. However, in each of the
four cases, the tapes showed some leakage and visual degradation.
Table 1. Summary of Duct Tape Failures
# of Test
Final leakage at end of
testing (fraction of unsealed
8 Aging 5 different
7 days 20%-70%
5 Aging 181B-FX 10 days 70%-100%
4 Baking 3 different
34 days 30%-80%
1 Baking 181B-FX 60 days 25%
Because the baking test does not stress the samples with low temperatures or a
pressure difference across the sealant, time to failure is longer than for the aging test. There
are some cases where duct tapes have failed the aging test, but the same tapes in the baking
tests have not. A visual inspection of these baked samples reveals that the duct tape samples
have delaminated and the heat has apparently caused the rubber adhesive to harden. It
appears that some of the samples have hardened in such a way as to maintain their seal rather
like a mastic material. Because this process of hardening to maintain the seal has happened
without any pressure being applied, it is unlikely to happen similarly in real installations (as
shown by the aging results).
Table 2 summarizes the results from all of the other sealants. These sealants did not
fail after several months and can be considered to have better longevity performance than the
duct tape. Significantly, the other tapes (butyl, foil and clear UL 181B) did not exhibit the
shrinking of the backing and the delamination shown by the duct tapes. The aerosol and
mastic showed no visible or measurable signs of degradation.
Table 2. Summary of non duct tape test results
# of Test
Description Duration1 Comments
1 Aging Butyl Tape 3 months 15mil; Foil Backed
1 Aging Aerosol 3 months
1 Aging Mastic 3 months 181A
1 Aging Foil Tape 3 months 181A-P only
1 Aging Foil Tape 1 month 181A-P & 181B
1 Aging Clear UL 181B Tape 3 months
1 Aging Clear UL 181B Tape 1 month 181A & 181B
1 Baking Clear UL 181B Tape 4 months 181
1 Baking Aerosol 4 months
1 Baking Foil Tape 4 months 181A-P
1- Note that duration does not indicate time to failure. It is the length of time the samples were tested in the
The aging results described above were all done with our first test apparatus and
mostly completed by 1999. Since those experiments were done, we have redesigned and
rebuilt the aging apparatus. The new apparatus conforms generally to the specifications of
the ASTM draft test and incorporates many improvements encountered during the first stage
operation. The major additional capability is testing at steady hot or cold temperatures (i.e.
no cycling) with the leakage site pressurized. We added this ability in order to determine if a
simpler longevity test of heating or cooling only could be used. The main appeal of a
simpler test is the reduction in equipment investment, set up and operating oversight. In
addition, the new apparatus can test a total of 38 samples simultaneously. The standard test
sections are 100 mm (4 inch) duct collars mounted in a 112 mm (4.5 inch) hole, however, the
apparatus has space for 150 mm (6 inch) ducts up to 700 mm (28 inches) long. We are
planning to test other types of duct connection, such as factory assembled duct board splitter
boxes in the future, and the apparatus has been designed to accommodate these larger sample
Preliminary results are the same as for the other tests discussed in this paper – i.e. the
only sealants to fail are duct tapes. The failures occur fastest when heating only (in about
one to three weeks), with slower failures during cycling. The tapes being cooled have not
failed yet, but their leakage is slowly increasing. These results indicate that heating only
may be a simpler alternative for longevity testing (compared to the complex cyclic testing we
have done for this study).
Codes and Standards
There are several codes and standards that are either relevant to duct sealing, or have used
the results of our duct sealing results. Both Underwriters Laboratory and ASTM are
concerned with laboratory testing of duct sealant products. The CEC and EPA include
restrictions on duct sealant materials in their duct programs.
Underwriter's Laboratory. The UL 181 standards are referred to in many codes and
specifications related to thermal distribution. Currently several products that have good
longevity fail to meet the appropriate standard or have no appropriate UL standard to
reference. Individual manufacturers are addressing this concern by either modifying their
products or working with UL to develop appropriate testing.
American Society of Testing and Materials. There is currently no consensus or ANSI-
approved standard for evaluating duct sealant longevity, however ASTM Committee E6.41 is
developing a test method. The test sections are of the plenum to collar joint type shown in
Figure 1. The test sections use ducts of 4 to 8 inch (100 to 200 mm) diameter round sheet
metal mechanically connected using sheet metal screws. The sealant is applied after
ensuring that surfaces to be sealed are clean and free from dust, dirt and excess lubricants
used in the manufacture of many sheet metal duct fittings. The test sections are tested before
and after they are sealed by measuring the leakage flowrate when the sample is pressurized
to 25 Pa. The test sections are removed from the longevity apparatus on a weekly basis to
have the leakage test performed. The longevity test apparatus is required to operate in a
similar way as the aging tests performed for this study:
1. The bulk (average) flow velocity through each test section is 5 to 7.6 m/s (1000 to 1500
2. Pressure difference between the inside of the test section and its surroundings is 100 to
200 Pa (0.4 to 0.8 inches of water).
3. The lowest test section surface temperature is 0°C to 5°C (32°F to 41°F).
4. The highest test section surface temperature is 66°C to 82°C (150°F to 180°F).
5. Cycle time is between 8 and 12 minutes.
6. Temperatures and pressures are continuously monitored.
California Energy Commission (Title 24). The version of the State energy code of
California, adopted in June of 1999 allows builders to get extra credit for building an
efficient duct system through the Alternative Calculations Manual (ACM) compliance
procedure. To obtain the energy efficient duct credit in the ACM the air leakage at 25 Pa
(0.1 inch of water) must be less than 6% of air handler fan flow (for comparison, the default
duct leakage is set to 22%), and the air leakage must be verified by measurement. Because
of the poor longevity characteristics of duct tape, the CEC believes that ducts will not stay
sealed when this product is used. Accordingly, the performance credit is not available for
ducts sealed with duct tape.
EPA ENERGYSTAR Ducts. EPA’s ENERGYSTAR duct program has been developed for
retrofit, repair and replacement applications rather than new construction, although it is
expected that this program will be applied in the future to new houses. The ENERGYSTAR
duct program has both a prescriptive specification and a performance specification. The
prescriptive method requires duct leakage to be less than 10% of air handler flow (measured
using fan pressurization) and duct insulation to be a minimum of R4 (RSI 0.7), but any ducts
with less than R4 (RSI 0.7) must be insulated to at least R6 (RSI 1). The performance
specification is an efficiency of 85%. The efficiency is to be calculated using the methods in
proposed ASHRAE Standard 152P [ASHRAE 1999]. In order to prevent the cases of duct
systems that achieve high efficiency using 152P, but would be considered poor for other
reasons, the EnergyStar program requires that the maximum allowable leakage is 25% of air
handler flow for systems that use the efficiency calculation option. This program also
specifies the required system airflows in order to reduce the duct (and equipment)
inefficiencies introduced by having airflows that are too high or too low. As with the CEC
ACM requirements, cloth backed rubber adhesive duct tape is not considered an acceptable
sealant in this program.
Other duct efficiency programs
The following programs currently give limits on allowable duct leakage.
City of Austin Electric Department (CAED). CAED specifies leakage to be less than 5%
of air handler flow and/or pressure pan (Conservation Services Group (1993) p. 44) readings
all have to be less than 1 Pa.
State of Oregon. The specification is for the leakage to be less than or equal to 0.06 cfm at
50 Pa (0.2 inches of water) per square foot of conditioned space (1.1 m3/hour per square
meter). For an air-conditioned California home with an air handler flow of about 0.7 cfm/ft2
(13 m3/hour/m2) (CEC 1998), this leakage specification corresponds to 6% of air handler
flow at 25 Pa (0.1 inches of water). An alternative is to have pressure pan readings less than
City of Irvine IQ+ program. The specification is that the 25 Pa leakage flow is numerically
less than the floor area in square feet divided by 20. This corresponds to an allowable
leakage of 50 cfm at 25Pa/1000ft2 (0.9 m3/hour/m2).
Pacific Gas & Electric (PG&E). The PG&E Comfort home program includes duct leakage
testing at 25 Pa, with a limit of 12% of the nominal air handler flow that is fixed at 400
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