Assessing the Longevity of Residential Duct Sealants
Published in Proc. RILEM 3
International Symposium: Durability of Building and
Construction Sealants, February 2000.
Iain S. Walker and Max H. Sherman
Energy Performance of Buildings Group
Energy and Environment Division
Lawrence Berkeley National Laboratory
Berkeley, CA, USA
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 (both physical measurements and
visual observations) of duct systems have shown that these seals tend to fail over extended periods of time.
In this study, three test methods were used to test the longevity of duct sealants: simple heating, heat
cycling and combined pressure and heat cycling (aging). The most advanced method was the "aging" test,
developed to evaluate the longevity of duct sealants by alternatively blowing hot (75°C) and cold (-5°C)
air through test sections, with the apparatus cycling between hot and cold air quickly. The temperatures
and cycle length were chosen to accelerate the aging process of the duct seals. The aging apparatus was
able to test eight samples at a time, with the test samples constructed from standard duct fittings. The
results of these tests were used to evaluate different sealants relative to each other, so that
recommendations regarding duct sealants may be developed. Typical duct tape (i.e. fabric backed tapes
with rubber adhesive) was found to fail more rapidly than all other duct sealants. The accelerated test
method is being developed into an ASTM Standard under sub-committee E6.41.
In the U.S. forced air systems are the dominant method of heating and cooling residential buildings .
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 [2,3,4,5] 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, there are 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  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 fastening
together of the duct system components does not usually provide an air seal. High pressure drops in the
vicinity of the air handler and associated plenum, make even small holes have potentially large leakage
flows. Therefore, standard practice  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 . 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.
This study was undertaken by the State of California because it wanted to be able to make
recommendations on acceptable practices for duct sealing for use in energy conservation programs and in
building energy codes. These recommendations needed to include the effect on future energy
consumption and therefore the longevity of duct system performance.
While some duct sealant technologies are rated (e.g. by Underwriters Laboratory [9,10,11]) 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 typically clear polyester backing and an acrylic adhesive. Clear
UL 181B tape is often used on factory-assembled duct systems.
• "Foil Tape" has metal foil backing and can have either acrylic or rubber adhesive (we tested only
acrylic adhesive tapes). Foil tapes are often used on rigid duct systems (e.g. duct board).
• "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 wet application, gooey adhesive 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 .
Three separate experiments were used to examine the longevity of these duct sealants:
1. Cycling tests. This experiment alternately blew heated and room temperature air through sample
duct connections. The pressure difference across the duct leaks was also cycled.
2. Baking tests. Samples were placed in an oven and held at a steady temperature with no air flow
through the test sections.
3. Aging tests. This was the most sophisticated experiment that alternately blew heated and cooled 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:
2. Evaluating sealant longevity performance
The longevity measurements in this study focussed on the properties of the sealants themselves. Therefore
considerable effort was made to ensure good initial seals by following good practice and manufacturers
instructions carefully. For example, the sample connections were thoroughly cleaned and dried before the
tapes were applied. This is particularly important for sheet metal that has an oily residue left over from
the manufacturing process that impairs a good initial seal and would presumably impair longevity
performance also. The ducts were not cleaned before the application of the 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; also, 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 [9,10,11] 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; continually cycling (there are
no long “off” periods during which the seals are not stressed) in the cycling test; and rapidly changing
from hot to cold conditions in the aging test. 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.
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 [13,14]. 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 cycling apparatus puts about 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 larger mechanical stress on the seal than it would
experience in a real installation.
The twenty minute cycle time of the cycling test was limited by the need to warm up and cool down the
test sample. Also the cycling apparatus could not subject the test sample to the cold temperatures that
might be expected either in the winter or in air conditioning supply ducts. The aging test was designed to
be able to overcome these limitations and provide accelerated longevity testing.
2.1 Sample connections sealed in this study
There are several types of connection commonly seen in duct systems. For sheet metal and duct board
systems there are linear joints between the individual sections making up the duct system. For round
sheet metal systems there are the round joints between pieces of duct. For plastic flex duct systems there
are the round joints between individual pieces of flex duct required for long runs. However, there is one
joint common to all systems: that is the perpendicular joint created when a duct connects to a plenum (for
duct board and sheet metal systems) and where a collar (connected to the end of a piece of flex duct)
connects to a plenum or junction box in flex duct systems. In addition, in field observations of duct
systems, it is often this type of connection that has sealant failure. This may be because it is difficult to
apply tapes to this interior right-angled joint, particularly for ducts of round cross section. It was also
expected that this joint may accelerate failure because the holes to be sealed can be relatively large (about
6 mm x 6 mm) compared to other system joints (e.g., the longitudinal seam in sheet metal ducts).
Figure 1 illustrates the sample connections constructed for the baking and aging tests. The sample
connection section used in the baking and aging tests was built from standard duct fittings. It consisted of a
flange and a collar with fingers to fold in and out of the hole in the flange. The gap between the flange and the
collar was 6 mm all around the perimeter. The collar was centered in the flange. Sheet metal screws were used
to mechanically connect the collar to the flange.
2.2 Leakage measurement method
All three test methods were designed so that individual sections could be removed from the test apparatus
and tested using a pressurization test procedure, as shown in Figure 2. For the leakage measurement, the
sample connection was blocked at one end and a fan was connected to the other end to pressurize the test
section to 25 Pa. The flow was then measured using a calibrated orifice. Because a large range of leakage
flows were expected, from about 15 m
/hour (25 cfm) @ 25 Pa with no sealant in place to close to zero
leakage when the sealants are initially applied, several orifices (from 6mm to 30mm in diameter) were
used depending on the measured flow.
The sample connections were all tested for leakage before any seal was applied because the failure criteria
was based on the fraction of unsealed leakage. The test sections that were used in this study had pre-
sealed leakage within a few percent of 17 m
/hour (10 cfm) @ 25 Pa. After sealing all the joints had close
to the same small amount of leakage, typically less than 0.9 m
/hour (0.5 cfm) @ 25Pa.
Figure 1. Sample Connection for Duct Seal Longevity Testing
2.3 Baking test
Sample sections, sealed according to manufacturers’ instructions, were placed in an oven set to a
temperature typical of a hot attic or a hot supply plenum on a heating system. The oven was set to operate
in the range of 60°C to 80°C. The leakage of each sample was measured every few days to get a
quantitative estimate of the failure rate with time. The sample sections were also visually inspected to
note any changes in the sealant.
The only samples that have shown degradation from baking are those with rubber based adhesives (i.e.
duct tapes). The visual inspections indicated that at the elevated temperatures of the oven, rubber backed
duct tapes have a tendency to delaminate. The tape is a sandwich of several layers: the rubber adhesive, a
mesh of reinforcing fibers and vinyl, polyethylene or cloth backing and a low adhesion outer surface.
Delamination occurred as the vinyl or polyethylene backing tended to shrink. This left only the
reinforcing mesh and the adhesive behind around the edges of the pieces of duct tape. Because the
reinforcing mesh was a fairly open weave, air could then leak out through this “unbacked” mesh. This
shrinkage also made the pieces of tape wrinkle up and pull away completely from the sample connections.
Visual inspection of the samples in the aging apparatus showed the same behavior. However, the addition
of pressure across the leaks made the effects of wrinkling and shrinkage worse because it forced the tape
off the sample.
sheet metal duct
and edges of hole in
sheet metal flange
with screws for mechanical
sheet metal duct
sheet metal flange
sheet metal flange
sheet metal screw
sheet metal plenum
sheet metal duct
50 mm (2 in.)
100 mm (4 in.)
in three locations
Seal under test
outside around circumference
Tabs alternate inside and
6 mm (1/4 in.) gap
between sheet metal duct
Figure 2. Apparatus for measuring leakage of sample sections
2.4 Cycling test
The baking test subjected the sample joints to heat only, without air flow through the samples or a
pressure difference across the seal. In the cycling test, the sample joints were subjected to cyclic
temperature and/or pressure stresses by blowing heated or room temperature air through the sample at
various pressures. The air is heated to about 93°C (199°F) and the resulting duct surface temperatures are
about 65°C (149°F). Development of this cycling apparatus was funded three years ago by the US
Environmental Protection Agency to determine the longevity of the aerosol sealant technique (developed
at Lawrence Berkeley National Laboratory) under accelerated conditions. This apparatus alternately
blows heated or room temperature air through half of the sample joints with about a 20 minute cycle time.
The other half of the samples experience pressure cycling only using room temperature air. The system
has an open cycle (i.e. the heated or room air exiting the test sections is not recovered by the apparatus
and simply enters the laboratory where the testing takes place. Eight sample joints are tested at the same
time so that different sample joints can be evaluated simultaneously. The apparatus puts about 200 Pa of
pressure across the sample connections, which is higher than the pressures measured in most residential
duct systems, but it acts to accelerate any failure by putting a larger mechanical stress on the seal than it
would experience in a real installation. Note that this is the total leakage for all the sample connections.
Before applying the sealant, the total leakage was approximately 170 m
/hour (100 cfm) at 25 Pa. After
sealing this was reduced to approximately 12 m
/hour (7 cfm) at 25 Pa.
The measured leakage has very little change over the 18 month measurement period. Not only has there
been no failure, slight reduction has been measured. This trend would indicate that the seal was getting
tighter with time. It is possible to speculate that this might be caused by dust build-up improving the seal.
The trend is sufficiently small, however, that it is more likely statistical or experimental bias.
2.5 Aging test
The design of the aging apparatus was intended to overcome many of the limitations imposed by the
cycling and ultimately to perhaps become a standardized way of testing the longevity of duct sealant
systems using accelerated methods. The specific design objectives included the following:
• Combined thermal and pressure cycling in a pressure range typical of residential duct systems.
• Rapid cycle times: 6 minute target to speed up the aging process.
• Maximum duct surface temperature should be as hot as the hottest attic, but under 93°C (200°F).
• Minimum duct surface temperature should be cold enough to form condensation and perhaps
• A standardized joint and assembly method should be used so that only the sealant is being tested.
100 mm (4 in.)
Seal under test
Fan and flowmeter
• Multiple sealant materials evaluated simultaneously.
• Automated data taking and leak monitoring.
The aging apparatus has a source of hot air (the hot deck) and a source of cold air (the cold deck) as
shown in Figure 3. A selector valve, directs air from either the hot deck or the cold deck to flow through
each test section. Air exiting the test section is recirculated to reduce the heating and cooling energy
requirements. Further energy requirement reductions were made by heavily insulating the apparatus
(except for the sample joints). A typical insulation level for most of the system is about RSI 3 (R19).
Figure 3 Aging Test Apparatus
The apparatus was designed so that half of the sample sections have hot air while the other half have cold
air flowing through them. When the selector valve changes position, the sections that had hot air blown
through them now have cold air and the previously cold air sections get hot air. This alternating of hot
and cold air provides the thermal cycling. In addition, the pressures changed in the system with each
cycle: as the previously hot section cooled, the pressure decreased from about 200 Pa to 100 Pa (these
pressure differences are all relative to the room - i.e. across the seals). Similarly, the previously cold
section pressure increased by a similar magnitude. An orifice downstream of the fan was used to control
the pressure at the leak site and was also used to monitor the system air flow. The hot and cold decks were
designed to have high (thermal) mass to stabilize the system load.
The mass in the hot air deck consists of multiple plates of sheet metal and weighs about 70 kg (150 lb.).
The flow resistance of this mass was calibrated so that it could be used as a flow meter. A location for air
(and flow meter)
to flow into the system was provided to allow outflow at the leaks, this make-up air was brought into the
system at the low pressure side of the fan. The makeup air to the hot deck was fitted with a calibrated
anemometer, which measured the total leakage flow of the hot deck and the four sections that were
currently selected to it. This total leakage was continuously monitored in order to detect catastrophic seal
failure and record the failure time. Every few days the test sections had their leakage measured
individually, using an orifice flowmeter, using the procedure discussed above. This measurement was in
addition to the four sample total measured by the orifice mounted in the system.
A data acquisition system was used to continuously monitor the following parameters for the aging tests:
total leakage, surface temperature of each sample, pressure difference across the samples, air temperatures
in the hot and cold plenums, ambient air temperature, ambient air humidity. This continuous monitoring
ensured that the temperature, pressure and humidity stresses for each test section were well known –
essential for a test that is used to compare one sample to another. The surface temperatures and ambient
air humidity allow estimates of surface condensation for the cold samples.
Figure 4 shows the temperature cycling for one of the sample connections. The hot plenum was operating
at 75°C (170°F) and the cold plenum operated at -5°C (23°F). The test sample surface temperatures had a
maximum of about 60°C (140°F) and a minimum of 0°C (32°F). These surface temperatures were not as
extreme as the plenum temperatures, but still provide a considerable stress for the sealants. The cold
plenum temperatures greater than 0°C (32°F) were measured during the defrost cycle for the cooling coil.
0 6 12 18 24 30 36
Figure 4. Temperature cycling in aging apparatus
3. 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). One exception to this criterion was that 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. Several samples were prepared: one each for the baking and aging tests and another
eight for the cycling 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).
Clear UL 181B tape is produced by a single manufacturer and is only available in the single type that was
tested. 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. Visual inspection and the aging
test results indicate that the tape 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 5 shows pictures of four of the first set of samples that were tested on the aging apparatus.
3.1 Duct Tape
Samples were obtained from several companies, only some of which were tested in this study. There is a
wide range of duct tape products available that claim to 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  tapes that were not listed in product catalogs when
this study was performed. While we have not investigated mastics as much, there seems to be fewer
grades. There are fewer grades of mastics for use as duct sealants, and a few mastics are currently UL
181B-M  approved although many are UL 181A .
4. 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 is maintained and then fails rapidly.
This is in some ways fortunate because determining an exact numerical failure criterion is somewhat
arbitrary. Nevertheless, the following failure criterion was selected based on the results of preliminary
testing such that a good seal is adequately differentiated from a failed seal. 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.
Figure 5. Four samples connections for the aging test. Clockwise from top left: clear UL 181B tape,
aerosol sealant, mastic and 181B-FX duct tape.
0 5 10 15
Leakage [m^3/hour] at 25 Pa
UL723 General Purpose Cloth Tape
UL181B-FX Cloth Tape
Clear UL 181B Tape
Figure 6a. Changing test sample leakage at 25 Pa, from the aging apparatus
Figures 6a and 6b show the change of leakage for some early samples with time in the aging apparatus.
The initial high leakage number (about 17 m
/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 Figure 7 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 rubber or vinyl adhesive 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
UL181 Foil Tape
Butyl Backed Foil Tape
Premium Cloth Tape
Figure 6b. Changing test sample leakage at 25 Pa from the aging apparatus
About 40 different samples have been tested in the three test apparatuses. Failure was determined either
from visible catastrophic failure (e.g., the tape falls right off) or by a measurement that showed that the
leakage was above 10% of the unsealed value. Because the only samples that have failed are duct tape,
their results will be treated separately.
4.1 Duct Tape Results
Table 1 summarizes the 18 failed duct tape samples. Samples having the UL 181B-FX rating have been
listed separately to determine if this rating can be used as an indicator of good longevity performance.
The aging and baking test results indicate that there is no clear advantage for the UL 181B-FX listed
tapes. Most of the duct tape samples failed within in a week in the aging test. One UL 181 rated and one
non-UL 181 rated sample lasted for over a month.
Because the baking test does not stress the samples with either low temperatures, or more importantly
pressure, time to failure is longer than for the aging test. There was a range of time to failure from a few
days to several weeks. This indicates that some tapes have better longevity than others due to differences
in materials or construction. Also, this indicates the possibility that manufacturers could reformulate
existing duct tapes in order to improve their longevity performance.
Although the duct tape samples from the aging test have failed, some of the duct tape samples from the
baking tests have not. A visual inspection of these baked samples reveals that most of 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 verified by the aging results. This
hardening of rubber adhesives with application of heat is why silicone or urethane adhesives are used for
more temperature resistant tapes.
Table 1. Summary of Duct Tape Failures
# of Test
Final leakage at
end of testing
8 Aging 5 different grades 7 days 20%-70%
5 Aging 181B-FX 10 days 70%-100%
4 Baking 3 different grades 34 days 30%-80%
1 Baking 181B-FX 60 days 25%
4.2 Other Sealant 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 good longevity performance compared with 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
Test Type Description Duration
3 months 15mil; Foil Backed
3 months 181A
3 months 181A-P  only
1 month 181A-P & 181B
Clear UL 181B Tape
Clear UL 181B Tape
1 month 181A & 181B
Clear UL 181B Tape
4 months 181 
4 months 181A-P
2 years Pressure only
2 years Heat and Pressure
1- Note that duration does not indicate time to failure. It is the length of time the samples were tested in
Most duct tape fails prematurely when exposed for an extended period of time to elevated temperatures.
Other sealants do not exhibit this premature failure. The test results did not show a correlation of
longevity with mechanical strength, and the tapes with low strength (e.g., clear UL 181B tape) did well in
this study. Because the purpose of a duct sealant is only to reduce leakage, the longevity test results are a
better guide when selecting duct sealants than the mechanical properties currently given in manufacturers
Future work will include
• a more detailed investigation of the effects of elevated temperatures by testing over a range of
• testing of more sealants of all kinds
• development of an industry standard test method through ASTM.
This study was sponsored by the California Institute for Energy Efficiency (CIEE), a research unit of the
University of California, (Award No. BG-90-73), through the U.S. Department of Energy under Contract
No. DE-AC03-76SF00098. Publication of research results does not imply CIEE endorsement of or
agreement with these findings, nor that of any CIEE sponsor.
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