"We tried as many different kinds of duct sealants as we could get
our hands on. Of all the things we tested, only duct tape failed. It failed reliably and
often quite catastrophically," says Sherman, who heads the Energy Performance of
Buildings Group in Berkeley Lab's Environmental Energy Technologies Division (EETD).
"On the other hand, while duct tape may not last long as a sealant, in the short run
it is strong, sticky, and fairly easy to use."
During World War II, before it was called duct tape, the U.S. military bought
quantities of the cloth-backed, rubber-adhesive tape for making emergency repairs on the
battlefield. In the movie business it's called "gaffer's tape," used for
everything from bundling cables to holding sets together. Contractors, however, are not
supposed to use it for structural purposes, such as suspending ducts -- although this
legal stricture may often be honored in the breach.
Walker notes that "tape manufacturers sell all kinds of colors and grades --
'contractor' grade, 'professional' grade, even 'nuclear' grade, whatever that means. But
performance doesn't seem related to grade." Although the Underwriters Laboratory (UL)
has developed ratings for duct tape, these refer not to longevity but to such
characteristics as strength and fire resistance. Houses are supposedly designed to last 30
years, and flex duct systems are often rated for 15 years, yet there is no UL rating that
addresses the longevity of duct sealants.
To test longevity, Sherman and Walker devised an aging test, an accelerated way of
mimicking the cycling of a home or office-building HVAC system from night to day and
winter to summer (although at less extreme temperatures). Eight identical "finger
joints" -- a standard method of fitting a smaller duct into a larger plenum by means
of metal flanges, leaving gaps -- are tested simultaneously, each sealed with a different
product.
The ducts are independently supported, and all other possible leaks are carefully
closed, tighter than most real-world systems. Hot air at 75 degrees Celsius (167 degrees
Fahrenheit) is forced through four of the ducts, cold air at 12 C (53.6 F) through the
other four; the hot and cold air flows are alternated every five minutes. Although the
industry recommends that some tape products be assisted by collars or clamps, these are
rarely used in the field and were not used in the accelerated testing of the sealants.
Sherman and Walker also performed a bake test in which the sample joints were baked at
temperatures of 140 to 187 F (60 to 75 C). In many parts of the U.S., Walker notes, air
conditioning units and duct systems are often placed in the attic, "just about the
worst place to put them. Attic temperatures can easily get up to 150 degrees F."
The researchers tested 19 different sealant samples in the aging rig and 13 in the
baking rig -- what Walker calls "our rogues' gallery." When a joint leaked 10
percent of the air that it had leaked before being sealed, it was declared failed; most
joints were tested until they were leaking 50 percent or more. Only one duct-tape product
survived three months of the aging test. Eleven failed within days; some fell right off
the joint. Clear tapes, foil-backed tapes, mastics, and aerosol sealant, although they
lack strength, formed good seals for the duration. "We can't prove it yet, but we
think that heat degrades the glue, and that's what's killing the duct tape," Walker
says. While five duct tape products survived the baking test, in some cases this was
because the backing separated from the glue, then fortuitously slid over the holes,
plugged them, and baked shut again. In both kinds of test, duct tapes -- the majority of
the products tested -- were the only sealants that failed.
The aerosol sealant system developed at Berkeley Lab's EETD was tested many times
longer than the others. The sticky vinyl polymer is designed to be pumped through ducts to
automatically seek out leaks, span them, and dry; in a program undertaken for the
Environmental Protection Agency, Sherman and Walker sealed leaks with aerosol, then cycled
the ducts from ambient air pressure and temperature to hot air at twice typical
duct-system pressures, every 20 minutes for two years. There was no significant change in
duct tightness. The aerosol's success motivated the researchers to try out other sealants,
because, says Sherman, "we realized we were putting the aerosol through tests that
more traditional sealants aren't put through."
Sherman and Walker are working to have longevity standards adopted by such agencies as
the California Energy Commission and the American Society for Testing Materials. Meanwhile
the researchers are proposing more rigorous tests and new and improved sealants.
As for the almost complete failure of the current crop of duct tapes, Walker says,
"There is no reason to believe that duct tape adhesives and construction methods
cannot be reformulated to work better at higher temperatures." In the meantime there
are products in the market that pass both industry standards and also have good sealant
longevity. Given its other advantages, Sherman and Walker would like to see better quality
duct tape, as well as an improved rating system.
Sherman and Walker report their findings in the July/August issue of Home
Energy magazine. The online edition can be reached at http://www.homeenergy.org/898ductape.title.html.
