Lora’s question seemed innocent enough, but it was enough to touch off a war of words and prove that building science isn’t always as dryly academic as you might guess. It can, in fact, get downright cantankerous.
Lora’s HVAC installer wanted to insulate the ducts in her house with double-wrapped bubble wrap “as a cheaper way to achieve R-6.” Fine, she thought, but does the stuff really work?
It “sort of works,” said Green Building Advisor Michael Chandler, but a better option is installing foil-faced duct insulation (not foil-faced bubble wrap) after ducts have been sealed with mastic and checked for air-tightness, preferably by an independent tester. Foil-faced bubble wrap “is not Energy Star or Manual D approved,” he wrote, “and is generally viewed as snake oil by reputable builders.”
Senior editor Martin Holladay was next, picking up the “snake oil” theme and going on to say any claim that bubble wrap is the equivalent of R-6 duct insulation is a “scam and a fraud.” He specifically cited a company called Reflectix, which makes a 5/16-inch thick material consisting of bubble-wrap plastic between layers of reflective material. “According to ASTM C518 tests commissioned by Reflectix, the product has an R-value of 1.04,” Holladay wrote. “(Reflectix does not mention this R-value of 1.04 on its Web site; however, it can be found in a laboratory report available from the company on request.)”
Installation is the key to performance
As Holladay points out, the R-6 insulating value is based on the complete assembly, not the material alone. In tests conducted by the ICC Evaluation Service, the bubble wrap was installed over a continuous 3/4-inch air space around the duct. “In fact,” he added, “the reported R-6 derives to a large extent from the air space, not the Reflectix.”
An installer would have to apply 3/4-inch spacers every two feet along the ducts to get the intended performance. “Most HVAC contractors are likely to consider the construction of a three-dimensional site-built air space around installed ductwork to be labor-intensive and awkward, especially considering the difficulties of working around hanger straps,” Holladay wrote. “Moreover, the R-value of the assembly depends on the long-term maintenance of the 3/4-inch air space; if the Reflectix ever sags, the R-value of the assembly will drop.”
Maintaining an air space on one side of a reflective barrier is essential to performance, Robert Riversong added. “Reflective ‘insulations’ work best with downward-flowing heat, such as from roofs to attics or from floor to cellar,” he said. “It’s possible to get as much as R-8 or R-9 in a floor with one foil surface enclosing a 4″ air space. As soon as you tilt the air space above 45° toward vertical, internal convection undermines the radiant benefit.”
Although the physics of radiant barriers is relatively simple, he added, “much of the manufacturer promotion is hype.”
If it’s good enough for space suits, why not ducts?
The “hype” and “snake oil” insinuations were too much for Kelly Myers of RFoil Insulation Products. “Contrary to the above comments, a foil-faced bubble insulation is a very good way to insulate ductwork, especially in hot climates,” Myers said. “Most people don’t understand how an reflective-faced material can insulate…”
Myers said his industry has been guilty of making “false or exaggerated performance claims” or recommending it for inappropriate situations.
“I’ve seen foil insulation marketed for applications where I know the product will not perform, such as where no airspace exists. These are times when foil manufacturers have to accept that traditional insulation is the better choice. Our industry hasn’t done that very well,” Myers said.
Even so, bubble wrap duct insulation is an effective insulator, he said, and not susceptible to the same condensation and mold problems as fiber-based duct insulation.
Then there’s the endorsement from outer space. Myers pointed out that foil is used in space suits because it reflects the intense radiant energy of space more effectively than a fiber insulation. That alone should be some indication the product works, Myers argued, and besides, conventional tests that measure R-values are misleading because they don’t take the transfer of radiant energy into account.
Some users of bubble wrap liked the results
The reference to the benefits of foil in space touched off an exchange of verbal broadsides (including claims that one thread contributor “had left a good part of [his] gray matter in outer space”) and a detailed debate over the nuances of R-value testing and physics.
There were, however, several first-person reports of bubble wrap being used successfully on ducts, the point of Lora’s original question.
Gary, for example, said what was apparently bubble wrap was installed around ducts that ran over a computer room where air conditioning equipment ran around the clock. Before the installation, condensation on cold ducts was a serious problem. Afterward, no condensation. “I can’t speak to R value or anything other than it provided what was necessary for the application we needed it for,” he said.
Likewise, Michael T. Lauer reported significant energy savings after wrapping existing R-6 duct insulation with a layer of bubble wrap radiant barrier in an attic.
In the end, Riversong may have summed up the efficiency of bubble wrap radiant barriers most succinctly: “It’s not a scam,” he wrote, “but it has limited value and only when used in the appropriate applications.”