More Q&A Spotlight
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.”
Radiant Barriers
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.”
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13 Comments
How ironic...
How ironic that as read this great article by Scott, on the same webpage there is an ad by Lo/Mit Spray-on Barrier. Maybe we should jump from the frying pan to the fire!!! It all “boils” down to $$$$….
It boils down to keywords, Armondo
Google serves the ads based on what the article is about. This may not be the best value for the people buying the ads though, huh?
reflectix
I read and understand that using bubble wrap as duct insulation without an air gap doesn't work. What if it is installed on the bottom of the floor joist with a 4" space between it and the fiberglass insulation. Is that still a waste of time. Unfortunately I have already purchased the bubble wrap and would like to be sure it is an ineffective product before returning it. To bad I found out about this site at the wrong time.
Response to Harry Corey
Harry Corey,
Reflectix has an R-value of about R-1. If it is installed adjacent to an air space, it can raise the R-value of the air space up to about R-3. That's not much.
You can get much more insulating value per dollar by buying real insulation. For example, $100 of XPS or EPS foam will do a much better job of insulating than $100 of Reflectix.
So if the retailer who sold you the Reflectix will take it back and give you a refund, by all means return it.
The duct feels cooler with bubble wrap - why is that?
My heating ducts were warm to the touch.
I installed some bubble-type duct insulation because I think fiberglass is the asbestos of the future.
Now when I touch the outside of the insulated duct it is cool to the touch.
I realize that this is not exactly lab-tested proof - but what other explanation is there except that the bubble wrap in insulating the air duct?
PHM
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Response to PoodleHead Mikey
PoodleHead,
You're right -- the bubble wrap is insulating the duct. It probably has an R-value of R-1.
It just isn't insulating very well. In most areas of the country, R-6 or R-8 duct insulation is standard (and often required by code).
Infrared Images of Bubble-Foil Wrap Duct Insulation
I was recently in an attic of a recent-construction multifamily building; I noticed that they had used two layers of bubble-foil wrap duct insulation on the duct trunks, and a fiberglass wrap on the runouts. Both of these ducts were sheet metal, but with some insulated flex at runouts. The bubble wrap was installed with no air gap between the bubble wrap and the sheet metal duct.
I happened to have my infrared camera with me in the attic. Of course, foil surfaces throw off temperature measurements by infrared, due to emissivity effects (that's why you can set the emissivity value on IR cameras). However, the ducts were both covered with a layer of cellulose insulation, which essentially negates the effect of low E coatings.
You can see the difference in surface temperatures--the bubble wrap foil duct has a much warmer surface temperature than the fiberglass wrapped ducts. So it looks to me like two layers of bubble foil wrap is performing worse than an R-6 fiberglass duct wrap.
Yes, I know, the ideal experiment would have been to clean off the ducts and run a strip of masking tape down the foil jackets of both to get a same-emissivity surface temperature test. However, I was trying to get an investigation done, not get duct surface temperatures.
Also, as a side note--if the low-E foil coating had a significant effect on my surface temperatures, it should make surfaces "look" colder, not warmer, to an infrared camera.
Response to Kohta Ueno
Kohta,
Thanks for sharing your revealing photos. Your conclusion -- "the bubble-wrap foil duct has a much warmer surface temperature than the fiberglass-wrapped ducts" -- is consistent with what one might expect from the dismally low R-value of bubble wrap.
You didn't mention whether the photos were taken during heating season or cooling season -- but I assume that these ducts were delivering heated air from a furnace or heat pump, not cool air from an air conditioner.
Duct Air Temperature
Sorry I didn't make that explicit--this photo was taken on a 30-40 F outdoor temperature day in New England; all HVAC equipment was running in heating mode. Heating was provided by 90%+ sealed-combustion gas furnaces.
Last year we finished off an attic room over our garage and had duct work added from our existing HVAC system to the new space. The contractor looked at the size of the unit and said it should handle the additional approx 230 sq ft of space. The system works somewhat... but it's definitely warmer in that room during the summer than the rest of the upstairs. My question from the start was with the type of insulation they used for the duct work, which was the thin bubble wrap type around the main trunk line. The main house has fiberglass insulation around the main trunk line. Both spaces, new and old, used fiberglass flex lines. My question is: Can I add fiberglass insulation over the existing foil bubble wrap to up the r-value and help the system handle the additional space footage? I'm concerned about the cool air being lost to the extreme temperatures in the attic. Or would that be a waste of money. Our house is in the southern Maryland region.
Thanks for any insight,
HD
HD,
Q. "Can I add fiberglass insulation over the existing foil bubble wrap to up the r-value and help the system handle the additional space footage?"
A. Yes. The existing bubble wrap has such a low R-value that it's basically worthless as insulation.
Thanks!
I have installed a lot of reflective barrier. One must remember that reflective barrier MUST be installed as a designed system or used to stop radiant heat transmission one direction or another.
Bubble foil in a crawl space under joists when sealed properly provides vapor barrier and thermal reflectivity two directions while giving a very low initial R value. It still works well for that purpose.
Multiple layers of foil barrier with air gaps can provide EXTREMELY efficient ending of radiant heat transmission. I have used it in southern and western walls as well as vaulted ceilings to end heat Transmission problems. Literally dropping temperatures of the interior surface 50 degrees or more.
We need to stop arguing about whether reflective barriers work and begin to address WHEN and HOW they work best. They are extremely inexpensive for their return on investment when used as a SYSTEM.
The question I have is why would one use bubble foils on an ac system when one can use sheet foils with air gaps for the same return on investment. With a proper air gap they will work two directions, without it they must have mass insulation under them and only work in one direction.....toward the air gap.
Possibly a better idea is reflective foil under rafters (or under roof deck and under rafters. Then standard foil faced ducting will have two layers stopping radiant heat transfer as well as cooling an attic to within a few degrees of exterior ambient. I have done this in Dallas MANY times.
The best results in Dallas were reflective barrier under the rafters PLUS over the insulation. This gives the best results for summer and winter. Many clients had rooms that were unusable due to heat loss and or gain prior to installation. Resolved issues at a very fair price.
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