Using radiant barriers as duct insulation, Aka "Bubble wrap" What value do you assign for R-value?

I recently inspected a house that I am rating and discovered the HVAC contractor used Big 8 bubble wrap on the metal main duct in a vented attic in Climate zone 5. I spoke to the contractor about this at the rough inspection and duct testing and told him he needed to add more insulation to the nice wrap job he had done. My experience has been that a materiel that thin + a thin air space can not be classified as a R-8. I found a few articles from some pretty knowledgeable building science guys to confirm my thoughts. The contractor however is insistent that this is a suitable method of insulating duct work. He sent me the manufacturers claims and I looked up a ESR -report. (ESR-1236).  First I must say I do not work on really any new buildings with duct work outside the thermal boundary and advised agaisnt it! 

What is the industry accepted method for crediting duct work wrapped with a radiant barrier and a airspace.?The ducts did pass a total leakage test, but they still leaked so, it is not in a vacuum either. I could not find any official language by RESNET or IECC on this.

Thanks for your Thoughts in advance!

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1st:  Read 4.2 and 5.0 of the ESR-1236.   Are all of the conditions for installation listed there fully followed. 

2nd:  Are all of the Manufacturer's Install instructions followed?

If they are, you must accept the R-Value Claims. Just as you would with a Batt saying R-19 or an insulated flex duct stating R-8 that is installed correctly.

If you have doubts about the claimed R-Value, there is not much you can do for this home.  You can send the data you have collected to the FTC.  They regulate sales claims related to R-Value.  You can check their R-Value Rule on their website. Their word on R-Value Claims is probably the final word.

No listing on the wrap it is a 0 in my book - but if the r-value is called out / shown on wrap, it is as John states, you list it as such

I commonly see this an assign it the Value the manufacture states. 

But, I completely agree and so do other HVAC professionals that it most likely does not perform to the assigned R-value. Very frustrating. Just explain to the client/homeowner your concerns. Can you bury the system in blown insulation. If it is air sealed very well then should perform fine w extra insulation on top

That is why I went into the detail of the install directions.  You only have to accept the Posted Value, if tt is installed accortding to manufacturers instructions.    I put a yardstick in a blown attic to check the height and I get a 1 inch tube of empty space around it -  I take a picture and cut the R-Value in half.

Well thank you all for your thoughts!

I did receive a little feed back from the MassSave program that I work with in MA. They will not recognize radiant air barriers that count the air, only the material. They also pointed out the International Mechanial code, 

IMC 604.7 Identification. External duct insulation, except spray polyurethane foam, and factory-insulated flexible duct shall be legibly printed or identified at intervals not greater than 36 inches (914 mm) with the name of the manufacturer, the thermal resistance R-value at the specified installed thickness and the flame spread and smoke-developed indexes of the composite materials. Duct insulation product R-values shall be based on insulation only, excluding air films, vapor retarders or other duct components, and shall be based on tested C-values at 75ºF (24ºC) mean temperature at the installed thick-ness, in accordance with recognized industry procedures. 

I think it is important that all HERS raters are consistent on this. I also brought this up with my provider who asked for a RESNET interpretation. We got a unofficial answer and a form to fill out for a official answer from RESNET. I would assume since RESNET has adopted the "I"codes it will be a slam dunk! I hope.

Thank you all!

I concur with your IMC interpretation. I have used this before with code officials and they have had to agree. Look forward to hearing what the "official" response is from RESNET.

Read the manufacturers claims and installation steps closely  The foil needs a dead space 

"To achieve a thermal resistance of R-8, 2-inch-wide spacer strips of rFOIL™ reflective foil insulation are wrapped around the duct to create spacers at 2-foot intervals. The Big-8 duct insulation is wrapped around the duct with the spacers creating an airspace between the duct and the insulation. All seams and joints are sealed using rFOIL Metalized Tape (see Figure 5 of this report)."  from the duct installation instructions."

If the installation is in a hot & humid climate -- that dead space could very well mean a place for moisture to condense when the air hits the duct... and that could mean water pooling and collecting between the duct and the foil.   Potentially perfect conditions for mold over time.  If the water leaks,  it can damage the building materials it resides on or around it.   Problems with condensation on the outside of cold ducts has been documented sufficiently enough to call it into question.   Those issues lead to additional research - as noted below (BTO)

The DOE through the Building Technologies Organization (BTO) and the Build America program has done testing on buried ducts... and the technique that works well  -- is foamed ducts (to keep air away from the duct work) - then burying them in insulation --OR- just making sure that all ducts are in the climate controlled space to begin with...

I'm with Sean Lintow -- while you can measure the R value in a lab for testing... there are simply too many ways it will fail when installed in the field... especially if the installers take short cuts or get sloppy.

@Dennis, without defending Big-8, I would point out that condensation (on duct surface) is a function of how well the vapor barrier (the outer layer) is sealed. This issue is exactly the same with conventional foil-faced fiberglass duct wrap. It, too, must be properly sealed at every seam and termination, otherwise condensation will form on the duct surface in all but the driest climates.

Correct,   but when testing has been done on sealing ducts with wraps... (See the BTO and Build America website for reports... plenty of research done by ASHRAE and universities also) the seals are never really tight enough to prevent the moisture from making it to the metal duct work.   Remember moisture can migrate through sheet rock and most building materials.  Only certain types of paint will seal against moisture migration.  There are a lot of vapor barrier techniques that are quite effective... but any time an insulation like the one described in the story is added AFTER the fact... it's almost impossible to completely seal around the duct and prevent humidity from getting in between the outside wrap and the duct.    This is one of the reasons why foamed AND THEN burying the duct has been tested AND tried for buildings with ducts in attics.   Its also why moving ducts into the climate controlled space is also now emphasized. 

Agreed, But this guy fought me all the way. I have completed the rating and the HVAC contractor added R-8 on Top of the bubble wrap. We had to hit a HERS of 55 or below and when I gave the duct insulation R-1.1 it increased the HERS score to 57. The Builder had no choice but to add duct insulation resulting in R-9.1 duct insulation. Still not great in my book but got a D+

It was a modular home and the main duct was elevated in the truss right down the middle. 100 % in unconditioned attic.

I just hope RESNET chimes in agreeing  with the Building code so all Raters are consistent going forward. 

Onward and Upward!

Links for Build America and Building Science Corporation:

Using a wrap to insulate a duct is almost impossible.  That's why new installations use the long lengths of duct insulation that is pulled over the duct work at installation and not wrapped.  

I've seen FAR too many wrapped duct installation jobs in which they use masking tape or cheap "duct tape" to seal up the wrap... and that fails within a few years of installation.

The third document (NREL) shows good installations -- and examples of bad duct wrap installations that fail over time.

And finally

Link above talks about high humidity areas... references work done by Dr Joseph Lstiburek  of Building Sciences Corp.

As a radiant barrier manufacturer I've been fighting this bad information for years. I've run into this claim for installing radiant barrier in attics many times.  Which is similar the claims made for wrapping ducts.  This is a video I did back in 2011 explaining how companies claim to get an R-11 with a 1/4" thick product. AtticFoil Compared To Bubble Foil Products



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