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Vapor Barriers

As a Building Official In Connecticut I am constantly confronted by builders choosing to use kraft faced fiberglass insulation. As we are now enforcing the IECC 2009 installation methods and practices that were wrongly accepted for thirty years need to change. Tabs stapled to the inside of framing members is unacceptable and has been wrong for 30 plus years but acce[ted as common practice and approved. Technically this also does not pass the "continuous vapor retarder required by the IBC and IRC. In reality the tabs are not vapor proof because they do not have the asphalt glue that binds the fiberglass to the paper and acts as the vapor retarder in conjunction with the paper facing. Most builders choose the fiberglass because of product cost and ease of installation which reduces labor costs. Is using unfaced fiberglass insulation properly installed, with a continuous poly vapor barrier a better way to handle the situation. It covers the code and properly installed and sealed should prevent moisture vapor from entering the wall cavity through the sheetrock. What are the negative aspects if any that you see. Many insulation contractors do not believe in poly vapor barriers, but these are the same people who continue with sloppy installation and stapling the facing to the inside of the framing members. If we are going to still accept fiberglass insulation is unfaced with poly a better choice.
Ed Palma, Town of North Haven, Connecticut Building Department, LEED, NAHB CGP,
Connecticut Contractor HIC #561267

Asked by Edward Palma
Posted Jul 19, 2012 11:06 AM ET


4 Answers

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I think you are focusing on the wrong issues.

The kraft facing on fiberglass batts is a perfectly adequate vapor retarder, even when the batts are inset-stapled, and even when the batts are sloppily installed. I don't think there is any justification for labeling these vapor retarders as less than continuous. Even if the vapor retarder has 3% gaps, it is still 97% as effective as a vapor retarder without gaps.

Problems due to outward vapor diffusion during the winter are extremely rare. Moisture problems in walls are caused chiefly by (1) rain penetration and (2) air leakage. So you should focus on the elements of the code pertaining to air leakage; don't worry so much about very minor problems with vapor retarders.

The main problem with the batt installation methods you describe has nothing to do with the vapor retarder. The problem has to do with the low R-value of the resulting walls. Unfortunately, batt-insulated walls perform very poorly because these walls usually have many air leaks and because the batts don't fill the stud cavities completely.

In your climate, installing polyethylene as a vapor barrier is a bad idea. If the home is air conditioned, the use of polyethylene can lead to moisture problems during the summer.

Answered by Martin Holladay
Posted Jul 19, 2012 11:52 AM ET


Thanks for your help Martin. I do stress proper air infiltration sealing to the contractors as well as proper installation techniques. We are also seeiing more builders subscribe to duct testing and blower door testing of their houses as this will be a mandatory requirement when we adopt the IECC 2012 next year. I would assume that your answer applies to the new poly- faced/fiberglass products that Johns Manville and Certainteed are offering?

Answered by Edward Palma
Posted Jul 19, 2012 1:30 PM ET


To the best of my knowledge, the main purpose of the poly wrapping of the batts sold by Johns Manville and CertainTeed is not to provide a vapor retarder; the poly is there to provide itch protection.

According to Johns Manville, the facing (which is "perforated to let moisture escape") is "twice as resistant to moisture vapor penetration as kraft paper."

The advice I provided on kraft-faced batts applies equally to these poly-faced batts.

Answered by Martin Holladay
Posted Jul 19, 2012 2:10 PM ET



I certainly agree with Martin that the issue is air leakage and not vapor drive. I'm in Washington State and our State code training and advising is preformed by Washington State University http://www.energy.wsu.edu/Home.aspx. I just attended their pilot air sealing workshop on this yesterday. It was exceptionally well done and will be rolled out to the rest of our state.

The upshot is that they also agree that air sealing is the issue and vapor drive seldom needs special handling.

We currently require blower door testing at 5.9 ACH 50. As it stands now, WA State will adopt the IECC 2012 and the requirement goes down to 3 ACH 50.

Air sealing to 3 ACH 50 will be a whole new game and I think where your knowledge and support will have the most impact. Stay current. At 3 ACH 50, it begins to make sense to look at spanning walls and lids with an air barrier membrane that is NOT polly. Something that is an air barrier & a class 2 vapor retarder.

You can still apply drywall directly to it, and use good http://www.airfoilinc.com/ or other electrical boxes and perhaps skip the gaskets for airtight drywall. However it's done, as things get tighter, membranes make more sense because they are faster and more reliable.

Membranes or not, it's going to be a difficult transition for many builders. We are seeing some insulation contractors starting to take over air sealing on projects. Perhaps that will be some good news as they can be more efficient with blown in + a membrane.

I great to see someone in your position engaged in the dialog.

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Answered by albert rooks
Posted Jul 19, 2012 2:15 PM ET
Edited Jul 19, 2012 2:22 PM ET.

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