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Cellulose Insulation And Vapor/Air Barriers

The Facts On Cellulose Insulation & Vapor Barriers

We receive many questions on the CIMA website Contact Us form every month. A recent one on cellulose insulation and air/vapor barriers (one of a flurry on the topic) tipped the scale toward a broader response as we continue to dispel this enduring myth.

“Good afternoon I live in buffalo NY and am remodeling my home. I want to dense pack the walls with cellulose insulation and loose fill the attic area. Do I need a vapor/air barrier in my climate zone for this product. Buffalo NY gets cold. Thank you for your time.”  Lou

DOE Double Wall Construction Diagram using Cellulose Insulation
The matter of vapor retarders was settled 40 years ago with the publication of two studies by George Tsongas. One of the studies is most reflective of Buffalo (but also applicable to most everywhere else in the norther continental U.S.) Buffalo is cold, but it’s not THAT cold. Heating degree days are 6747. The Spokane climate, where Tsongas did his study has 6842 HDD. Almost identical, but Buffalo is just a tad warmer. Tsongas made one mistake – which he has since acknowledged – in his paper. He said that in new construction installing a vapor barrier is a worthwhile precaution since it’s easy to tack up when the walls are open.

Numerous additional real world and computer model studies have been published since the 1970s. Building scientists are now in universal agreement that prescriptive use of vapor barriers, i.e. a distinct component of a building shell assembly with a perm rating of 0.1 or less, has been a serious mistake in most climates south of the arctic. They are more likely to cause problems by trapping moisture in walls than to prevent them by keeping moisture out. If there is such thing as “settled science” this is it. Vapor diffusion retarders of Class II and III – materials with permeability ranging from greater than 0.1 through 10.0 – are suggested in most of the contiguous US. They help keep moisture from migrating into walls by diffusion, but still allow for drying so it doesn’t get trapped. The US Department of Energy classifies cellulose insulation as a class III vapor retarder. It’s actually a “smart” vapor retarder, since its permeability decreases as moisture content increases.

Anyone interested in learning more about moisture management in building assemblies should check the Building Science Corporation website. Joe Lstiburek is one of the world’s leading expert in the field of moisture management in buildings. He is actually the person responsible for the climate zone map that has become a fixture of building and energy codes, and for the vapor barrier/retarder classification that is now in common use. In 2002 Lstiburek published an apology for and retraction of all the bad advice about moisture experts (like himself) have given over several decades. Since he published this document Lstiburek has raised his vapor retarder threshold to 9000 HDD. That puts it into the center of North Dakota and far northern Minnesota. For cellulose we would push this a little farther north since cellulose has superior moisture management properties compared to other fiber insulation materials.

For those with eyes now glazing over from technical language, here’s the bottom line:

Homes and buildings located in a colder northern climates of the continental U.S. likely do not need an air/vapor barrier where cellulose insulation is used. Of course, it’s important to always check as some local building codes may still require an air barrier or vapor barrier be installed. Work with your local officials regarding compliance…and share this blog post with them since old habits and myths about cellulose insulation just seem to die hard.

Dan Lea, CIMA Executive Director

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