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Fine Homebuilding: The Magazine

Fine Homebuilding: The Magazine

What's the Difference: Vapor Barriers and Vapor Retarders?

comments (2) March 5th, 2009 in Blogs
RYagid Rob Yagid , senior editor

Click To Enlarge Photo: Krysta S. Doerfler

by Rob Yagid

You don't have to be a building-science expert to know that trapped moisture is bad for houses. To help slow moisture diffusion through roof, wall, and floor assemblies, many experts-and in some parts of the country, building codes-demand the use of vapor retarders.

Across the building industry, however, the term vapor barrier is commonly used in place of vapor retarder. This misuse raises the question of whether the two terms categorize the same products and whether those products have the same performance traits.

Understanding perm ratings 

Permeability, which is the amount of moisture that can pass through a material, is measured in perms. The lower the number,  the less permeable the material and the more moisture it will block. Contact the manufacturer to get the perm-rating  information for the product you are thinking about using.

Vapor retarders are all-encompassing

The International Residential Code (IRC) defines a vapor retarder as a vapor-resistant material, membrane, or covering with a  perm rating of 1 or less. However, the 2007 IRC supplement recognizes some materials that have ratings of 1 perm and higher  as vapor retarders. Based on their perm rating, building products fall into one of three classes of vapor retarder.

The IRC does not mention vapor barriers, but some manufacturers and some people in the building-science industry use vapor  barrier to distinguish a class-I vapor retarder, or an impermeable material.

Why terminology is important

As building science progresses and influences the way houses are built, attention to detail and accuracy are critical. An  assembly that calls for a vapor barrier is designed to stop moisture on one surface-under a concrete slab, for  example-while more permeable vapor retarders allow some movement of moisture. If walls, roofs, or floors are configured  with the wrong vapor-retarding products, a structure can trap moisture. Misusing these terms leads to confusion in product  choice, which ultimately can lead to failure where it matters most-in your home.





posted in: Blogs, framing, roofs, water and moisture control, walls, floors
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Comments (2)

mpmarchese mpmarchese writes: Help
I live in a 4 unit, 23 year old condo building. I found I have windows that leak. The contractor I called reported that a silver paper used as a vapor barrier between the siding and the plywood had disintgrated and now the plywood is getting wet and rotting. He checked different sections of the building, not just where I'm having problems and it is gone from most of the building. What information is available for me to give my other condo owners to tell them this must be fixed?

Posted: 2:55 pm on June 8th

stevede stevede writes: If I am building a house with stucco and stone exterior in Delaware. Would the following insulation be ok and prevent moisture trapping? Starting from the outside: Tar paper, sheething, 1" closed cell srpay foam, Kraft paper faced fiberglass (thickness determined by 2x6 wall) with the combination to give R19+. In the attic I would like to use 1" closed cell spray foam topped with blown in fiberglass combination to give R 39+.
Posted: 10:17 am on November 7th

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