vapor diffusion

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My Top 10 Building Science and Energy Efficiency Ideas of 2015

A year-end review of the articles and ideas that made me think

Posted on Dec 30 2015 by Allison A. Bailes III, PhD
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The year 2015 is almost finished. I've written 70 articles in the Energy Vanguard Blog and this one makes 49 here at GBAGreenBuildingAdvisor.com. I've been to a bunch of conferences and talked to a lot of people. A lot of thoughts about building science, energy efficiency, indoor air quality, and more have gone through my head. (Not to mention all the thoughts about skiing, Little Baby’s Ice Cream, and those things that I never let out of the confines of my skull!)


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  1. Energy Vanguard

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All About Vapor Diffusion

Even if a material has no holes, water can often move right through it

Posted on Jun 12 2015 by Martin Holladay
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Building scientists talk about several different moisture transport mechanisms. Most of these mechanisms — for example, water entry due to a roof leak — are easy to understand. Other transport mechanisms, like vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. , aren't quite as intuitive.

First, some basic definitions. Water vapor is water in a gaseous state — that is, water that has evaporated. It is invisible.

Water vapor diffusion is the movement of water vapor through vapor-permeable materials. Vapor diffusion happens through a solid material even when the material has no holes.


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Image Credits:

  1. Building Science Corporation

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The Return of the Vapor Diffusion Bogeyman

For years, some builders assumed that they didn’t really need to worry about outward wintertime vapor diffusion — but it turns out that they might have to worry after all

Posted on May 15 2015 by Martin Holladay
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Fully aware that I am engaging in gross oversimplification, I’m going to offer a cartoon version of the History of Vapor Barriers. (I’m not a cartoonist, though, so someone else will have to make the drawings.) Here goes:

Panel 1: In the late 1940s, residential building codes in the U.S. began requiring the installation of vapor barriers on the interior side of walls and ceilings. These requirements had complicated historical origins but were not based on credible building science.


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The Physics of Water in Porous Materials

Water has unique properties that govern its interaction with wood and other construction materials

Posted on May 6 2015 by Allison A. Bailes III, PhD
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I like to tell people I'm a recovering academic. The truth is, though, that I haven't left physics behind. That would be impossible since I've been making a career in the world of building science. So today I'm going to delve into that subset of building science called building physics as we take a look at the physics of water in porous materials. You'll also learn about the fourth state of water, the one that's not liquid, not solid, and not vapor.


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Image Credits:

  1. Image #1: Building America Solution Center

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Do I Need a Vapor Retarder?

Someday, builders will stop asking this recurring question — but unfortunately, that day has not yet come

Posted on Jan 18 2013 by Martin Holladay

UPDATED on May 15, 2015

Every couple of weeks, someone sends me an e-mail with a description of a proposed wall assembly and an urgent question: “Do I need a vapor retarder?” Energy experts have been answering the same question, repeatedly, for at least thirty years. Of course, even though I sometimes sigh when I read this recurring question, it’s still a perfectly good question.


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Image Credits:

  1. Matthew H

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When Sunshine Drives Moisture Into Walls

Because of inward solar vapor drive, vapor diffusion from the outside inward is often more worrisome than vapor diffusion from the inside outward — so you need a good vapor barrier strategy

Posted on Jul 2 2010 by Martin Holladay

Builders have worried about wintertime vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. ever since 1938, when Tyler Stewart Rogers published an influential article on condensation in the Architectural Record. Rogers’ article, “Preventing Condensation in Insulated Structures,” included this advice: “A vapor barrier undoubtedly should be employed on the warm side of any insulation as the first step in minimizing condensation.”


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Image Credits:

  1. Steve Bostwick

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Forget Vapor Diffusion — Stop the Air Leaks!

Building codes are finally beginning to recognize that air barriers matter more than vapor barriers

Posted on Mar 19 2010 by Martin Holladay

Last week’s blog answered some common questions about vapor retarders. This elicited a comment from Bill Rose, research director of the Building Research Laboratory at the University of Illinois in Champaign. “We might imagine a future in which the building code sections that address the vapor barrier would all go blank,” Rose wrote.


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  1. Martin Holladay

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Vapor Retarders and Vapor Barriers

Answers to persistent questions about vapor diffusion

Posted on Mar 12 2010 by Martin Holladay

UPDATED on May 15, 2015

Although building science has evolved rapidly over the last 40 years, one theme has remained constant: builders are still confused about vapor barriers.

Any energy expert who fields questions from builders will tell you that, year after year, the same questions keep coming up: Does this wall need a vapor barrier? Will foam sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. trap moisture in my wall? How do I convince my local building inspector that my walls don’t need interior poly?


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Image Credits:

  1. Martin Holladay / IECC

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Air Barrier or Vapor Barrier? - Building Science Podcast

Air leaks are a big source of moisture problems that lead to bugs, mold, and rot in homes

Posted on Mar 10 2010 by Joe Lstiburek

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This podcast series is excerpted from a two-day class called "Building Science Fundamentals" taught by Dr. Joe Lstiburek and Dr. John Straube, of Building Science Corporation.


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Image Credits:

  1. Anna Robinson

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