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Worries About Trapping Moisture

While some types of wall assemblies and roof assemblies are risky, many worries about ‘trapping moisture’ are baseless

Posted on Dec 2 2016 by Martin Holladay

A significant number of questions posted by readers on the GBAGreenBuildingAdvisor.com site are variations of, “Will this wall detail (or roof detail) trap moisture?”

When I entered “trap moisture” into the GBA search box, I got 182 results. The search terms “trapping moisture” yielded another 104 results. Clearly, there is a high level of concern around the issue.

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Image Credits:

  1. Kontrol

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Smart Vapor Retarders for Walls and Roofs

Can a smart vapor retarder be used to make an otherwise risky assembly safe?

Posted on Jan 29 2016 by Martin Holladay

During the winter, when indoor air is usually warm and humid, most wall 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. is cold. Under these conditions, we really don’t want water vapor to move from the interior of our homes toward the exterior. That’s why builders in the 1980s installed polyethylene on the interior side of walls.

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Image Credits:

  1. Photo #1: Alex Wilson
  2. Photo #2: Oliver Klein

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Plastic Production Rises But Recycling Can’t Keep Up

Production continues its 50-year trend upward, but millions of pounds of plastics end up in landfills and the oceans

Posted on Feb 20 2015 by Scott Gibson

Plastics serve many purposes, but millions of tons of discarded plastics end up buried in landfills, floating in the world's oceans, and burned in poorly regulated incinerators, a report from the Worldwatch Institute says.

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Image Credits:

  1. Michal Manas/ Wikimedia Commons

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Polyethylene Under Concrete Slabs

Does the poly vapor barrier belong above the rigid foam or below the rigid foam?

Posted on Jul 4 2014 by Martin Holladay

What goes under the concrete in a slab-on-grade home? In the old days, not much — just dirt. Eventually, contractors discovered that it made sense to include a 4-inch-thick layer of crushed stone under the concrete. The crushed stone provides a capillaryForces that lift water or pull it through porous materials, such as concrete. The tendency of a material to wick water due to the surface tension of the water molecules. break that reduces the amount of moisture flowing upward from the damp soil to the permeable concrete.

Since the crushed stone layer provides a fairly uniform substrate, it also may also reduce the chance that a concrete slab will be poorly supported by random pockets of soft, easily compressible soil.

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Image Credits:

  1. R. Ian Parlin / Oyster River House

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What Happens When You Put a Plastic Vapor Barrier in Your Wall?

A look at the physics of humid air and cool surfaces

Posted on Apr 30 2014 by Allison A. Bailes III, PhD

A lot of people have heard advice about vapor barriers and vapor retarders. Many of them have walked away confused. A big part of the problem, I think, is that they've been told what to do — "Put it on the warm-in-winter side," or "Never use one" — but they haven't had the physics of what happens explained to them.

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Image Credits:

  1. Energy Vanguard

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Smart Vapor Retarders

Modern replacements for the troublesome old poly vapor barrier have low permeance in cool, dry weather, and high permeance in humid weather

Posted on Jul 31 2013 by Alex Wilson

Nowhere in building design has there been more confusion or more dramatic change in recommended practice than with vapor retarders. Thirty years ago, we were told to always install a polyethylene (poly) vapor barrier on the warm side of the wall. Then we were told to forget the poly and go with an airtight layer of drywall (airtight drywall approach). Insulation contractors, meanwhile, often said to skip the vapor barrier; we need to let the wall or ceiling cavity dry out.

It made for a lot of confusion. And I’m not sure we’re totally out of the woods yet.

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Image Credits:

  1. Alex Wilson

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How to Deal With a Vapor Barrier Edict

When a local code inspector insists on an interior polyethylene vapor barrier, plans for a simple addition to a Canadian home suddenly get more complicated

Posted on May 6 2013 by Scott Gibson

Christopher Solar had a simple plan for an addition to his Ottawa home. The one-room structure would have a shed-style roof with a cathedral ceiling and vertical board siding. Solar liked a wall assembly he'd read about at GreenBuildingAdvisor, which consists of exterior foam, batt or blown insulation in the stud cavities and airtight drywall on the interior. An interior polyethylene vapor retarder never entered the picture.

And that's where his story gets complicated.

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Image Credits:

  1. CertainTeed

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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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Vapor Barriers Redux

Are there any risks if you build an exterior wall without an interior polyethylene vapor retarder?

Posted on Sep 24 2012 by Scott Gibson

Few topics in building science seem to have caused as much confusion as the use of a polyethylene vapor barrier in exterior walls.

Once routinely used by builders to prevent the migration of interior moisture into wall cavities, polyethylene is no longer recommended for houses unless they’re built in extremely cold climates.

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Image Credits:

  1. Fine Homebuilding magazine

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Can Polyethylene Be Used as an Air Barrier?

A material that's fallen out of favor might still have a legitimate use

Posted on Oct 24 2011 by Scott Gibson

Polyethylene sheeting has had its ups and downs as a preferred building material over the last 20 years.

At one time, it was routinely used in wall assemblies as a vapor barrier. As building scientists learned more about air and moisture movement through walls and ceilings, however, they began to advise builders that an interior vapor retarder is better than an interior vapor barrier, and the perceived usefulness of poly plummeted.

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