I Have The Vapors (Just What is a Vapor Barrier)?

by Erik North on October 3, 2011

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What is a vapor barrier (incidentally, vapor barrier is a both an outdated term and a bit of a misnomer. The permeability of most building materials fall well short of a ‘barrier’)?

First off, most folks hear vapor and think gases. For buildings, vapor means the gaseous form of water. A barrier that controls water movement, i.e. moisture, is called a vapor barrier. Several different materials can be used as vapor barriers throughout your home, including polyethylene plastic, foil faced board insulation and latex finishes.

Simply put, the vapor barrier or vapor control layer controls vapor movement (clever name, no?) across the building enclosure. Vapor barriers are found on wall, ceiling and floor sections that have been insulated and are intended to be the edge of a building enclosure. Due to water’s high permeability, vapor barriers simply slow, or retard, the amount of moisture.


A brief aside on technical building issues

Aggggh! Technical Mumbo Jumbo

Warning: Tech Talk!
I like to say materials are tested by very smart guys in lab coats. Vapor permeability is tested by the ASTM (American Society for Testing and Materials) using the dry cup method (formally known as ‘ASTM E-96 Test Method A’…sexy).

The current building code has the following definitions:

  • Class I Vapor Retarder – 0.1 perm or less (Aluminum foil, Polyethylene plastic)
  • Class II Vapor Retarder – Less than 1.0 perm and greater than 0.1 perm (Plywood, vapor barrier paint)
  • Class III Vapor Retarder – Less than 10 perms and greater than 1.0 perm (Unpainted drywall)
  • Not a vapor retarder – greater than 10 perms
  • A perm is defined as 1 grain of water vapor per square foot per hour per inch of mercury (1 inch mercury = approximately 0.49 psi).

    Finally, the Class I Vapor Retardants are called ‘vapor barriers’ by the code.

    In other words, even a ‘barrier’ lets water vapor through. Just very very slowly.
    End of Warning: Tech Talk

    Tracking Vapor Movement

    Tracking incoming moisture in your home can be difficult, but one place to check is where you are experiencing air leakage in your home. Air can often be the vehicle that transports moisture into your home. Unfortunately, when you see it is usually the first time you realized there are moisture issues. Driven by stack pressure and temperature, warm, moist air moves through the building frame. If it comes in contract with solid surfaces beneath the dew point, the water vapor will condense.

    Water Vapor – Your Home’s Number One Enemy

    It may seem that fire is your home’s number one enemy, but it’s actually water. Water can be an invisible enemy, rotting wood and leaking into foundation cracks. It can cause short-circuits in appliances, take out your power, and create breeding grounds for molds, mildew and bacteria. It’s difficult to fully dry out and restore water damaged areas. Yes, despite it being the elixer of life, water can be a challenge for your home.

    This is where vapor barriers can be both important and potentially hazardous. A properly placed and wholly intact vapor barrier will keep water vapor from condensing within the conditioned space (which can lead to M-O-L-D, Shhhhh!) and OSB turning to mush. Hooray.

    OK, I just read how angry the last two paragraphs sounded…water vapor is an issue but there’s rarely been a case of structural failure because of acute moisture. So there’s that.

    The Challenges of Vapor Barriers

    It is the water/vapor problem every builder has faced since forever: How do you keep water out of a building but allow it to dry out if there’s a temporary moisture problem? The thinking behind vapor barriers is sound…keep walls dry. In practice, builders and building scientists have concluded that some level of permeability is desirable to allow the wall assembly some drying potential. Windows leak, homeowners can find themselves hosting many people (thus many more showers) or decide they want an indoor vivarium…in other words, the building enclosure needs to deal with whatever happens.

    Environmental factors play a huge role with the vapor controls as well. Very (and I mean very) generally speaking, ‘properly placed’ for vapor control layers means on the interior in cold, northern climate zones, some permeability in both directions in temperate zones and exterior vapor controls in hot, humid climates. The issue is that its nowhere near that simple. Minnesota and Maine are among the coldest states in the continental U.S. and both get very hot and very humid come July and August. Homeowners break out the air conditioners (or fire up the central system) and the exterior heat drives moisture inward. If your vapor controls is a Class I Vapor Retarder like a polyethylene plastic, there’s a strong possibility moisture could condense inside the wall assembly.

    There are other factors like types of insulation, interior and exterior finishes and cladding/siding. Houses are most commonly clad with vinyl siding over a weather resistant building wrap. However, some cladding like stucco or brick are vapor porous and hold a good deal of water. Doesn’t it make sense in these cases to design a certain amount of drying potential into the wall system (yes)?

    Properly located and designed vapor controls working with the insulation and air barrier can help reduce (and eliminate) potentially harmful condensation issues. Older homes were so leaky and transfer so much energy through the building enclosure that they dried out like crazy. Newer homes have tighter, much more well insulated building shells which means less drying. Knowing more about the science of vapor barriers can help ensure your new home avoids moisture related problems.

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