Thursday, August 14, 2008

Vapor Barriers

By Gaius Hennin, PE

Someone recently asked me “What is the most common mistake in residential construction?” Without hesitation I responded: “Vapor barriers.” Vapor barriers are the most common mistake in residential construction? Yup-everything about them is often wrong. They are even improperly named. The word ‘barrier’ implies that a vapor barrier stops the movement of vapor, when in reality it only retards that movement. All membranes installed in residential construction as vapor barriers allow some vapor to pass through, for two reasons.

The first is on a molecular level; vapor moisture travels through these membranes via diffusion. This is the process by which water molecules travel from where there are many of them to where there are fewer of them as a result of their ‘kinetic’ properties. Molecules are constantly in motion, bumping into each other, and will therefore mix until evenly distributed. If there is an abundance of vapor moisture on one side of a vapor ‘retarder’, it will work its way between the molecules of the retarder until there is an even quantity on both sides; even a six-mil polyethylene membrane, even a sheet of glass, given enough time. The amount of vapor moisture that passes through the membrane is a function of the discrepancy of vapor moisture and vapor pressure; if one side is very wet and the other dry, more moisture will pass through than if one side is slightly damp and the other dry. Adding heat (and therefore pressure) to the wet side will also accelerate the rate of diffusion.

The second reason is much more practical. Materials manufactured for use as vapor retarders are sold in long rolls (long enough to go around the house several times), but are not wide enough to cover the inside or outside of a house without seams. These seams will leak air and therefore vapor. Also, the vapor retarders are installed with staples, and get pierced thousands of times with drywall screws; more opportunities for vapor to get through. Perhaps most significant, though, are all of the necessary and planned wall penetrations. Every electrical outlet can represent a large hole in the vapor retarder, as do direct vent combustion units used for heating, the all important cable TV wire, the cookstove hood, dryer vent, windows, doors, light switches, air conditioning piping, solar water heater piping and direct vent boiler exhausts, to name a few. Even with careful detailing using acoustical sealant tapes and specialty products that self seal to the vapor retarder, all of these penetrations make it more likely to leak.

In addition to the inherent problems with the vapor retarder itself, there is also significant confusion about when it is appropriate to use a vapor retarder and where to place it in the wall assembly. This confusion is well deserved. The ‘rules’ (by rules, here I mean scientific rules, not building codes which heretofore have been static on the topic of vapor retarders) for vapor retarders change based on the climate in which you build, the type of vapor retarder you use, the type of wall assembly you use, the (variable) climate inside the house, who you ask and which building code is being enforced in your area. It is a contentious subject, with far more misinformation than fact readily available. Figuring out the ‘right’ answers to vapor retarder questions such as whether or not to use one and where to place it in the wall assembly can seem overwhelming and is more critical than ever with today’s highly insulated walls. The fact is there are no short answers or simple generalizations that can be applied to vapor retarders: vapor transport in buildings is complex. However, there is a solid body of knowledge based on partial and full scale testing of different wall assemblies in different climates. The two most important aspects of the wall assembly are the type of vapor retarder and the type of wall cladding (exterior siding). Wall claddings will greatly influence the selection of a vapor retarder. For years, the four model building codes defined a vapor retarder as “a material having a permeance rating of 1.0 (a ‘perm’ of 1) or less when tested in accordance with ASTM E96”, and 4 mil rated polyethylene was the standard bearer. For interest sake, and I know you are interested or you would have stopped reading in paragraph 1, a perm is defined as a hairstyle with long lasting waves or curls produced by treating the hair with chemicals.*

As the result of a five year study at the Oak Ridge National Laboratory and aggressive educating of the establishment figureheads by Joseph Lstiburek and Betsy Pettit of The Building Science Corporation and Achillles Karagiozis from ORNL, the building code has finally been modified to reflect reality. The predominant code in use today is produced by the International Code Council. The ICC has multiple publication (The International Residential Code, The International Building Code, The International Energy Conservation Code, etc) which are collectively referred to as the I-Codes. I will be writing more on these codes later as most states are now adopting the I-Codes and making their enforcement mandatory; it is the most widely used code in our history and it is here to stay.

The IRC (which governs construction of 1 and 2 family dwellings) recognizes 8 zones that the Department of Energy has developed for moisture control recommendations (see accompanying map from the DOE). The 2004 changes to the code have eliminated the requirement for vapor retarders in zones 1 (Southern Florida), 2 (the gulf Coast Region), 3 (such as Oklahoma, Arkansas, South Carolina) and 4 (such as Missouri, Kentucky, Virginia, Maryland). The 2007 (to the IRC and the IECC) change moves from the old definition of a vapor retarder (see above) to three classes of vapor retarders :

  • Class I - Less than or equal to 0.1 perm (such as polyethylene)
  • Class II - Greater than 0.1 perm but less than or equal to 1.0 perm (such as kraft-facing)
  • Class III - Greater than 1 perm but less than or equal to 10 perm (such as latex paint)

The code now requires that zones 5, 6, 7 and 8 have a Class I or Class II vapor retarder. There are several exceptions to this, the most far reaching is that a vapor retarder is not required on walls constructed of materials that cannot be damaged by moisture or freezing.

For the first time we have rules regarding vapor barriers not dictated by politics, but actually based on fact. Getting any building code to change is a colossal undertaking, and usually involves going toe-to-toe with some of the largest and most established organizations in the country (the National Association of Homebuilders, for example, who fought this change to the very end). The facts of the studies, however, and common sense make the necessity of this change obvious. We have known for years that a one size fits all approach to vapor retarders simply doesn’t work, and finally it is no longer a part of the document that governs how we build.

1 comment:

  1. Very good article on "Vapor barriers/retarders", and you didnt evem say "warm in winter side" once (ha ha). You let me know there is a little more to this topic than I had previously thought.

    And thank you so much for the description of a "perm" - very useful for a bald guy like me... I always thought a perm was a therm with a lithp!

    Seriously, keep blogging

    -Larry Owen
    student, aug '83