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Install a plastic vapor barrier

QuestionI'm preparing to frame an unfinished basement and have been told both that I need a vapor barrier and that I don't. I've tested the walls and floors with a sealed-down piece of plastic and did not get any condensation. I've also been told I can put my framing directly against the poured concrete wall and also that I should move it in to leave a 2-inch air gap. Who is right?

AnswerEveryone seems to have an opinion on this subject; however, here are my recommendations, with some help from Mark S. Wiser, Residential Energy Services Supervisor for the Chelan County PUD, and Tom Craig, certified plan examiner with Spokane Building Services. All of us have seen damaged walls and framing members when framing members, insulation, and vapor barriers are improperly installed.

The purpose of the vapor barrier is to stop the warm moist air from condensing on the cool concrete walls. This is accomplished by installing the vapor barrier to the warm (room) side of the insulation. Without a vapor barrier, water vapor flows through the insulation and if the concrete wall is cold enough, the water vapor condenses on the concrete and drips down the wall. That's when the damage starts to occur—mildew, mold, rot, and water stains.

In a basement, I would install a continuous vapor barrier; i.e., 4-mil plastic sheeting, over unfaced insulation.

You could also use encapsulated insulation such as ComfortTherm by Johns Manville (which eliminates the itching associated with exposed fiberglass) or kraft- or foil-backed insulation. To learn more about ComfortTherm encapsulated insulation, visit www.jm.com. If you go this route, staple the insulation tabs (stapling flanges) to the face of the framing members—not to the inside—to create the barrier. Work carefully and don't rush the stapling process. Doing it correctly will ensure that you create the efficient continuous barrier you want and meet the energy code in force in your area.

To save money on framing members, construct the walls from two-by-fours and hold them out from the foundation 2 inches. The 2-inch space permits wall adjustment if the foundation is not plumb. Even more important, it allows space for any moisture trapped between the foundation and wall to evaporate.

If your slab is not sitting on a "Passive Radon System," then the 6-mil plastic required for this system will not be there to prevent moisture from passing through the concrete.

In this case, you will be required to use treated material for the bottom plate in the two-by-four wall construction. Even if it's not required or you do not know whether your slab sits on a passive radon system, it's good practice to use treated material for the bottom plate, but be sure to check with your local building department.

Wiser sums it up by saying, "In a perfect world we can assume the basement is perfectly dry and with proper drainage pipes installed on the exterior. Reality, though, is that it doesn't happen and people water their lawns or let snow build up or don't have adequate overhangs, etc. For these reasons, I would always recommend, in a retrofit situation, pressure-treated plates and an air space between the wall and framing."

He likes to use rigid foam for the extra R-value per inch but fiberglass is less expensive to use. I also want to point out that this project will require a building permit and inspections.

As I mentioned earlier, everyone seems to have an opinion on this subject. Here are some websites you may want to visit for further information:

  • A thorough discussion of the need for vapor barriers complete with diagrams showing specific building circumstances from renowned building scientist Dr. Joseph Lstiburek—click on "Understanding Vapor Barriers" once you get to www.buildingscience.com/resources/articles.
  • A knowledgeable site that deals with pressure diagnostics in homes—www.energyconservatory.com
  • Building envelopes program at Oak Ridge National Laboratory—www.ornl.gov/sci/roofs+walls
  • Building standards and programs, energy codes—www.energycodes.gov
  • Energy and Environmental Building Association—www.eeba.org
  • Home Energy Magazine with very good practical information. Started by building science researchers at Lawrence Berkeley National Laboratory—www.homeenergy.org
  • Home page for Washington State University Energy Programs—www.energy.wsu.edu

Copyright © 2003, 2006, & 2007 LAF/C.R.S., Inc. All rights reserved.
Question answered by Leon A. Frechette.

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