The following are questions and answers provided by the University of Wisconsin Cooperative Extension.

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Answer:  Vapor barriers are intended to stop water vapor in a home from passing through the drywall into the wall cavity and condensing on the inside of the sheathing. Vapor barriers always go on the warm side of the wall. In this case that is the inside of the new stud wall. The stud wall you add will make the foundation wall colder and therefore condensation is more likely to form on it if vapor enters the wall cavities. The condensation will decrease the effectiveness of your insulation, and can cause fungal growth and decay. Vapor barriers don't necessarily take care of the major source of water vapor entering wall cavities which is air leakage. Be sure to seal all penetrations of the wall carefully. If possible, avoid having electrical outlets and switches on the outside walls.

If you aren't absolutely sure that the foundation wall will be dry you may want to run a vapor barrier half way up the foundation side of the wall as well.

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Vapor Barriers in Ceilings

Answer: The vapor barrier always goes on the warm side of the insulation, which is between the drywall and the insulation. Seams between sheets of the vapor barrier should be sealed (usually taped) and any penetrations for electrical fixtures must also be sealed. As much as possible the wiring and electrical fixtures should be installed on the room side of the vapor barrier. A vapor barrier is not needed or desired between the insulation and the air space above it. One of the functions of the air space is to permit any moisture that migrates through the insulation to be carried away.

Question: Is it true that vapor barriers in ceilings cause problems?

Answer:  Building codes require a vapor barrier on ceilings to prevent moisture from moving through the sheet rock or plaster into the insulation. Some contractors have had problems installing vapors during the winter. When they do the drywall finishing, the drywall on the ceiling may sag from moisture. This is usually because they have not insulated the attic before beginning the drywall finishing. Moisture from the drywall finishing condenses against the plastic and dampens the drywall. If the ceiling had been insulated, the vapor barrier would have been warm and condensation would not have occurred.

Another argument used against vapor barriers is that the house needs to "breathe." The vapor barrier is intended primarily to deal with moisture movement. It only affects air movement to the extent that it makes the ceiling less leaky. Encouraging water vapor to get into the attic will lead to wet, ineffective insulation and possible damage to the house. It is much better and cheaper to provide some type of mechanical ventilation if you are concerned that the house will not "breathe" enough.

Question: When reinsulating an existing attic is it important to add a vapor barrier?

Answer:  Most older homes have enough air leaking in and out that a vapor barrier is not as important as it is in a new, tightly-constructed home. If you add a vapor barrier, do not try to pull back the insulation and install plastic between the ceiling joists. It's better to use a vapor barrier paint. Vapor barrier (more accurately called vapor retarder) paints are available from paint companies. These paints are formulated to stop water vapor from moving through the wall. To be effective, they must be applied according to directions. This means putting on a thick coat. It is probably safest to put on two coats to assure enough thickness.

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