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Vapor Barriers and Dealing with Moisture

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

Humidity in the basement
Vapor Barrier on Basement Walls
Vapor Barriers in Ceilings
Dealing with Dry Rot
Drying out a Crawl Space
Proper Humidity Levels in the House
Moisture Problems

Humidity in the basement

Question: Will opening windows help reduce the humidity in a basement in warm weather?

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Answer:  Usually opening windows is not a good idea. Humidity in basements is high in the summer because the basement is cool due to the temperature of the earth surrounding it. When warm outside air enters the basement its relative humidity increases as it cools. Opening a window usually does not provide enough ventilation to increase the temperature but it does provide more air with more moisture. The best way to keep basement humidity under control in the summer is to keep the basement as tightly closed as possible and if necessary run a dehumidifier.

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Vapor Barrier on Basement Walls

Question: Where does the vapor barrier go when finishing a basement wall?

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

Question: Do you put a vapor barrier between the roof and the insulation and/or between the insulation and the drywall?

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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Dealing with Dry Rot

Question: What can be done about apparent dry rot damage to basement beams and joists?

Answer: Dry rot is a term for decay that occurs when no decay mechanisms are apparent. It is still decay and requires moisture to occur. According to scientists at Forest Products Laboratory in Madison, wood needs at least a 25 percent moisture content for decay to occur. You must find a way to reduce moisture to stop the decay. In the basement, this means stopping leaks and possibly running a dehumidifier.

You should check the beams to see how far the decay has progressed by probing them with an ice pick or screw driver. If the instrument penetrates, consult an engineer or housing inspector to determine what needs to be done.

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Drying out a Crawl Space

Question: What can be done to dry out a crawl space when open windows aren't drying it out?

Answer:  Open windows or vents don't usually provide enough ventilation to warm a crawl space adequately to prevent condensation. They introduce warm air which is cooled by the cooler walls and ground to the point where moisture condenses on the coolest surfaces. The best solution is to close the windows and otherwise stop air from leaking into the crawl space. To be sure that moisture is not evaporating from the soil cover the soil with heavy plastic. Overlap the plastic and weight it down at the joints and around the edges.

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Proper Humidity Levels in the House

Question: What is the recommended indoor relative humidity for homes is Wisconsin?

Answer:  In most homes, relative humidity should remain between 30 and 40 percent during the winter. Higher relative humidities are likely to cause condensation problems on windows and outside walls. Lower relative humidities may be uncomfortable and cause static electricity.

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Moisture Problems

Question: What would cause wet spots on ceilings during the winter?

Answer:  There area two probable causes - ice dams or condensation. Water coming from ice dams or other roof leaks will stain the ceiling before you notice evidence of moisture on the ceiling surface. Moisture on the surface without the accompanying stains probably is condensation resulting from a cold spot on the ceiling. If this occurs in isolated spots, check the attic. You may find the insulation missing or bunched up so that it isn't doing its job in the affected area.

Question: Why would a house have condensation problems after a new furnace is installed?

Answer:  Your humidity problem does not mean that your furnace is defective. The old furnace drew air from inside the house to support combustion. It also sent a stream of air up the chimney and out of the house when it operated. Therefore, in addition to providing heat it also provided ventilation for the house. Your new furnace doesn't provide this ventilation. It appears you need to find a way to replace the ventilation system you lost. You can do this by running a kitchen range hood or bathroom exhaust fan several hours each day until the condensation problem vanishes. Open a window on the other end of the house an inch or so to provide replacement air. Adding more fresh air is more effective than a dehumidifier. A dehumidifier will typically only lower humidity to about 50%. Winter condensation problems usually require that humidity levels be kept below 40%. Bringing a controlled amount of additional fresh air into your house should also be cheaper than running a dehumidifier.

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Used by permission of John Merrill and the University of Wisconsin Cooperative Extension

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