Moisture Problems


Attic Ventilation 

How important is attic ventilation?

The most common justification given for venting attics is to prevent ice formation during the winter. A research review published in ASHRAE Transactions, 1999, 11-4, by Anton Ten Wolde and William Rose concluded that while ventilation plays some role in keeping attics dry controlling indoor humidity is much more important. Another justification for attic ventilation is to keep attics cool and prevent ice dams. According to Ten Wolde and Rose, it appears that ventilation makes little difference in ice dams formation unless large amounts of heat are escaping into the attic from below. Good insulation is much more effective in keeping the attic cool. The third reason for ventilation is to extend shingle life by keeping roofs cool during warm weather. In reality the temperature of the sheathing directly beneath the shingles was only 10 degrees Fahrenheit lower for ventilated attics than it was for unventilated attics. Sunlight and wind were the primary predictors of shingle temperature.

Cathedral Ceiling with Musty Smell

What can be done for a cathedral ceiling that shows water stains and has a musty smell?

First, check whether condensation is occurring on the ceiling or whether the problem is in the roof cavities. Condensation on the ceiling indicates a combination of high humidity and a cool ceiling. Reducing humidity in the home or increasing the insulation value of the roof system are ways to address this.

It is probably more likely that the problem is in the cavities above the ceiling. There should be an air space above the ceiling insulation to allow air to circulate over the insulation and carry away moisture escaping through the ceiling from the house. In theory, air is sucked through soffit vents and is pulled along the surface of the insulation to the ridge of the house, where it is exhausted. In reality, the insulation often fills the entire space and restricts this air flow from the soffit. This means that moisture that escapes through the ceiling is not removed. The ridge vent may even draw in additional air from the house. You may be able to remove a section of soffit to determine if air flow is blocked. Correcting the problem requires entirely redoing the roof from the inside or outside to provide channels to maintain the air flow. In the short term, reducing relative humidity in the home to around 30 percent, carefully sealing all penetrations into the roof cavities and applying a vapor retarding paint may reduce the problem. 

Condensation Problems with a New Furnace 

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

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.

Condensation on Windows

How can we reduce the condensation on the inside of our windows when it is cold outside?

Condensation and ice form on windows because the window surface is below the dew point for the air near the window. To eliminate condensation you either need to raise the temperature of the window surface or reduce the relative humidity in the room. If the condensation is on a double glazed window and the temperature outside is above zero the problem is probably high indoor relative humidity. The best way to deal with condensation on high quality, double glazed windows is to reduce indoor relative humidity to no more the 40 percent.Several conditions can increase condensation problems. Closing drapes causes the glass temperature and the temperature of the air between the drape and the glass to become cooler, making condensation more likely. If the thermostat setting is dropped substantially at night, this will cause an increase in relative humidity and may increase condensation. This condensation should vanish once the temperature rises in the morning.


Are dehumidifers a good way to get the damp feeling out basements in the summer?

The damp feeling is probably due to high humidity in the relatively cool basement air. Conventional dehumidifiers are meant to operate with temperatures at 65 degrees and warmer and only drop humidity levels to about 50% which means they are fine for summer basement conditions. When the temperatures drop much below 65 degrees the coils freeze and the units cease to operate. Dehumidifers are rated by the number of pints of water they collect per day. Small ones remove up to 25 pints per day while large ones can remove 40 to 50 pints a day. The Association of Home Appliance Manufacturers provides sizing recommendations for dehumidifers. It recommends that for moderately damp conditions (a musty smell all summer and damp spots on walls) in a basement of less than 1500 square a dehumidifer rated at 25 pints per day should do the job. Consumers Report, (June 1995) on the other hand, reports that larger models may actually be more cost effective.

Fan Vented to Attic 

Is it acceptable for a bathroom fan to vent into the attic rather than outside?

The Wisconsin single-family building code, which sets the requirements for new construction, now requires that all exhaust systems be vented outside the house. The reason is that during the winter, fans such as the one in your bathroom would send air saturated with moisture into the attic directly onto frigid roof boards. It is unlikely that under these conditions, normal attic ventilation would effectively remove this moisture-laden air before ice formed on the roof decking.

Heat Recovery Ventilators

How do we determine if a heat recovery ventilator (HRV) makes sense for us?

The first question is whether you need whole house ventilation. Many older homes have enough natural ventilation without adding a powered ventilation system. In others running existing exhaust fans will be enough. However, for new tightly constructed homes a whole house system may be needed. For cold climates an analysis done by scientists at the Lawrence Berkeley Laboratory in California (LBL) found that a balanced ventilation system that has fans to bring in air as well as to exhaust stale air is most cost effective. In fact, they recommend a heat recovery ventilator system that recaptures some of the heat from the exhausted air.If you are not sure whether your house is tight enough to justify the cost of installing an HRV consider having an energy assessment of your done. Be sure the assessment includes an air leakage test. In Wisconsin you can find out about energy assessments by contacting the Focus on Energy Program. See links below.

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Moisture Dripping from Light Fixtures

Why would moisture drip from recessed light fixtures?

There is a good chance that it is condensation. New recessed light fixtures have a housing that keeps insulation away from the hot surfaces of the light fixture. The seal between the inner fixture and the housing or the fixture and the ceiling may not be air tight. Warm moist air from the house may travel into the housing and condense on the cold housing surface and then drip back into the house. The solution is either to buy a new fixture that is designed to be airtight or to try to make your current fixture air tight be sealing holes. The light fixture should also be installed on the warm side of attic insulation. This must be done in a way that avoids fire hazards.

Musty Smell in Basement

What is the best way to get rid of a musty smell in the basement?

Musty smelling basements are a common summer problem. The musty odor is caused by dampness. Some times the dampness comes from summer rains that result in storm water finding its way into the basement. Sometimes it doesn’t enter as liquid water but as water vapor coming through the basement wall from saturated soil on the outside of the wall. More often it comes from condensation against cool surfaces in the basement. Because basement walls are in contact with the soil and soil temperature several feet below the surface remain at a constant temperature of 60 degrees or less, basement walls and floors tend to remain cool. While walls in newer basements are insulated, floors generally are not. Many folks assume that opening windows and letting more warm air into the basement will warm the air and decrease the relative humidity and hence the condensation problem. Unfortunately, it is difficult to provide enough warm air to increase the surface temperatures of uninsulated basement walls and floors. The added air actually provides an increased supply of moisture, which make condensation worse. If the moisture problem results primarily from condensation then the best solution is to close the basement off as much as practical to minimize the amount of warm humid air that enters it and use a dehumidifer to reduce the moisture level of the basement air.

Placement of Vapor Barrier

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

The vapor barrier always goes on the warm side of the insulation, which in cold climates 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. 

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

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.

Preventing Dry Rot

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

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, Wisconsin 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.

 Recommended Relative Humidity

What is the ideal relative humidity for a home during the winter?

The ideal is a compromise between what is good for the home and what is comfortable for you. Too little humidity and you will have dry skin and notice static electricity. Too much and you will have condensation on windows. To balance these extremes a relative humidity of about 35% is recommended. If you have persistent condensation on uncovered, double glazed windows when the outside temperature is above zero your relative humidity is probably well above 35%.

Reducing Winter Humidity Levels

How can winter humidity levels be reduced?

In the winter you can effectively remove moisture by running an exhaust fan and opening a window on the opposite side of your home. If you are using a range hood be sure that it isn’t just recirculating the air but is actually ducting the air outside. The cold outside air will act like a sponge as it warms and passes through the house. The cost in terms of heat loss will be just a few cents a day if you run the fan for a few hours which is probably all that is needed. When you first ventilate it may take a while to reduce humidity levels because it is likely that a great deal of moisture is stored in the walls and furnishings. This means that you have to dry out the walls and furnishings before you can dry out the air.

Removing Mushrooms

What should be done when mushrooms start growing on the floor around the base of our toilet?

Mushrooms are the fruit of a wood infesting fungus. When the fungus fruits it is well established. Chances are the wood flooring around the toilet is decaying. Leaks in your tub caulking may be helping it. You can remove the visible fungus by washing bathroom surfaces with a strong detergent and then rinsing away the residue. You can then treat the area with bleach water to slow the growth of the fungus. Unfortunately, surface treatments are not likely to get enough fungicide down to the wood to cure the problem. I expect you will have treat the infested wood in the floor from underneath in order to solve your problem. If the fungus is well established it may have weakened the wood enough that it needs to be replaced.

Vapor Barriers in Ceilings

Is it true that vapor barriers in ceilings cause problems?

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 cold 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.

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

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 many 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.

Ventilating Crawl Spaces

 How much ventilation does a crawl space need to keep humidity in check?

Research done by the University of Illinois Building Research Council and elsewhere questions the wisdom of ventilating crawls spaces. The results of this research suggests that it is better to carefully seal and insulate the crawl space particularly in warm, humid climates. These researchers have found that vents can actually introduce more moisture than they allow to escape. In warm weather the air in crawl spaces is usually cooler than the outside air. This is because the earth floor which is usually below grade is protected from solar heating and affected by the cooler temperatures of below grade soil. When air from the outside enters through the vent it, therefore, cools off. As it does, its relative humidity increases. In some cases the crawl space can be cool enough that relative humidity rises to 100% and condensation occurs on cool surfaces. The situation is even worse if air conditioning is running in the home above the crawl space. This will mean that the floor above will be one of the cool surfaces on which condensation occurs. The venting lets in humidity both through air circulation and also through diffusion of water vapor from the humid outside air to the air inside which may be lower due to the condensation. Note that not all building codes allow a sealed crawl space at this time.

Wet Spots on Ceilings

What would cause wet spots on ceilings during the winter?

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.

Winter Condensation Problems 

What can I do about mold growing on the outside corners of our bedrooms in the winter?

The mold is a sign that you have too much moisture in the air in your home. The first step is to determine the source of moisture. Some steps to take include:

    1. Check for plumbing leaks under sinks, tubs, toilets, around water softener, furnace, and washer for drips and wet places.
    2. Look for roof leaks.
    3. Be sure the clothes dryer is venting outside.
    4. Make sure that water is draining away from the outside of your home so that your basement is not a source of moisture.

The next step is to reduce the moisture you add to the air through daily activities. You can cover pans when cooking and remove any firewood that is being stored in the house. Discontinue the use of humidifiers. Run exhaust fans for 15 minutes after taking a shower or bath, washing a floor or washing dishes. Make sure that the fans you use vent to the outdoors, as opposed to recirculating air or venting to the attic.

If you have reduced the sources of moisture as much as practical and still have a problem you will probably need to increase ventilation. Use existing exhaust fans to do this. In a tight house ventilation will be more effective if you open a window at the other end of the house a crack to provide replacement air. Cold outside air can hold very little humidity. When you bring this air inside, it can absorb a substantial amount of moisture as it warms up. If you then exhaust the air, you will effectively be removing moisture from the home and reducing relative humidity. If this doesn’t solve your problem you may want to investigate a whole house ventilation system.