Excessive Moisture in the House Q&A


“For the past few weeks, I’ve been having a problem with moisture inside the house. I’ve noticed lots of condensation on my windows and mold starting to grow on the window frames, and at the base of several walls. When I clean the mold up, it goes away for a while, but then comes back. I’ve even noticed mold on some of the indoor flowerpots. I don’t know where all of the moisture is coming from, and what I need to do to correct this. Any suggestions?” – L.J., Oakland



Excessive moisture in a house is never a good idea, and over prolonged periods can allow mold and mildew to grow. What you are experiencing is common, and tends to occur after winter rains. During the winter the air and exterior elements of a house are cold and damp. To stay warm, we tend to keep our homes sealed up by closing all of our windows and doors. This eliminates natural ventilation and traps moisture inside the house. Without ventilation or enough temperature, moist air cannot circulate, and the moisture cannot evaporate.

Moisture inside a house only occurs if it is generated by the occupants of the house, or from some external source such as a plumbing or roof leak. Believe it or not, normal living conditions can generate several gallons of water vapor a day. Every time we cook, shower, wash or dry clothes, or even breath inside a house, we are releasing water vapor into the air. Unless we provide some means for this vapor to escape, it will remain in the house and condense on window, wall, and ceiling surfaces.

Ventilation is really the key to controlling moisture in a home, and when combined with some heat, it can ensure that a home remains comfortable and dry. Without ventilation, wet or humid air is drawn to cooler surfaces such as walls, windows, ceilings, or personal belongings, where it condenses on the surface. If the moisture does not evaporate, it creates a cool moist environment for the mold spores in the air to grow.

The first thing you should do is to determine what the sources of moisture can be. The most common sources are from bathing and cooking. Does condensation form on your walls and ceilings after cooking a meal or bathing? If so, you need to increase the ventilation in the area by using an exhaust fan or opening a window. Do you have indoor plants or aquariums? These are also big generators of moisture indoors.

Occasionally, indoor moisture can be the result of standing water under the house, or excessive moisture in the attic. Often times during the winter a home can have some amount of standing water under it, or excessive moisture in the attic from a roof leak or from a lack of ventilation. Moisture from the sub area and attic can infiltrate into the interior of the building through vapor pressure and condense on wall surfaces and personal belongings. It is important to check these areas of the house to ensure that they are well ventilated and basically dry.

If standing water is noted in the sub area, it may be necessary to have it pumped out, or to have fans installed to accelerate its drying. If excessive moisture is noted in the attic, it may be necessary to install additional eave or roof vents.

Moisture and mold growth on the inside of the house usually forms on surfaces that are not exposed to any air flow or movement of heat such as behind dressers, beds, and in closets. Moisture is more apt to condense on exterior walls that are not insulated and in rooms that do not have their own heat register or source of heat. Many homes built in the 1940’s through the 1960’s only had a centrally located wall or floor furnace. These heating units could not efficiently get heat into most of the bedrooms and bathrooms like the forced air furnaces used today. Because of this, it was common for mold to appear.

Once the sources of the moisture are realized, then you have the ability to deal with correcting the mold conditions. Minor amounts of surface mold can be cleaned and removed with a mild solution of chlorine bleach and water. Start by mixing one part bleach and 7 parts water and spraying the solution on the mold. Let this set for a few minutes, and the color of the mold will begin to disappear. Then, wash the area with soap (cleaning detergent) and water and dry. This will remove any mold spores that remain on the surface.

Finally, keep in mind the importance of keeping the interior of the house well ventilated. In the morning open the blinds so that any condensation on the windows can evaporate, and open doors to bedrooms so that air can circulate. And remember to either open a window or use the exhaust fan after bathing or cooking. Following these simple suggestions will normally keep mold from occurring.

Experience: Mold and Mildew

Mold and mildew were indications of leakage in wall cavity that could not be readily seen.