Condensation and Moisture Problems in the Home

Do you have persistent problems with condensation on windows, mold on ceilings and walls, and musty odors? These problems are annoying and damaging to the home and may cause some serious health problems if they persist. It is important to understand that household mold growth may come from small roof and other leaks but most often are caused by humidity factors, especially in the winter months. There are several of these humidity factors that must be examined to determine cause and therefore lead to some sort of solution or combination of solutions. This article will only address condensation reasons for the moisture, focusing mainly on windows since most mold problems I see are on window components. A future article will address ways to reduce the problem.


Relative humidity is the amount of moisture the air is holding relative to the maximum that it could possible hold at any given temperature. For example, if your indoor air temperature is 70 degrees and the relative humidity is 40%, the air at this temperature contains only 40% of the moisture that it is capable of holding. If the temperature would drop from 70 degrees to 55degrees, the relative humidity will climb to nearly 70% even though the actual amount of moisture has not changed in the air. This is because colder air cannot hold as much moisture as warmer air. If the temperature drops low enough, the humidity will eventually raise to 100%. This point is considered the dew point in which the humidity condenses from water vapor to water. Condensation that occurs on windows and other household surfaces in the winter results from moist air coming in contact with surfaces that are below it’s dew point.


Condensation on bare, double glazed windows that persists during the day when outside temperatures are zero or above and inside temperatures are approximately 70 degrees indicates humidity levels above 40%. If you keep your home cooler, the humidity level that yields persistent condensation will be somewhat lower; for example, 34% for indoor temperatures of 55degrees. Single glazed windows will result in a higher condensation rate because the glass of the window is not insulated as well. The glass surface temperature is close to that of outside air. When even relatively low humidity air hits this surface, the air drops below the dew point and condenses.


Covering the window with a loose fitting drape would make the condensation problem worse because it would lower the temperature of the window surface without keeping room air from contacting the surface. This is similar to the problem that sometimes occurs behind furniture positioned against outside walls or in closets with outside walls that are poorly insulated. Condensation and mold may grow on these wall surfaces because the heated room air doesn’t circulate there to warm them yet they are open just enough to room air so that moisture moving though the home can condense on the wall surfaces.


Mold forming on the corner between the outside walls and ceilings is caused when attic insulation is thin at this point because there either isn’t enough adequate space between the ceiling and the roof deck to accommodate the full amount of insulation, or there are additional structural members at this point which displace the insulation. Other possible reasons are: wind through attic vents may blow insulation away from this area; wall insulation may settle in corners; or air circulation to this area of the room is limited.


Replacing the windows with triple glazed glass will make a considerable difference. This option if often out of the question for most people. Reducing the humidity in the home and placing plastic over the windows will help in most cases and be explored in the future article, but in many cases it may not be enough. As you can see with the example above, even a home with a humidity level at 30 to 40% can cause moisture problems if the air comes in contact with a cold surface. Even a home with a indoor temperature of 70 degrees and a relative humidity of 20% will have moisture produced on a single glazed glass window if the outside temperature hits 10 degrees above zero. Poorly insulated walls in a home will become wet and support mold growth for the same reason.


For more information on mold, visit my link to frequently asked questions regarding mold at http://www.environmentalhealthguy.com/faq_category.php?catID=31


Christopher A. Hinz
Environmental Health Specialist
Waushara, Green Lake, and Marquette Counties.