Causes of Attic Moisture

Causes of Attic Moisture

Once a moisture problem has been detected either by obvious visible signs of condensation, mold or through elevated wood moisture readings in the attic, the next step will be to determine the possible causes and solutions of the moisture issue.

Below a wide variety of moisture sources are identified.  All of them have one thing in common that must be understood.  Air acts like a sponge soaking up water vapor.  The warmer the air, the more moisture that it can hold.  The challenges in attic moisture occur when the warm air is cooled down by lower attic temperatures and no longer able to hold its same moisture level.   That moisture is released from the air in the form of condensation on the attic surfaces.

Every attic and home is different, but here are some general causes to attic moisture problems:

Moisture Generated in the Home

There are many sources of moisture in the air created from everyday living. Some are hard to change as we still need to use our homes for cooking, showers, baths and cleaning clothes. We all still need to breathe, which also releases moisture into the air. There are other sources that may be adjustable such as the overuse of a humidifier in the winter, unvented heaters or unmanaged moisture in a basement or crawlspace.

All of this moisture migrates up toward the attic through something called the stack effect. We all remember from our science classes as children that warm air rises. The warm air in the home effectively acts as a transport for the moisture in the air moving up. This air continues to rise through the house and escapes into the attic through poorly sealed cavities and gaps between the attic and house. In the most climates during the summer, this is not an issue as the attic temperature is likely to be hotter than the air in the house allowing the attic air to easily hold the moisture. In most climates during the winter, the temperatures are reversed with much colder temperatures in the attic. These cooler temperatures keep the air from holding the moisture and in effect trapping the moisture in the attic. With such cold temperatures in the attic, the moisture coming in will continue to be an issue if it is trapped in the attic with nowhere to go.

Hot Humid Air from Outside

In the summertime, attics that stay very hot will generally be able to hold moisture in the air without concern for condensation. However, problems can occur when you have big fluctuations in temperature from daytime to nighttime for example. When the outside air is hot and humid, it will migrate into the attic space bringing the humidity and moisture with it. This is fine as long as temperature remains high, but what happens when the outside temperature drops substantially at night? The cool outside air will cool the inside surface of the roof. This will cause the temperature in the attic to be reduced, but the moisture gets trapped in the attic space and without the higher ambient temperature in the attic it can no longer be held in the air and begins to condense on the surfaces inside the attic. Once this occurs it then takes much longer to get the moisture out the following day. If this continues to happen over a long period of time it can lead to major problems.

Improper Venting

Many homes have exhaust fans in the kitchen to remove steam and moisture from cooking areas. Most homes also have exhaust fans in bathrooms to remove moisture from showers and baths. When working properly, these fans are great for removing excess moisture from within the home living area. The challenges occur when they are not properly vented to the outdoors. It is not uncommon for these fans to be set up improperly with the exhaust blowing directly into an attic space. Check where your exhaust fans are venting to.  

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