About Relative Humidity

Author: Pacific Northwest Inspections Group, LLC   Date: 25-Nov-2013.   Category: Mold   Add to Favorites 

What is Relative Humidity (RH)?

Relative humidity is defined as the ratio of the water vapor pressure or water vapor content to the saturation vapor pressure or the maximum vapor content at the temperature of the air or gas. The saturation vapor pressure in the air varies with air temperature: the higher the temperature, the more water vapor it can hold. When saturated the relative humidity in the air is 100% RH. Relative humidity recommended for houses is between 30% and 50%. This level of humidity is not always easy to maintain, especially in certain “problem” areas of the home or building. A dehumidifier is often used in basements and crawlspaces where humidity can be very high. However the use of the furnace may contribute to areas of your home suffering from the opposite problems: lack of humidity. Therefore, monitoring RH levels in your home will allow you to maintain areas and deal properly with seasonal variation conditions.

Examples of How Relative Humidity Changes can Occur:

On a 30 degree F day with outdoor RH of 60%, the indoor RH can drop to levels as low as 15% when the air is heated to 70F (as the air is heated it will expand giving it the capacity to hold more moisture. If moisture is not added to the air as it is heated the RH level may drop). If the outdoor temperature is 10 degrees F, the indoor relative humidity may fall to 8% or less. These are extreme levels of air dryness.  In the summer, outside air at 70F and 60% RH flows into a crawlspace at 60F causing an increase in RH up to 82%. You can see from the above examples why it is important to measure and control relative humidity levels throughout the entire home or building, since humidity levels in the air may change drastically through the heating and cooling processes. These processes can then lead to extreme highs or lows in the RH levels indoors. If you suspect unhealthy conditions in your home due to humidity being too high or too low, it is prudent to utilize a tool known as a thermo-hygrometer. This handy instrument calculates temperature and measures indoor relative humidity levels. Using a thermo-hygrometer is the best way to learn about and understand your indoor environment. This unit allows you to evaluated the RH levels in your home and understand the RH patterns, such as highs and lows, that exist. If you do have excessive humidity, and see evidence of a mold problem, then you may need an air purifier to remove mold spores from the air.