Why the dew point matters in a home inspection

For those who frequently check weather reports, the dew point is likely a well-recognized but poorly-understood figure. According to the official definition provided by the National Oceanic and Atmospheric Administration's National Weather Service, the dew point is "[a] measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant). A higher dew point indicates more moisture present in the air. It is sometimes referred to as Dew Point Temperature, and sometimes written as one word."

Just like any other weather phenomenon, the dew point has implications for the proper functioning and maintenance of many systems throughout a home. Real estate professional publication WorkingRE went in depth on how these effects impact a home inspection.

The dew point determines the point at which water vapor condenses into a liquid. Imagine a glass filled with ice - assuming the surface of the glass is 32 degrees Fahrenheit or less, and the surrounding air is warmer. This will cause water vapor that impacts the glass to turn into liquid moisture, because it will have been lowered below the room's dew point.

This can also be observed on window panes during the winter. As WorkingRE explained, some condensation often forms on windows when the outdoor temperature is significantly different than the indoor temperature. When it's cold outside but warm inside, condensation tends to form on the lower edges of a window pane. Since cold air tends to fall relative to warmer air, the coldest air at the bottom of the window may freeze during periods of extreme cold. This can cause problems with the area surrounding windows including warped frames and mold growth, according to WorkingRE.

A home inspector may recommend one of two fixes for the issue. For one, the temperature of the glass itself could be raised. This may require a fan or the application of more direct sunlight to the window. The other solution is to reduce the humidity of the interior air. This might be the less practical solution, since dehumidifiers require high amounts of electricity and may make the inside uncomfortable.

Dew point and mold

Understanding dew point, condensation and humidity is central to fixing issues related to mold growth. Mold likes to grow in warm, damp areas. However, some homeowners may be confused to find moisture, and thus mold, growing where there are seemingly no holes where rain could enter. As WorkingRE noted, this is a common reason for mold growth on a roof deck. Very small leaks allow warm, moist air to enter a space in a roof or attic. This air is then cooled below the dew point, causing moisture to form. Voila, a mold problem has begun.

To stop this mold from forming, WorkingRE recommended finding and sealing these air leaks. This may be accomplished by installing air seals between the window and the frame. using this simple fix, many common mold problems may be alleviated.