Taking back the moisture

How much moisture does outside air possess?  This question has a few answers.  The first question that should be asked before giving an answer is "what is the temperature outside?" A given quantity of air will occupy a certain volume at standard atmospheric pressures and temperatures.  Double the temperature and allow the space that this quantity of air occupies to increase so that the pressure of the air remains at standard atmospheric values, and the space the air occupies will approximately double.  This is the reason unconditioned air in your home (no humidification) is relatively dry in the winter.

Very cold outside air at standard atmospheric pressures contain much less moisture per given volume of air than warmer air.  This is specifically true when cold and warm air at the same relative humidity levels are compared. Relative humidity is a measure of how much moisture a given quantity of air contains compared to how much moisture that same quantity of air could potentially hold.  As an example, say a 100 cubic foot volume of air at 70 degrees has a relative humidity of 50% and contains 1 cup of water.  At 100% relative humidity this same volume of air would contain 2 cups of water. Now lets take a look at why our homes are so much drier in the winter.

Let's say the outside air is 0 degrees F and is at 20 percent relative humidity.  If we take a 100 cubic foot sample of this air and assume it holds 5 ounces of water when it is at 0 degrees and 20 percent relative humidity, what will happen to that air when we bring it into our house and raise the temperature to 70 degrees?  First of all, the 100 cubic foot space this air occupies will increase dramatically when it is heated to 70 degrees.  Instead of occupying 100 cubic feet this same quantity of air will occupy a much greater space.  If air is allowed to expand, heated air will always occupy a much greater space.


Hopefully the picture is starting to become clear.  Let's go back to the outside air that contained 5 ounces of water at 0 degrees.  Now that we've brought the air inside the home and raised it to 70 degrees lets assume it occupies 200 cubic feet of space.  As you can see, five ounces of water contained within a much larger 200 cubic feet of space thins the solution out considerably.  All of those airborne water molecules are much farther apart and the air begins to feel dry.  Also the relative humidity of the air has decreased significantly, since not only has the air expanded to a much greater volume, but also warmer air is able to hold much more water at similar volumes as cold air.

Our homes are continually bringing in outside air to make up for air that is lost to the outside. There are many reasons our inside air is expelled to the outside such as exhaust from hot water heaters and furnaces.  When new air from the outside is brought in it immediately expands and the relative humidity of that air goes down.  This is why we need humidification in the winter.

Written by Randy