Why does air become more humid as it cools down and more dry as it heats up? Think of it this way, evaporate one cubic meter of water into the warm humid outdoor air inside a space the size of your back, side, and front yard and you will not detect much humidity in that vast amount of warm expanded air, cool that volume of air and it shrinks, let’s say you shrink it to the size of the interior of you home, obviously your home is smaller than the yard it sits on. Now you have less air volume but the volume of humidity ( evaporated water in the air ) remains the same and that humidity is squeezed closer together now, the result is more humidity in the air per given area and the relative humidity goes up. Cool that same air down even more, cool it down to such a degree that it occupies a space the size of one cubic meter.

Now that air that was the size of your front back and side yard and contained one cubic meter of water vapor now is one cubic meter of air and contains one cubic meter of water. Now the air is saturated with water, it is at 100% RH, we call this dew point and the water vapor molecules join together to form droplets of water we call condensation and mold now has water to grow.

There is a gradual progression of wetness in that ever changing volume of air and that gradual progression of wetness is measured in Relative Humidity percentage when testing your air. Around 60% to 65% Relative Humidity is all that is needed for mold to grow; we do not need 100% or condensation for it to grow.
Open doors and windows are often not a problem in northern or western regions but in hot humid Florida outdoor air often causes mold problems if the AC is on or off, but with the cold AC turned on condensation problems becomes even more of a problem in and around AC units and ducts.



To open or close my windows

January 30, 2013


Often our clients with mold problems are told that opening doors and windows improves indoor air quality because it dilutes pollutants. Well that is true if you want to dilute indoor air pollutants.

If it is humid outdoors, for example around 60%RH or higher, then opening doors and windows can very easily cause mold problems that far out ways the benefits of the influx of fresh outdoor air. Hot humid air is even more of a threat than cool humid air.

Humidity problems comes from outdoor humid air when doors and windows are open and it becomes a serious problem when the AC unit is not used on a regular basis to help dry the humidity from the air.
If doors and windows are open when the AC is in use, then some of that hot humid outdoor air that enters the property be dried by the AC but an AC unit cannot typically dry all the air entering the property and entering the AC unit. The humidity that is not dried will become condensed under these new cooler conditions especially inside the AC unit, this is because most fluid media including air condenses or becomes smaller when it is cooled. Because the hot humid air is now condensed inside the cold AC the humidity in that air squeezes closer together and this causes the humidity in that air to rise to an even higher level than it was when it was outside.

Simple quick answers that are always correct are few and far apart in regard to mold problem advice, but in most cases it is not advisable to open doors or windows for extended periods of time if it is above 60%RH outside. If the outdoor air is rain free, dry, and clean and you are safe from intruders climbing in your window then opening your window may be beneficial as it may let in fresh outdoor air and dilute your moldy indoor air.