Healthy Indoors Magazine
Issue link: https://hi.healthyindoors.com/i/1056074
Healthy Indoors | 9 condensation occurring on the interior walls, which are typ- ically constructed of cellulose-rich wallboard. A classic ex- ample is the application of vinyl wallpaper in hotels located in hot and humid environments. Hotels like vinyl wallpaper because it looks good and is easy to maintain and clean. One minor problem, though, is that vinyl wallpaper is also a very effective vapor barrier. Since hotels tend to be un- der negative pressure, thanks in part due to the centralized bathroom exhaust system, they pull unconditioned outside air through the building envelope. Moisture contained in the infiltrating air is deposited on the gypsum board surfaces enclosing the wall's cavity, migrates to the wall covering/ gypsum board interface, and gets trapped there. Now, if the dew point of the infiltrating air is, say, 75 o and the rooms AC is set to 72 o , it's pretty simple to guess what happens next. Jack Springston, CIH, CSP, FAIHA, is the Industrial Hygiene Services Manager for ATC Group Services' metropolitan New York area operations. Jack is an in- dustrial hygiene consulting professional with over 30 years of experience in recognizing, evaluating, measur- ing, and controlling employees' exposures to health haz- ards in their workplace. He has been a Certified Industrial Hygienist since 1993 and is one of less than 50 CIH's who also currently have a sub-specialty certificate in Indoor Environmental Quality. Jack has participated in and over- seen hundreds of indoor environmental quality studies. Contact him at: John.Springston@atcgs.com within a given amount of air is purely a function of how much energy is in that air. Relative humidity is a measure of how much of the energy available for evaporation has actually been used to liberate water molecules and change them into vapor. So, a relative humidity level of 50% means that half of the energy has been used to evaporate water from whatever available sources there are, and half of the energy is still available to do more evaporation. Pretty simple, right? In a gas such as our atmosphere, what we refer to as "temperature" is really a measure of the how fast the vari- ous molecules in the gas are moving. The faster the mole- cules move, the more kinetic energy those molecules have and, therefore, the higher the temperature of the gas. When a faster moving air molecules crashes into a slower moving water molecule, it transfers some of its energy to the water molecule, thereby making it move faster. The faster these water molecules move, the less likely it is that they will be attracted to, and bind with, adjacent water molecules. Con- versely, the lower the temperature, the slower the molecules move and, at some point, the attractive forces between the water molecules become strong enough to make them stick together and condense. The temperature at which this hap- pens is commonly referred to as the "dew point" — think condensation on a cold drink glass or a on a window. Numerous cases of poor or misguided construction in humid areas has resulted in vapor barriers being installed on the wrong side of the exterior wall. This, in turn, results in i