b'YOUR TOOLKIT FOR BUILDING EXCELLENCEPhoto 1surface of the component.point temperature of the air in contact with the surface, the Psychrometrics providessurface will remain dry (no condensation).anunderstandingofThe air state point is located where the air temperature line the relationship of air- intersects with the relative humidity line.A humidity ratio line moisture mixtures andthat intersects with the air state point is followed horizontally dew point temperature.to the line to the 100% relative humidity line to determine Basicpsychrometricsthe dew point temperature at the air temperature and relative can be studied using ahumidity used to find the air state point.simplified psychrometric chart shown in Figure 1.When the relative humidity is less than 100%, the air The science oftemperature will be higher than the dew point temperature. psychrometrics The difference between air temperature and dew point temperature as a function of relative humidity is shown in Airtemperature(redFigure 2.lines) increases from leftFigure 2to right, and the amount of moisture that air can holdincreasesasair temperature increases.The relative humidity (green lines) indicates how full the air is of moisture at a given air (dry bulb) temperature.The humidity ratio (blue lines) increases from bottom to top and indicates the actual amount of moisture in the air.Figure 1The bottom lineTo minimize condensation on inside surfaces of exterior building components, one wants to operate a building such that the inside dew point temperature is lower than the inside surface temperature of any exterior building component.The inside air temperature will be similar, to somewhat higher, than the inside surface temperature of all exterior building components.A large difference between air temperature and dew point temperature exists by maintaining a low indoor relative humidity (Figure 2).The required difference between dew point temperature and inside air temperature depends on the weather conditions and The dew point temperature (purple letters) is found where theR-value of the building component.The climate zone map, humidity ratio line intersects with the 100% relative humiditygiven in Figure 3, provides some insight into the required line with the dew point temperature being the air temperatureminimum R-value of a window or door frame.Colder zones at the intersection.Dew point temperature is the air-moistureshould have higher window and door minimum R-values.mixture property that indicates when condensation will occur.A higher R-value will result in a higher inside surface A surface that is cooler than the dew point temperature oftemperature. This allows indoor air to have a higher dew the air in contact with the surface will have condensationpoint temperature before condensation begins to form.form of the surface.If a surface is warmer than the dew continued on page: 20 FRAME BUILDER - VOL5 4 / 19'