Example: to find where in the exterior wall the dew point will fall.
Using a sling psychrometer the inside air wet bulb temperature is determined to be 52 degrees and the dry bulb temperature is 73 degrees.

1. On the wet bulb scale (read off 100% relative humidity line) find 52 degrees, from this point, draw a line following constant enthalpy line sloping diagonally down to the right.

2. On the dry bulb scale (bottom axis) find 73 degrees, draw a line vertically up from this point.

3. The intersection of these two lines is the relative humidity of the air, in this case, 20% R.H.

4. From this point of 20% R.H. draw a horizontal line to the left. following a constant humidity ratio of .0035 lbs/lb of dry air.

5. Where this horizontal line crosses the saturation curved line, is the dew point of the air, 30 degrees.

Thus, air with a wet bulb temperature of 53 degrees and a dry bulb temperature of 73 degrees has a dew point of 30 degrees.
Next we need to find where 30 degrees falls in the exterior wall assembly of the building that is being looked at. This will take some simple calculations which will be shown in the next post.