In the Field with Neil: Calculating Dewpoint Temperature

By Neil Moyer
October 13, 2014

Cold Drink with Lime Slices
Condensation  on my glass is OK, but not so much in my wall assembly.

Summer is nice and hot, and wet.  We build enclosures to keep us at a comfortable 55 degree dewpoint temperature (typically 75F and 50%RH).  Florida has an outside dewpoint temperature that is in the low 70s during the summer (and after a rain – shoots up to where the temperature and dewpoint temperature are nearly identical).  It is all about the dewpoint – when do things start sweating.  Buildings don’t sweat like people do, but they most certainly can have condensation problems.  Consider that nice glass of ice tea sitting on the patio table – yep it is condensation on the exterior of that glass.  (Dewpoint temperature is that special temperature when water in the vapor form turns into water in the liquid form.  It is a relative humidity of 100%.)  It is OK on my glass, but not so much in my wall assembly.  That can lead to “green buildings”; the one that no one likes except maybe lawyers and building forensic guys.  So understanding dewpoint temperature is important when we design and modify our buildings.

The dewpoint temperature is governed by the amount of moisture in a specific volume of air. This means that as the amount of moisture increases, so does the dewpoint until saturation (RH = 100%).

So how can I determine the dewpoint temperature while in the field away from my fancy psychrometric chart or computer program.  If I have a smart phone/tablet, I might use an Apple app or Android app to quickly and accurately calculate the dewpoint temperature.

So what if my smart device is dead, no computer nearby and I forgot to bring my psychrometric chart…well there is a relatively quick way to get an approximate value (assuming you know the temperature and relative humidity). So let’s see how it works given 75F and 50%RH.

  1. Subtract the relative humidity from 100.
    ( 100 – 50 = 50 )
  2. Divide the result by 3
    ( 50 / 3 = 17 ) approximately
  3. Subtract the result from the temperature
    ( 75 – 17 = 58 ) pretty close for back of hand calculation

Remember this is only an approximation, but can be handy for those impromptu “gee, I wonder ifs”.

Enjoy and try to keep dry out there.

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