Testing Relative Humidity

Dew (condensation) occurs when the outside airborne humidity is forced to condense because the air temperature drops too low for it to hold the humidity that it held before the temperature drop. (An example is a cold glass gets beads of water on it because the glass is colder than the air surrounding it). Similarly, when humid air in any space "sees" a cooler temperature than the dewpoint of that air, condensation occurs. ( have you ever noticed the fog when the refrigerator or freezer door is opened in a hot room?). Avoiding condensation in any enclosed space is simply a matter of keeping the dewpoint in the interior air low enough that it doesn't exceed the lowest surface temp in the space.

Along the bottom is "normal" or dry-bulb air temperature, just as you'd read it on any thermometer. The sloping line along the upper left side is the dewpoint, or temperature at which airborne moisture will condense. The curving lines show "relative humidity" by percent. The vertical scale on the right is "absolute humidity." Since the air can hold more humidity the warmer it is, given the same absolute humidity, relative humidity goes down the warmer the air. This should be obvious in this chart. (Don't worry, it took me a while to read this chart too!)

Note the example dash line starting at 92 along the bottom "normal" temp line. Follow this up to the dotted horizontal line, where it says 46% RH. Now follow the dotted line left to the dewpoint sloping line and it looks like about 67. This means that 92 degree air that has 46% RH will condense on any surface below 67 degrees.



		Relative Humidity (RH) Calculation
		A recent discussion in the ListServe E-mail generated significant interest in how to measure the Relative Humidity (RH) in a room and how to determine the dew point. The following discussion is from John Robbins along with an accompanying RH chart he found in one of his text books. So here goes:
		Dew (condensation) occurs when the outside airborne humidity is forced to condense because the air temperature drops too low for it to hold the humidity that it held before the temperature drop. (An example is a cold glass gets beads of water on it because the glass is colder than the air surrounding it). Similarly, when humid air in any space "sees" a cooler temperature than the dewpoint of that air, condensation occurs. ( have you ever noticed the fog when the refrigerator or freezer door is opened in a hot room?). Avoiding condensation in any enclosed space is simply a matter of keeping the dewpoint in the interior air low enough that it doesn't exceed the lowest surface temp in the space. How to use the Psychobetric Chart: Along the bottom is "normal" or dry-bulb air temperature, just as you'd read it on any thermometer. The sloping line along the upper left side is the dewpoint, or temperature at which airborne moisture will condense. The curving lines show "relative humidity" by percent. The vertical scale on the right is "absolute humidity." Since the air can hold more humidity the warmer it is, given the same absolute humidity, relative humidity goes down the warmer the air. This should be obvious in this chart. (Don't worry, it took me a while to read this chart too!) Note the example dash line starting at 92 along the bottom "normal" temp line. Follow this up to the dotted horizontal line, where it says 46% RH. Now follow the dotted line left to the dewpoint sloping line and it looks like about 67. This means that 92 degree air that has 46% RH will condense on any surface below 67 degrees.

		Let's try another example, one that allows you to do this backwards. What if it's a nice cool 65 degrees outside, but foggy. Fog occurs when it's 100 % RH. So follow the 65 degree line from the bottom all the way up to the top where it meets the sloping dewpoint line coming up from the left. Now move straight to the right from there. Can you see that 65 degree air that is 100% RH is over 80% RH if the air temp is 70, and over 70% RH if the air temp is 75? So if you are having a problem with condensation inside your living space, maybe your problem is in certain "spots". These are specific places where the interior surface temperature can be lower than elsewhere. Maybe there's slightly less insulation there for one reason or another. Maybe there is increased exterior surface areas outside that spot to conduct more heat loss. Maybe the space directly around that inside surface has bad air circulation, such that warm humid air is getting trapped there instead of mixing with the rest of the air in your space. Funny thing is that if air "puddles" in a spot next to a slightly cooler surface, it can slightly reduce that puddle's air temperature while holding "absolute humidity" constant, thereby creating a condensing condition. If you carefully monitor exactly what's going on inside and outside in terms of temperature and humidity, you can use this chart to come up with a prescription to minimize your condensation potentials. You can also use this chart to help minimize mold growth. If you're having a mold problem, mold is sustained if relative humidity is 60% or higher. Again, this percent must be considered relative to the surface temperature where mold seems to want to grow. The way to avoid mold growth is to keep relative humidity at these cold spots lower than 50%. And CONSUMER REPORTS recommends closer to 40%! John F. Robbins, CEM Passive High Performance Home Design, Energy Analysis & Consulting Website: http://www.queencity.com/people/jrobbins Voice Phone: (859) 363-0376 [local Cincinnati Ohio call] Mail Address: 3519 Moffett Road, Morningview, KY 41063-8748 Co-Chair, SW Ohio Alternate Energy Assn (AEA) www.aea1.org Residential Chair, SW Ohio Chapter Assn of Energy Engineers (AEE) www.aeecenter.org Member, Ohio Assn of Building Performance Professionals (OABPP) Member, Kentucky Solar Partnership www.a-spi.org/solar Member, Appalachia - Science in the Public Interest (ASPI) www.a-spi.org Member, International Dark-Sky Association (IDA) www.darksky.org

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Relative Humidity (RH) Calculation

How to use the Psychobetric Chart: