Is there an equation that will tell me how relative humidity varies with temperature?
mfroberts
| Posted in General Questions on
I live north of Seattle and the winter humidity in my partially earth berm home runs 70%. The outside air temp tonight is 45 degrees and 95% humidity. I am trying to decide between ventilation and a dehumidifier. I would like to know at what RH that outside air will have after I warm it up.
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Mike,
The information you seek can be determined using the psychrometric chart.
For more information on how to use the psychrometric chart, see:
Are Dew-Point Calculations Really Necessary?
Psychrometric Chart Use
In your example, find the point on the chart corresponding to 45 degrees dry bulb and 95% relative humidity. Then move horizontally to the right to find out what happens to that air at 70 degrees F. You'll find that the relative humidity of the air drops to about 38% or 40%.
One of many dewpoint calculators on the web: http://www.dpcalc.org/
Air with a higher dewpoint has more moisture.
Ze formula est:
http://en.wikipedia.org/wiki/Clausius%E2%80%93Clapeyron_relation#August-Roche-Magnus_approximation
A quick & dirty approximator that will work in a spreadsheet tool can be found here:
http://andrew.rsmas.miami.edu/bmcnoldy/Humidity.html
But in Seattle the answer to the real question is "heat recovery ventilation":
Typical outdoor dew points in Seattle are in the 40s & 50sF. The dew point of fairly healthy 50% RH/70F conditioned space air is about 50F. Any time it's under 50F outdoors the ventilation air is GUARANTEED to have a dew point no more than 50F. So ventilating with air when it's 50F or colder outdoors will bring the interior air's RH down to 50% or lower @ 70F.
Only rarely do outdoor dew points hit the 60s in Seattle, which would be 70%RH @ 70F.
Heat recovery ventilation... maybe... but the price tags I have seen for installed systems still seem very high. We have cheap hydro power and who knows, he might have low loads to begin with. Seattle is quite similar to the climate here, and outdoor air does a great job of drying houses in the winter.