[Greenbuilding] indoor humidity levels and how to lower them

nick pine nick at early.com
Fri Dec 30 13:28:00 PST 2011

Reuben Deumling <9watts at gmail.com> writes:

>... Are there other tricks for lowering humidity indoors?

Turn on a kitchen or bathroom exhaust fan when the house RH rises to 60%. 
Andersen windowfolk say an average family of 4 evaporate 2 gallons of water 
per day, ie 0.67 pounds per hour... 70 F house air at 50% RH has Pa = 0.374 
"Hg and wa = 0.62198/(29.921/Pa-1) = 0.00787 pounds of water per pound of 
dry air. If 60C0.075(0.00787-0.0025) = 0.67 (with outdoor humidity ratio 
00025 in Phila in January), C = 28 cfm. ALMOST ALL US HOUSES NATURALLY LEAK 

>I'm not sure where they'd get the makeup air.

Through cracks and crevices in the house envelope. For a lot less heat loss, 
turn on a reversing fan in a house partition wall to draw air in through 
half of the house envelope and exhaust from the other, then vice-versa, 
turning all the cracks and crevices into extremely efficient bidirectional 
air-air heat exchangers, if your house leaks less air than 1 in a million US 
houses. Or use an exhaust fan and add enough solar siding to make up for the 
heat loss.


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