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Relative Humidity Indoors

It has been my experience that in this climate a "normal" indoor/outdoor air exchange (15 CFM per person of 0.35 air changes/hr.) supplied by mechanical ventilation will generally be enough to maintain relative humidity at or below 50% during the heating season because warmer air can hold much more moisture. As outdoor air warms up when it gets inside, the RH drops. It is more common to see problems with humidity levels being too low on cold days. And outdoor RH is generally low during the summer here. But during those "shoulder" months, when temperatures are mild and outdoor RH climbs,

Ventilation can have very little affect on RH, and it can rise to 60% or more indoors. Fortunately, this doesn't happen often or stay that way for long enough to develop chronic humidity problems. For people of average sensitivity it shouldn't be a problem, but certain bioaerosols and other triggers may be elevated during these periods, and those with allergies or asthma may be bothered. It may be helpful to have a dehumidifier plugged in and set at 50% for these people.


Scott Finley
Energy Options Northwest

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