Question-and-Answer Resource for the Building Energy Modeling Community
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You did not mention the climate or the time of year during which you see the low RH, however it is common for RH to be low in many climates during the heating season. As well, many climates (SW USA, for example) have low humidity year 'round.
Regardless, with no HVAC, there is no way to control it. (Unless you consider some sort of evaporative humidifier as "not HVAC")
