Ventilation will keep indoor humidity down if the outdoor dew point temperature is at least 20 deg F lower than the inside temperature. For example, the outdoor dewpoint right now is 58 deg F at the Oakland airport. If the indoor temperature is below 78 deg F, the house will be humid inside. If the house is poorly ventilated, it will be even more humid inside.
If the outdoor dewpoint reaches 70 deg F, the house will be a swamp inside. The solution is a dehumidifier. No need for ductwork, just put it anywhere in the house and set it for about 50% RH.
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