People need fresh air and buildings need to stay dry. Depending on the climate, there are many ways to properly ventilate a home, including using an energy recovery ventilator (ERV) or a heat recovery ventilator (HRV). Like an exhaust fan, an ERV ventilates indoor air to the outside but also brings in an equal amount of fresh outdoor air. When the two airstreams pass each other, most of the heat in the indoor air is transferred to the incoming cooler air. ERVs also transfer moisture in the air, helping maintain appropriate humidity levels within the home. HRVs just assist in fresh air exchange, and don’t exchange moisture.
The standard set by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers is 0.30 air changes per hour. This means that, about every three hours, almost all of the home’s indoor air is exchanged with fresh air. Along with the fresh air, the indoor relative humidity should be kept below 60% to inhibit mold growth.