Provide Supplemental Dehumidification or Humidity Controls in Humid Climates

In climates that have frequent or extended periods of high humidity, even a properly sized air conditioning system may not operate often enough to adequately maintain acceptable indoor humidity levels. As discussed throughout this section, high humidity can lead to the formation of mold on surfaces inside the home as well as condensation on building surfaces. Plus, occupant comfort is diminished with high or very low humidity even if the air temperature is comfortable.

• Provide supplemental dehumidification or humidity-based controls that can maintain indoor relative humidity at or below 60%, for Climate Zone 1 and those portions of 2A and 3A below the white “warm humid” line in Figure 5–6. Also consider these systems for homes in Zones 3A, 4A, and 5A which have high performance building envelopes where central cooling will be less frequent.

Why? Many parts of the country see at least some muggy weather during portions of the year. Basements often may be quite humid either during and after construction or during summer months when the temperature remains low relative to the outdoors thereby elevating relative humidity (RH). A maximum 60% RH level balances durability in the home, occupant comfort, and operating costs during these humid periods of the year. During cold winter months lower limits apply (see text box to the right).

There are several types of dehumidification systems available:

1. Portable dehumidifiers requiring manualemptying of condensate buckets unlessplumbed to a drain,
2. Whole-house dehumidifiers thatsupplement the moisture removalcapacity of the air conditioning systemand utilize the central ductwork,
3. Ventilating dehumidifiers that draw airinto the home, remove moisture, andthen deliver this air to the return side ofthe central air handling unit, and
4. Variable speed central air conditioningequipment that includes adehumidification mode to operate basedon indoor humidity levels (regardless ofwhether there is a call for cooling.

The cost of each type of system increases as its functionality becomes more sophisticated, and as with ventilation systems homeowners need some basic education in using the systems correctly.