Indoor Environmental Control in the Hot, Humid South

Design Conditions

Building Codes require that residential cooling systems be designed to maintain an interior temperature of no less that 75º on a design day as defined by ASHRAE. Design day temperatures for Charleston, South Carolina are 92º dry bulb (thermometer reading) and 78º wet bulb (indication of relative humidity). These "standard" design conditions have been developed by ASHRAE and are incorporated into ACCA’s (Air Conditioning Contractors of America) Manual J Load Calculation Procedure. The design conditions reflect average comfort conditions and take into account the limited moisture removal capabilities of standard residential cooling equipment.

Although it is certainly conceivable that the summertime exterior temperature in Charleston can exceed 92º, ASHRAE warns against sizing equipment to meet the cooling requirements during the most extreme conditions. Rather, ASHRAE recommends that in addition to temperature control, cooling systems be sized to provide adequate comfort dehumidification control as well, particularly in humid climates such as coastal South Carolina. To accomplish this, they recommend the use of an outdoor design temperature for which only a small percentage of predicted seasonal temperatures are expected to exceed. Since standard residential equipment only removes moisture when the compressor is running in an attempt to satisfy the thermostat, this method provides for improved dehumidification via longer run times and improves part-load performance.

However, this approach does little to remove moisture during high humidity/mild temperature periods such as during the Spring or Fall. This shortcoming of standard air conditioning equipment needs to be understood if humidity control is desired.

Low Operating Temperatures

Although environmental control system are designed to maintain an interior temperature of 75º on the design day, systems are often capable of achieving temperatures below that setting, particularly during off-peak days. Temperatures below 75º can have negative implications, the most common of which are sweating ducts and elevated substructure/crawl space moisture. Excessive substructure moisture can lead to mold and decay, result in cupped hardwood floors, invite termite activity and is a major contributor to poor indoor air quality. If air conditioning ducts are located in the crawl space, health effects are amplified as mold, moisture and other crawl space contaminants are readily drawn into the living space.

Substructure Moisture Control

Traditional building practices in the south have incorporated natural ventilation into homes built on crawl spaces. Although ventilated crawl spaces are still common, great improvements have been made in how to realistically address substructure moisture concerns before they result in damage. The most common alternative to natural ventilation (and frequently used to correct excessive moisture in existing construction) is a closed and dehumidified crawl space. Properly installed and maintained, closed crawl spaces prevent wood floors from cupping (moisture is equalized above and below the floors), prevent wood decay/rot (wood stays dry, 10-15% wood moisture content), prevent mold (no excess moisture, relative humidity is maintained at 50-60%) and deter termite activity.

Infiltration

Infiltration is one of the most unaccounted for aspects of indoor environmental control. Often associated with wasted energy, high rates of infiltration can significantly impact the performance of heating and air conditioning equipment and negatively impact indoor air quality via the introduction of unconditioned, unfiltered, moisture-laden outside air. High or fluctuating interior humidity is often traced back to excessive infiltration. Although leaky windows are often blamed for high infiltration, in many cases they are not the most significant contributor. Particularly prone to excessive infiltration are homes with vented attics and numerous canned lights or other ceiling penetrations. Homes with volumous "knee wall" attic spaces are also particularly susceptible to excessive infiltration.

Fresh Air

Although mechanical fresh air ventilation is recommended (ASHRAE Standard 62.2-2003) for today’s tighter homes, fresh air can not be directly introduced into a home in the southeast without first considering outdoor humidity and the added moisture load to the interior. Although energy recovery ventilators may provide satisfactory results, ventilating dehumidifiers are available that are capable of bringing in filtered outside air, putting the house under a slight positive pressure and dehumidifying the air as necessary. In addition to dehumidifying the air brought in for ventilation the units also maintains interior humidity 24/7 independent of the air conditioning system.

Unoccupied Periods

Although it seems to make sense to adjust the thermostat when we leave for an extended period of time, doing so can have negative implications. What limited moisture capabilities the air conditioning system has when it is running are non-existent when the thermostat is raised (or unit is cut off) for an extended period. Dehumidification systems that run in conjunction with (but independent of) the central air conditioning system offer the best option for maintaining humidity control even during unoccupied times.