Air and Moisture Control in Hot, Humid Climates

Moisture Control in Humid Climates

A well-designed air and moisture control system can add decades to a home’s life, save the home buyers thousands of dollars on energy and maintenance costs, reduce health risks like mold, and generally add to the value of the home and the quality of life and comfort of the buyers.

The best practices for air and moisture control vary depending on climate: it isn’t always cut-and-dry. Climate dictates the primary direction-of-flow for moisture, where moisture is in danger of becoming trapped, and where in the walls it is best to place moisture retarders. In this article we will go over the specifics of proper air and moisture control in warm, wet climates, where constant humidity threatens to erode building materials and promote mold growth.

In hot and humid climates, the largest source of moisture comes from outside air. Infiltrating moisture condenses on cool indoor surfaces, forming droplets of water and promoting mold.

While designers and builders can’t control the occupants’ ventilation practices, you can impact building tightness in important ways. A report on air and moisture control from the Clemson Extension discusses “12 Golden Rules for Builders in Warm and Humid Climates.

Here are a few key points from the “Golden Rules” to ensure air and moisture control in climates with high humidity:

  1. Use an air infiltration barrier on the outside of exterior walls. Vinyl coverings or other moisture-retarders placed on the inside of exterior walls will trap moisture inside the walls and promote mold.
  2. Eliminate crevices in exterior walls with tight building practices. The envelope should prevent outside air from entering the stud space and the house interior.
  3. Place a vapor retarder (at least 6 mils thick) beneath concrete slab floors to stop moisture and gases from entering the house. Any tears or holes must be tightly sealed.
  4. Air conditioning and heating ducts should be metal with exterior insulation. Ducts with interior fiberglass linings trap dust, which feeds mold.
  5. Install return air ducts in each bedroom or undercut the doors. Unless bedroom air can enter the return air stream, a negative pressure will form in other parts of the house and pull humid air in from outside.
  6. Provide air circulation into and through each closet and enclosed space via louvered doors, open shelving, or air conditioner supply vents to discourage moisture retention.
  7. Cure poured concrete and dry the structure before applying wallpaper, paint, or floor coverings. Wet building materials can create future problems and destroy the strength of the structure.

Preventing air and moisture infiltration and diffusion is vital in hot, humid climates, where the air is wet and warm. When moisture infiltrates the walls of a structure, the warm temperatures allow mold to thrive, and even more so when it can feed off organic building materials (wood, cloth, or paper).

Mold and other fungi pose serious health risks, as well as threatening the integrity of the structure’s materials. Ensure a tight enclosure and properly placed moisture retarders to protect the life and value of the home.


Cosella-Dörken delivers innovative, high-performance air and moisture barriers for commercial and residential construction sold under the DELTA® brand name. A North American manufacturer based out of Beamsville, Ontario, Cosella-Dörken Products, Inc. is a subsidiary of Ewald Dörken AG, a leading European developer and manufacturer of waterproofing and drainage products sold worldwide. Cosella-Dörken is known for delivering premium products while providing educational programs and full technical support. For more information, call 1-888-4DELTA4 (433-5824) or visit

  • Thomas Dugan

    While understand and appreciate that your goal here is to focus on moisture infiltration to the building (so as to sell your product), I take issue with that being the only source of moisture to be concerned with. I build precast insulated concrete homes that are about as tight as you can get (ICF and SIPs buildings are tight as well). To adequately control Relative Humidity in these homes you must also allow for the human generated humidity. Showers, toilet evaporation, cooking and even our breathing can contribute several quarts or more of water each day. Even in summer months when air-conditioning is running, there is a need for more latent capability than a properly designed HVAC system can provide. During the spring and fall transition months when AC if not running, the problem is even greater. With a really tight house, winter can still be a problem.

    The solution is to have a separate dehumidifier added to the system. It can utilize the existing ductwork as long as the system fan control can be controlled by a dehumidistat. In multi-story homes such as we have at the North Carolina Coast, I use a separate duct system for the inline dehumifier that also pulls hot air from high in the top floor and pushes it downward to the lowest level. This helps to overcome the inherent “stack effect” that is common with multi-story buildings without airlocks.

    My personal home has this type of system installed and I am able to dial in my desired RH within a few percentage points. I try to maintain an RH in the low 40s as this is supposed to help control dust mites (I am allergic) and black mold that can form in toilet bowls and tanks.

    When we used to only build leaky stick-framed houses, none of this was of much concern as the house was able to “breath” as they used to say. Unfortunately, the houses would have to breath in as well as out.