Most of building science tends to have a cold-climate bias. Although I am originally from New York, I have spent my entire professional life as a contractor and consultant in Georgia. In this mixed-humid climate (Zone 3A), the most critical issue facing builders is moisture. We experience high humidity most of the summer and periodically in spring and fall, and dry weather in the winter. Regular heavy-rain events make designing for bulk water management key. In my new home, I addressed these moisture issues with air sealing, weather barriers and flashing, HVAC design, ventilation, large roof overhangs, and site work.
The science of climate and humidity
The Atlanta area has similar total Heating Degree Days and Cooling Degree Days. We have moderate winters with few days below freezing, and long, humid, but not oppressively hot summers with few days over 90°F. The temperature differential between the interior and exterior is rarely more than 20 degrees in the summer and 40 degrees in the winter, requiring less insulation than is needed in colder climates to maintain comfort.
Humidity control, however, is a significant issue, particularly in our shoulder seasons, when we don’t need heating and cooling. Comprehensive air sealing helps manage moisture by not allowing air—and the moisture in it—to infiltrate into the house during humid weather or exfiltrate out during dry weather, helping to maintain comfortable relative humidity levels. To manage this, I designed my house for the most straightforward air sealing possible.
Designing for moisture and bulk water management
I chose a simple volume (no cantilevers or vaults) on a slab foundation; the wood-framed walls were covered with Zip System sheathing, which made for an excellent air seal with minimal effort. On the top floor, I installed Zip on top of the ceiling joists, and taped at the…