William Robinson, Reichel, Claudette H.
Hot-humid climate ideal wall assembly: In a hot-humid climate, moisture flows from outside to inside most of the year. In mild, short winters, any moisture flow in the opposite direction is not enough to cause problems because it dries out soon enough. Therefore, the ideal assembly of a frame wall keeps the interior side fully permeable and locates both an air barrier and water vapor retarder or barrier to the exterior side to minimize the entry of outside water vapor. (See Figure 1: Classic Hot-Humid Climate Wall Assembly)
Therefore, in a hot-humid climate, good moisture-managed frame wall assemblies include: sealed foam sheathing or sealed housewrap over wood sheathing or building paper on wood sheathing with spray foam within the wall cavity. With brick veneer or stucco, sealed foil-faced or skinned foam sheathing is a good solution. ICF, Shotcrete and SIPS walls have built-in vapor and air flow retardant properties, needing attention only to joints and penetrations. SIPS do require a drainage plane; concrete and foam systems don’t.
Mixed-humid climate ideal wall assembly: In a mixed-humid climate, moisture flow changes direction in cooling and heating seasons. In summer, it flows from outside to inside; in winter, from inside to outside. As a result, in mixed climates, the wall should be able to dry in both directions, changing with the season. Therefore, the ideal frame wall assembly for a mixed, humid climate locates an air barrier toward the interior side, a permeable interior finish and a water vapor retarder (not barrier) that adapts to the seasons. (See Figure 2: Classic Mixed-Humid Climate Wall Assembly.)
A seasonally adaptable vapor retarder assembly can be achieved with either of the following:
The air barrier system is best located toward the interior, but any location is better than none. The interior side of the wall should be permeable -- meaning no vinyl wallpaper, no oil-based interior paints no plastic or no foil vapor barrier on the inside of the wall assembly -- so it can dry to inside in summer.