Walls that Work in the South and Why

Figure 1, Classic Hot-Humid Climate Wall Assembly

Figure 2, Classic Mixed-Humid Climate Wall Assembly

Hot, humid climate ideal wall assembly: In this 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. (Figure 1, Classic Hot-Humid Climate Wall Assembly)

Vapor Retarders and Air Barriers: With nonabsorbent claddings, a semi-permeable sheathing will sufficiently control water vapor diffusion, so an additional vapor retarder is optional. If the cladding is brick or stucco, however, an exterior low perm vapor retarder or barrier becomes more important to block the higher vapor pressures from sun-heated, wet brick.

Building paper (# 30) and wood sheathing (OSB is less permeable than plywood) are semi-permeable vapor-retarder materials, but are not air barriers. Sealed housewrap can form an air barrier, but types vary widely in permeability. Sealed foam sheathing (with a skin or foil facing) can serve as both air and vapor retarders. Having a continuous air barrier system is more important than its location; anywhere is better than none if the wall can dry to the inside.

Permeable interior finish: In air-conditioned homes, any moisture that gets in a wall must be able to dry to the inside (cooler and dehumidified space). The interior side of the wall should be as permeable as possible - meaning no vinyl wallpaper, no oil-based interior paints and no interior vapor barrier on insulation. Having no vapor barrier on the interior side is more important than having a vapor retarder on the outside; none is preferable to one on the wrong side.

Therefore, in a hot, humid climate, highly moisture-managed frame wall assemblies include unfaced insulation in the wall cavity with sealed foam sheathing OR sealed low-perm housewrap over wood sheathing. Another option is building paper-on-wood sheathing with spray foam within the wall cavity. With brick veneer or cement siding, sealed foil-faced foam sheathing is an optimal solution. ICF, shotcrete and SIPS walls have built-in vapor and air flow retardant properties, needing attention only to joints, openings and penetrations. SIPS do require a drainage plane; concrete and foam systems don’t.

Mixed, humid climate ideal wall assembly: 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.  (Figure 2, Classic Mixed-Humid Climate Wall Assembly) 

A seasonally adaptable vapor-retarder assembly can be achieved with any of the following:

  •  R-5 or higher insulating foam sheathing on the exterior side functions as an exterior vapor retarder in summer. Yet, in winter, its inside surface acts as a warm “middle-of-the-wall” vapor retarder that does not get cold enough to cause condensation.
  • Kraft-paper-faced insulation toward the interior can act as a vapor retarder in winter (in low humidity conditions), but become semi-permeable in summer (in humid conditions).*
  • “Smart vapor retarder” technology is now available in a membrane form that is installed on the interior side of the studs and insulation. The membrane changes properties with the season, becoming a true vapor barrier in winter (when relative humidity is low) and fully permeable in summer (when relative humidity is high).*

The air barrier system is best located toward the interior (the ADA system described in Energy Efficiency section), 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 foil vapor barrier on the inside of the wall assembly so it can dry to inside in summer.

*Note: The water vapor permeability of absorbent materials (wood, paper, etc.) increases as the quantity of absorbed water on the surface of the material increases. That means permeability increase as RH increases and is the reason such materials have different “wet cup” and “dry cup” test perm values.

6/21/2008 12:46:09 AM
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