The 2009 International Residential Code (IRC) classifies vapor retarders into three categories I (0.1 perm or less), II (0.1 to 1.0 perm) and III (1.0 to 10 perm).  Vapor retarders are classified using the ASTM E 96 desiccant method or Procedure A.  Class I and II vapor retarders are required in climate zones 4C, 5, 6, 7 and 8.  Exceptions are provided for basement walls, below-grade wall sections and construction where moisture or freezing conditions will not damage the building materials.  Guidance is provided for the allowance of Class III vapor retarders when design conditions exist that promote drying through the use of ventilated claddings or reduce closed cavity condensation potential through the use of exterior insulating sheathings.

The chart below presents water vapor permeance results for several common interior building materials over a wide range of mean relative humidity.  If building materials are placed into four categories with respect to water vapor permeance; vapor barrier (0.1 perm or less), vapor retarder (1 perm or less), semi-permeable (1 to 10 perms) and permeable (greater than 10 perms), then products can be described as fitting into one or several categories.  Historically, building codes required that vapor retarders have a value of 1 perm or less.

The United States can be categorized into several hygrothermal regions, which account for exterior air temperature, relative humidity and precipitation.  Generally speaking, building envelope design in cold and extreme cold climate zones focus on heating systems, while building envelope design in hot-dry and hot-humid climates focus on air conditioning systems.  These climate zones also dictate how construction must focus on moisture loads as well as keeping moisture out of buildings.   Areas labeled “mixed” experience both hot and cold climates and often can be heating- or cooling-dominated.

The 2009 International Energy Conservation Code (IECC) and the 2007 ASHRAE Standard 90.1 “Energy Standard for Buildings Except Low-Rise Residential Buildings” climate zone maps divide the continental United States into seven climate zones for energy efficiency and moisture control.  Regions of Alaska are considered climate zone 8.

Mold requires four things to grow – moderate temperatures, a food source, oxygen and moisture.  ASHRAE Standard 160-2009, “Criteria for Moisture-Control Design Analysis in Buildings” provides guidance on how to recognize favorable surface conditions for mold growth and corrosion.  The standard recommends that the 30 day running average building material surface relative humidity be maintained below 80 percent when the 30 day running average surface temperature is between 5ºC (41ºF) and 40 ºC (104ºF) to minimize problems associated with mold growth.

Liquid water and water vapor can move into buildings from various sources through four moisture drive mechanisms – gravity flow and capillary suction of liquid water, and diffusion and airborne transport of water vapor.  Roofing surfaces and underlayments, wall claddings and water resistive barriers, as well as foundations with water separation planes and drainage systems all protect a building from the outdoor environment.  Internal moisture sources include kitchens, bathrooms and mechanical rooms, as well as moisture generated by the occupant’s daily activities to live and maintain the building.

Traditional attic ventilation techniques have been shown to be beneficial in managing moisture at the underside of roof decks.  An adequately balanced ventilation system that moves air from the soffit to the ridge effectively transports water vapor out of the roof – attic assembly.  As the size of homes have increased over the years, traditional attic ventilation has become more and more difficult to achieve due to architectural details that include open attics, vaulted ceilings and cathedral ceilings.

Joint research conducted between Pennsylvania State University, Oak Ridge National Laboratory and the University of Waterloo examined the benefits of ventilation spaces between wall claddings and water resistive barriers in wood framed wall assemblies (ASHRAE 1091-TRP).  The research suggests that a 10 mm (~3/8 inch) clear air space may be sufficient to promote ventilation drying with wall claddings.  Traditional residential cladding systems combined with water resistive barriers promote drainage and drying through a ventilation space.