Placement within the wall, floor, or ceiling assembly
The need for vapor retarders and their proper location within a wall assembly is influenced by the interior and exterior environmental conditions as well as the wall’s thermal and vapor flow characteristics. It is important to note that each building is fairly unique in terms of wall construction, interior use, and environmental conditions, and should be evaluated individually by the building designer.
When a vapor-pressure differential exists, water vapor will move toward the lower pressure independently of air. For instance, with a winter condition of 0°F and 75 percent relative humidity, an outside vapor pressure of 0.027 inHg would exist. Inside a building heated to 70°F and with 35 percent relative humidity, vapor pressure would equal 0.259 inHg. The vapor pressure inside would be nearly 10 times as high as outside. Like other gases, water vapor moves from an area of high pressure to an area of low pressure until equilibrium is established. During cold weather, the difference in pressure between inside and outside causes vapor to move out through every available crack and directly through many materials that are permeable to water vapor. When vapor passes through pores of homogeneous walls, which are warm on one side and cold on the other, it may reach its dew point and condense into water within the wall.6
Vapor retarders are applied to the warm side of an exterior structural assembly. In cold climates this occurs toward the interior of the building, whereas in hot climates this occurs toward the exterior of the building. Generally speaking, the dividing line can be drawn between the southern tip of Texas and the Florida-Georgia border on the Atlantic Ocean. (Always verify with local building officials as to the proper placement.) Two vapor retarders on opposite sides of a single wall can trap water vapor between them and create moisture – related problems in core materials (Fig. 4.2).
A cold climate is an area that has more than 2200 heating degree- days. (A degree-day is a unit that measures the extent to which the outdoor mean daily dry-bulb temperature falls below or rises above an assumed base, normally 65°F (18°C), for heating and for cooling. Although specific exceptions may apply to a particular area, it is best to verify with local practice.3 Unless relative humidity is extremely high, moisture will not condense on a warm surface. This is usually accomplished by using vapor retarder-faced insulation or unfaced insulation plus a separate polyethylene vapor retarder.7
This practice prevents wintertime condensation, but summertime conditions on buildings that are cooled instead of heated also must be examined. Even though an interior side vapor retarder is on the “cold”
Perm Ratings of Different Materials
(Rating of 1 or less qualifies as a vapor barrier)
Figure 4.2 Vapor retarder placement. (Southface Energy Institute)
side in summer, the dew-point temperature of hot, humid summer air is nearly always lower than the temperature to which we cool buildings. That means that an interior-side vapor retarder will nearly always be warmer than the dew-point temperature, and therefore, condensation will occur only very rarely and will be of very short duration, causing no problems.3
In warm, humid climates, the flow of water vapor will be reversed, and vapor will flow from the outside to the inside.6 These areas are defined as cooling-dominated climates below the 2200 heating degree – day [base 65°F (18°C)] mark set by the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE).6 In some areas of the South, it may be difficult to determine where the vapor retarder should be placed. Where there is uncertainty, it is best to follow local practice and local codes. In southern coastal areas with a long cooling season and high exterior humidity, air conditioning causes continuous moisture flow from the exterior toward the interior cooled area. If a vapor retarder is used, it should be on the exterior of the wall.
Many professionals believe that in such situations a continuous air/vapor retarder such as polyethylene should not be used. The reason is that there may be times when the weather becomes cool and vapor movement reverses and moves from the interior toward the exterior (winter condition). When this happens, it is best to have a discontinuous retarder such as that provided by faced fiberglass insulation, to retard the passage of vapor but permit some vapor to pass. This approach is reinforced by a number of sources that state that in humid climates where very little heating is required, vapor retarders can be placed on the exterior side of insulation without causing problems in winter.7 Under such circumstances, it is necessary to avoid use of interior wall coverings, i. e., vinyl wallpaper, that have low permeance to water vapor.6 Avoid using low-perm products on the outer skin of a wall in areas with high indoor humidity. This includes vinyl or metal siding without vents, metal siding used on uninsulated homes in cold climates, insulated sheathings with foil coverings, and low – perm plastics that are substituted for breathable building papers.8
Some professionals dispute the need for a continuous, sealed, vapor diffusion retarder in warmer climates. ASHRAE, on the other hand, makes no recommendation on where or whether to install a vapor diffusion retarder in the fringe zone. In the remaining zone, one generally hotter and more humid, ASHRAE recommends omitting a vapor diffusion retarder.3 In a fringe zone nearest where interior vapor diffusion retarders are recommended, the placement gets a little trickier. According to North American Insulation Manufacturers Association (NAIMA), the highest dew-point temperatures in the United States occur in places like Biloxi, Mississippi, and Galveston, Texas, where dew-point temperatures sometimes reach 78° F. Fortunately, this happens rarely; dew-point temperatures are nearly always 75°F or lower. And because winter temperatures in Biloxi and Galveston drop below freezing temperatures on occasion, there is some justification for interior-side vapor retarders there.7
NAIMA recommends another option of using either an interior – or exterior-side vapor retarder with moderate permeance. Inset stapled (or unstapled) kraft facing, with a permeance of about 1 perm, meets this requirement. Foil and polyethylene do not; their permeance ratings are much lower. (Omitting the vapor retarder entirely would work but would not allow energy-efficient cooling. However, if the building structure itself is a vapor retarder, unfaced insulation would be suitable.)7 Where winter heating loads and summer cooling loads are equal, the vapor diffusion retarder will be on the wrong side for half the year. An air retarder, described later in this book, may be a better choice.
Cellulose Insulation Manufacturers Association (СІМА), along with the year 2000 editions of the International Building Code (IBC), the
International Residential Code (IRC), and the International Energy Conservation Code (IECC), has provided several exceptions to the prescriptive vapor retarder requirement. These include moisture-resistant materials, certain geographic areas, and “where other approved means to avoid condensation in unventilated frame wall, floor, roof, and ceiling cavities are provided.” A growing number of experts are of the opinion that the matter of moisture control is too complex to be addressed by an absolute universal prescriptive requirement for a 1-perm vapor retarder. As studies continue, more will be learned about the complicated task of limiting moisture-related damage inside walls. Breathable walls, moderate climate zones, and the actual water vapor transportation aspects of air movement, as opposed to vapor diffusion, are of interest. As stated earlier, if unsure as to the vapor retarder placement within a wall, floor, or ceiling assembly, it is best to follow local practice and local codes.