Technical Updates

Posts Tagged ‘vapor retarder’

In hot, humid climates, one membrane can often act as the secondary weather barrier, air barrier, and vapor retarder. The most common of these membranes is “peel-and-stick” bituthene membrane (self-adhering composite membranes of rubberized asphalt bonded to polyethylene film) installed in masonry wall cavities or directly behind envelope finish materials, such as fiber-cement siding or stucco on lath.

In temperate climates, such condensation can easily occur in the winter, wetting the wall components. Even with low indoor RH levels, the wide temperature differential through the wall generally ensures that a first plane of condensation will be within the wall. Not only does condensation in such conditions cause mold growth, but the wetting of insulation reduces the wall’s thermal effectiveness.

Thus, the building envelope plays a vital role in minimizing uncontrolled moisture and air movement into a building and in preventing moisture entrapment within the wall. Although the building envelope contributes to moisture-related IAQ problems in hot, humid climates, infiltration of humid outside air and vapor diffusion through the envelope is not usually as great a factor in more temperate climates.

However, in temperate climates, the building envelope plays an important role in minimizing rainwater intrusion into the building, and in avoiding the subsequent mold growth that can result from such intrusion. In very cold climates, vapor diffusion or exfiltration of humid indoor air during colder months can also be a problem in wall cavities.

(To be continued…)

Comparing the latent and sensible loads for several major cities in different geographic regions (Peart and Cook 1994) helps illustrate the new definition. A study was done showing the monthly average latent and sensible loads from outside air for Orlando, Florida; Atlanta, Georgia; and Columbus, Ohio. During the cooling season in Orlando, the latent load far exceeds the sensible load of outside air. The effect of these conditions, which occur for more than half a year, is that any outside air drawn into the building envelope or occupied space will likely cause moisture accumulation and microbial growth problems. Furthermore, because this outside air is used for ventilating the building’s occupied spaces, it presents a huge dehumidification challenge for the makeup air system. Clearly, under these conditions, Orlando is highly susceptible to moisture intrusion problems.

Atlanta was shown to be less susceptible to moisture intrusion problems than Orlando because, on average, the difference between sensible and latent load is small, particularly during the peak cooling months. Standard AC systems have a better chance of accounting for the latent load in Atlanta than in Orlando. Nevertheless, the latent load in Atlanta represents enough of a moisture accumulation risk that it belongs within the upper boundary of the humid zone. However, according to the ASHRAE-defined humid zone, Atlanta is outside the critical zone for humid conditions.

When looking at Columbus, the latent load from outside air is consistently less than the sensible load. The reversal of the load relationship explains why buildings in Columbus are not likely to develop moisture-related problems from outside air intrusion, because any outside air that infiltrates into buildings in Columbus will be adequately dehumidified before it is cooled.

The new definition also explains why, in certain areas of the country, building commissioning procedures are more critical than in others. For example, if the building exhaust systems are started before the AC and makeup air systems, as is typical, huge amounts of moisture may infiltrate the building, depending on the outdoor conditions.

In applying the new humid climate definition, however, two qualifications must be made:

• The definition is based on average climatological data. At certain times during the summer, the latent load of outside air can exceed the sensible load to a much greater extent than was reflected in the study. Such episodes of extreme high moisture entering the building can cause problems despite seemingly safe average conditions and must be considered in problem prevention.
• If the building envelope has an improperly located vapor retarder, moisture accumulation problems can occur, even if a favorable sensible/latent load relationship exists. Condensed moisture behind the vapor retarder will never reach the AC system for proper dehumidification but will accumulate in the wall system. Thus, architectural aspects of the building work in conjunction with outside conditions to create problems.

To be continued…

In the summer of 1988, construction of a large luxury resort was coming to a close. Because the vinyl wall covering on the interior side of the exterior walls had an impermeable finish, it functioned as a vapor retarder (also referred to as a vapor barrier). The HVAC system consisted of a continuous toilet exhaust and packaged terminal air-conditioner (PTAC) units. The outside air exchange rate in each guest room averaged six times an hour, all from infiltration. In this case, problems developed inside the building and inside the wall.

The combined effect of excessive outside air infiltration and an improperly located vapor retarder caused $5.5 million in moisture and mold damage, even before the facility was opened. If these same design combinations had occurred in a more temperate climate, the problems would have been limited to increased energy consumption and possibly to complaints about guest comfort.

This is one example of how hot, humid climates present unique challenges that are often overlooked by the design and construction community. However, challenges also occur for buildings located in other climates. Meeting these challenges depends on understanding a building’s local climate conditions and how those contribute to IAQ problems.

Cold climates offer challenges for moisture flow through the building envelope that are similar to those in hot, humid climes. Cold climates are defined by the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) as those that experience at last 4,000 heating degree days (HDD at 65 degrees Fahrenheit [18 degrees Celsius] base) per year. Most problems occur during the winter, when the warm and relatively moist interior air is forced (due to high differential vapor pressures between indoors and outdoors) to the dryer and colder outdoor conditions. Moisture flow can be trapped and condensed on an improperly located vapor retarder. In addition, if the building is air-conditioned during the summer, the wall systems designed to address the heating condition can experience moisture damage inside the walls during the air-conditioned months. Therefore, few locations in the United States are completely free of potential moisture problems.

To be continued…