Technical Updates

Posts Tagged ‘building commissioning procedures’

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…