How to Avoid Costly Air Quality Problems

           As I approached the club's main entrance, the automatic doors opened and a rush of air filled the lobby. I noticed the receptionist's hair rustle as she placed her hand on the papers on the desk to keep them from blowing on the floor. As I waited for the club manager, I could smell a slight mustiness in the air, furnishings felt sticky, and the air conditioning diffusers were sweating. Discussions with the club manager revealed that members complained frequently of being warm. The card rooms were stuffy and the humidity was so high in the locker rooms that mold was growing on some of the golf shoes. Further investigation revealed that the building was operating under a severely negative pressure condition.

Few things damage a clubhouse more profoundly than a negatively balanced HVAC system. It is most acute in humid, Southern climates.Operating a clubhouse with severely negative pressure can set off an avalanche of bad things that can result in serious indoor air quality problems over time. Correcting these problems after the microbial growth has started can be a very expensive process.The good news is that these problems can be averted with proper HVAC equipment selection and a good maintenance program.

How it Happens

      A negatively pressurized building draws unconditioned outside air through every available opening. The lobby area is usually the first to feel the impact. Automatic doors opening and closing allow a huge amount of humid air to enter with each cycle, making the receptionist sweat in the summer and freeze in the winter.The lobby diffusers begin to sweat and soak into the surrounding hard ceiling where mold starts to grow.The humidity level everywhere inside the building rises.

Members feel warm because the evaporative cooling they get naturally from their skin under lower humidity conditions is reduced, so they call the facilities manager to lower the temperature in the space. Under elevated humidity conditions, the temperature usually needs to be maintained at 68-69 degrees for the majority of people to feel comfortable. When this happens, the building is in its most dangerous state for microbial growth. When the interior temperature of the building maintained is lower than the outdoor dew point temperature for a significant amount of time, water vapor will be drawn through the exterior walls. It usually condenses on the first impervious membrane it encounters. This can be the back of the wall board or some vinyl wallpaper. Once condensation occurs, mold, and the inevitable remediation costs, will not be far behind.
              Negative pressure is a condition found in clubhouses with alarming frequency. Clubhouse buildings, by their nature, employ significant numbers of constant volume exhaust fans. Usually the largest component of exhaust is located in the food service facility. It is not unusual for a major cooking facility to be exhausting 10,000-20,000 cubic feet per minute (CFM) of air from the cooking hoods. A properly balanced cooking hood system should be replacing approximately 80 percent of the exhausted air with fresh make-up air. This is usually supplied through a make-up air plenum located near the front of the hood. This leaves a net exhaust of 3,000-4,000 CFM from the kitchen area.

Add to this another 600-1,200 CFM from the dishwasher hood and another 500-1,000 from a display kitchen hood, and the aggregate exhaust can be in excess of 6,000 CFM from the food preparation area alone. Another 2,000-3,000 CFM likely will be added from the locker rooms and restrooms throughout the building. The net result is that a large clubhouse building can have a constant volume exhaust rate of 8,000-10,000 CFM. This is particularly problematic for clubhouse buildings because the exhaust rate is large compared with the square footage of the building, and units are run continuously.
To counteract the exhaust and maintain a neutral or preferably positive pressure in the building, an offsetting amount of outside air must be introduced. Because exhaust fans are at

Operating a clubhouse building with severely negative pressure can set off an avalanche of bad things that can result in serious indoor air quality problems over time.

constant volume (which means they run continuously with no variation in the flow rate), the make-up air also must be introduced in a similar manner.This is done through the air conditioning system- and therein lies the challenge.In humid Southern climates the humidity in the outside air is normally very high and goes through dramatic seasonal swings.The HVAC engineer is faced with the daunting task of providing a stable temperature and humidity condition inside the building with wildly varying outdoor temperature and humidity conditions.

Air conditioning systems maintain inside conditions normally by increasing or decreasing the air flow and/or the supply air temperature.There are a number of techniques engineers use to accomplish this task and some are more successful than others. Chilled water systems can use variable frequency drives on the air handlers to vary the air flow. Direct expansion systems, such as package roof top units, simply turn on and off based on the sensible demand. The challenge for clubhouse engineers is that neither of these methods are effective in maintaining neutral or positive pressure in the building when exhaust fans are running continuously.

What to Do?

Clubhouse buildings must be equipped with a dedicated 100 percent outside air pre-conditioner. This unit runs continuously and supplies a constant and equal volume of dehumidified outside air to the building. It should be interlocked to run when the cooking hood exhaust system runs. There are a number of manufacturers that produce this type of equipment. It can be done with chilled water or direct expansion equipment. The units are designed to remove large amounts of moisture from the air stream.

 The units also include a method of reheating the supply air to prevent overcooling the space. It can be done with a non-energy absorbing hot gas reheat coil or with electric heat. Sizing the unit is a subject of design as long as it can deliver an amount of outside air at least equal to the exhaust.

A very useful system to include is a cooking hood variable air volume system. This system uses variable frequency drives on the exhaust and make-up air fans to slow the exhaust rate down when there is no significant cooking being done on the line. The reality is that a fairly small amount of the hood's total daily operating time involves heavy grease and smoke-laden cooking. There is no need for the hood to be running full speed when there is no smoke being generated.

To provide comfort at the cooking line, it is advisable to provide temperature control for the hood make-up air. In the summer, the ambient air temperature can easily be 95 degrees plus. Dumping this air down on the chef at the cooking line can raise the temperature in this area to 110 degrees. This alone frequently drives the chef to turn the make-up air fan breaker off. Cooling half of the makeup air stream to 55 degrees and re-mixing with the remaining air will drop the temperature from 95 degrees in the summer to 75 degrees and 70-80 percent humidity.

With the basic equipment in place, the unit needs to be maintained regularly. All of the above discussed equipment can fail due to lack of maintenance and put the building into a negative air balance. The problem is that the equipment can fail and not be immediately noticed until serious problems start to develop. An aggressive maintenance program will keep the building properly balanced and healthy for many years.

About the Author

Robert Davenport is president and principal Engineer of RGD & Associates Inc., a South Florida firm specializing in clubhouse construction and renovation projects. 

 For more information about our services, visit RGD Consulting Engineers corporate website. 

Understanding Air Barriers

Understanding Air  Barriers 

By Robert Davenport P.E., RGD Consulting Engineers, Jupiter, FL

The envelope of an air conditioned space inside a building is required to have an air barrier. Without an effective air barrier, the space may and probably will experience moisture related damage in time. An air barrier is required by section 502.3.5 Building Cavities, the 2010 Florida Building Code – Energy Conservation. The code further specifically excludes batt insulation and lay in type drop ceilings as effective air barriers. Reference the term “Air Barrier” in Section 2 Definitions. This requirement is frequently overlooked in building design and construction.

The air barrier’s primary purpose is to prevent the commingling of conditioned and unconditioned outside air masses. For most of the year in South Florida, the outside air has high humidity. The temperature at which the water molecules begin to condense to the liquid state for much of the year is 73 degrees F. This is called the Dew Point. Since most air conditioned spaces operate at or near this temperature, it is easy to see that when the two air masses come into contact with each other, there is a high probability that liquid water can form. Water droplets condense on the nearest building material which may be the mineral fibers of batt insulation, the lay in ceiling or the roof structure. The potential for damage when this happens is well documented.

To be an effective air barrier, it must prevent air movement between conditioned and unconditioned spaces. The code specifically identifies gypsum wall board with taped joints to be an effective air barrier. Other systems may also be acceptable but are not identified in the code.  It is essential that the barrier be able to prevent air movement across itself when subjected to a pressure differential. Buildings can experience positive and negative pressure differentials for a variety of reasons. Properly designed air conditioning systems will typically maintain a positive pressure differential on the space. However both conditions have the potential to create serious negative effects on the building if the space is not supplied with an effective air barrier.

The condition observed most frequently in the authors experience is a commercial building with a wood truss roof structure. (Figure 1-1) The batt insulation is installed on the bottom chord of the truss and the lay in ceiling installed below that. The condition is exacerbated by code required attic ventilation above the batt insulation provided by soffit vents and or ridge vents. Conditioned spaces that have this condition usually operate with elevated humidity levels and with moisture related damage occurring at or above the suspended ceiling. The most obvious characteristic of spaces with this condition are sagging ceiling tiles with mold growing on the back side in many cases. Reference figure 1-2.

To insure optimum building performance and longevity, the air conditioned envelope must be provided with an effective air barrier. The building code identifies drywall with taped and mudded joints as one method of providing the air barrier. Figure 1-3 illustrates the same configuration with a gypsum layer added to the bottom chord of the truss.  

Alternatively, the code allows the insulation to be applied to the top chord and the attic space to be sealed. This method is becoming popular as research indicates that sealed attic spaces are preferable in South Florida.  However the requirement for an effective air barrier remains. Applying batt insulation to the top chord of the truss is problematic and it is still not approved by code for use as an air barrier. This is especially true if a mechanically induced pressure differential such as an exhaust fan is involved.

One system that is gaining popularity is Icynene spray insulation.  This material is sprayed on the roof decking and top chord of the trusses. It can be applied in a range of R values, usually R-30 on roof structures and is accepted as an approved air barrier by most building officials.  

For more information about our services, visit

 RGD Consulting Engineers corporate website - www.rgdengineers.com.