By Bob Baker

It is important that building use and HVAC system design be carefully coordinated. I recently encountered an interesting project that illustrates this point dramatically.

In this case, a detached medical office and surgery center open for less than three months, the Physician was examining a patient and water started dripping on them. This happened because significant quantities of moisture are condensing out of the air in the attic on various surfaces located above the suspended ceiling throughout the facility.

Facility description

This is a free-standing medical practice facility located in Florida. It is concrete block/stucco construction with wood frame tile roof. The total of area of about 6,000 sq ft is separated by a fire rated wall into two sections of roughly equal size. The first is the administrative/waiting area and the second is an outpatient surgical facility. Each side is served by independent HVAC and exhaust systems. Both have 5 ton heat pumps with a outside air pre-processing unit supplying 735 CFM of dry outside air to the surgery center.

The facility was designed with perforated soffits and roof vents with insulation above the suspended ceiling. During construction, the design was changed to conform to health facility design code that requires the air-handlers and air ducts to be within the conditioned space. The soffits were removed and replaced with solid panels and the roof vents were closed up. Insulation was removed from above the ceiling and installed between the rafters.

Walkthrough Inspection

An inspection of the attic spaces revealed the following: A. Water stains were observed at several locations throughout the facility. B. The overflow pans below both AHU1 and AHU2 contained water. C. Moisture was noted at the following locations: AHU tops and bottoms, register boots, several of the connection points between flexible duct sections and the duct board plenums. D. Relative humidity varied from 68-72% and dew point ranged from 72-76 degrees F. The thermostat set points in the below ceiling space were near the Dew Points recorded in the above ceiling spaces. As a result, un-insulated surfaces that interface with both spaces were condensing surfaces on which a moisture film was accumulating. Both RH readings were above the level where EPA, ASHRAE and others recognize that active mold growth is highly probable. E. A gray fibrous buildup later confirmed to be fungal growth was observed on several of the wooden structural members in the attic.

What happened?

The difficulties being experienced in this facility rise from two general causes: a) The conversion of the building design from a cross ventilated attic to a sealed attic design. b) Negative pressurization of the building envelope relative to the exterior. Each of these is discussed more fully below:

Design intent change - Under the initial design concept, the ceiling of the facility served as a combined thermal, air and vapor barrier between the work spaces and the attic area. Although, a suspended ceiling has poor thermal, air and vapor barrier properties, insulation bats with a vapor retarder facing to the conditioned space dramatically improve the barrier performance of the suspended ceiling system. The combined materials provide enough thermal, air-flow and vapor movement resistance that the area above the ceiling and that below the ceiling are effectively isolated from each other; a common situation in light commercial construction.

Under this arrangement, the below ceiling structure is built "tight" to limit infiltration of air, moisture and heat in hot and humid climates. The above ceiling space is built "loose" and incorporates vents added to encourage the free exchange of air with the exterior of the building. The roof design serves as a barrier to liquid (rain) while permitting the passage of air, water vapor and to some extent, heat. When the use of the building was changed (surgery center added), codes required that all HVAC airside components be contained within the conditioned space. To do this, the contractor moved the insulation and vapor retarder from the top of the ceiling to a position between the rafters. In addition, the roof vents were sealed and the perforated soffits replaced with solid soffits panels. Although this might have decreased free cross ventilation of the attic space considerably, it did not bring that space to the level of "tightness" of the below ceiling space. Thus the above ceiling space tended to seek balance with the exterior psychometric conditions (temperature, moisture content and dew point). Although, the ceiling is no longer an effective thermal, air movement and vapor barrier, it retains enough retarding properties to maintain a significant differential between the psychometric conditions above and below the ceiling. Thus, the ceiling and other structural separation components serve as a thermal bridge and "mini-environments" are set up where dew point is reached and water condenses out of the air.

Building Pressurization - It is common (and preferred) design practice in buildings in hot and humid climates like Florida to maintain building pressurization positive to the exterior. This is done by bringing in more outside air than you exhaust. Naturally, it must be pre-conditioned by cleaning, cooling or heating and removing excess moisture. Under this situation, conditioned air pushes against the building envelope and holds out air with high moisture content and/or particulate contamination. If the building becomes negatively pressurized, there is a risk that both moisture and contamination will be pulled into the building through unsealed penetrations and other entry points. The incompletely sealed soffits provided significant opportunity for infiltration of both moist air and contamination.

Healthcare regulations require, in order to promote good infection control, that an exhaust system is installed in such a facility to move air toward less clean areas (restrooms, soiled utility, closets) and away from clean areas. It is essential that this system operate continuously when ever the building is in use. The system in this building creates a combined outflow of air totaling 775 CFM (cubic feet per minute). This outflow must be overcome by outside air brought in by the HVAC systems or the building will "go negative" leading to infiltration of unconditioned air. Figure 1 is the building pressurization under various states of system operation.

Figure 1:

Under all conditions except when both systems are running, the building interior is negatively pressurized and there is risk of moisture and contamination being pulled through the building envelope. This risk is especially critical due to the current loose nature of the soffit seal. The result is that the above ceiling area has a ready supply of un-conditioned outside air with high moisture content virtually all of the time. This led to the massive moisture condensation that was taking place.

Conclusion Both building construction and HVAC system design and operation must be closely matched to the needs and use of the space within the building or indoor conditions will not be satisfactory. As this article reveals, any change in space use must involve a complete re-evaluation of both building and mechanical system design to assure that the new use will be properly supported. In a later article, we will discuss what was done to bring this building to a satisfactory condition and how successful that was.

Bob Baker is a member of IAQA, ASHRAE, CSPA and Chairman and CEO of BBJ Environmental Solutions, Inc., "The Standard of Care for Indoor Air". BBJ has offices in Tampa and Hong Kong and Mr. Baker follows indoor air quality developments throughout the world. For additional information, Mr. Baker can be reached at (800) 889-2251 or through the company web site at www.bbjenviro.com.