June 5, 1999

Abstract: 

Understanding the significance of the complex flow and pressure distribution problems created by the interaction of the building envelope with the mechanical system and climate can lead to changes in building design, commissioning, operations, maintenance, diagnostics and rehabilitation.

1. Introduction

Understanding the significance of the complex flow and pressure distribution problems created by the interaction of the building envelope with the mechanical system and climate can lead to changes in building design, commissioning, operations, maintenance, diagnostics and rehabilitation.

Diagnostic protocols can be based on the enhanced understanding of pressures and air flows in buildings and the measurement techniques presented in this thesis. These diagnostic protocols can be used to aid in identifying problems in buildings related to indoor air quality, smoke and fire spread, durability of the building envelope relating to air transport of moisture, operating costs relating to energy use, and comfort issues related to humidity, temperature and odors.

Additionally, rehabilitation approaches can also be developed that allow an assessment of the existing interactions in buildings to be rehabilitated, provide the designer with choices as to desired interactions given the constraints of the existing building, give the commissioning agent performance guidelines to compare against after rehabilitation is complete and provide the building operators with building operating instructions and required operating air pressure relationships.

The designer can now have a choice to either prevent accidental coupling of the mechanical system to the building envelope by design changes to the building envelope and the mechanical system or to deliberately couple the mechanical system to the building envelope to provide enhanced control of air transported moisture control, odor control, smoke control or heat transfer control.

The commissioning agent can now have the ability to determine the interactions of the building envelope and the mechanical system in a systematic, repeatable manner and compare the interactions to design intent.

Finally, the building operators can now have the ability to determine the interactions of the building envelope and the mechanical system on an ongoing basis in a systematic, repeatable manner and compare the interactions to design intent, the initial commissioned state and building operating instructions.

The following five examples are applicable to:

  • Indoor Air Quality
  • Smoke and Fire Spread
  • Durability (moisture)
  • Comfort
  • Operating Cost (energy)

2. Indoor Air Quality

A school located in Trenton, NJ provides a good example of using air pressure measurement techniques to diagnose and remediate indoor air quality complaints associated with the facility.

Description of Facility and History of Problems

The facility in question is a single story masonry school building constructed over a crawl space foundation. The facility consists of several wings constructed at different periods over the past 60 years. Each wing has a separate foundation system, although communication between the various crawl space foundations was present. The crawl space in the affected area of the facility consists of a perimeter cast concrete foundation wall on concrete strip footings. The floor deck consists of cast concrete supported on precast concrete beams supported on the perimeter foundation walls and interior cast concrete bearing walls. The crawl space floor surfaces were uncovered earth. Crawl space ventilation consisted of numerous 20 cm x 30 cm vents, distributed in an approximate ratio of 1/1500 between vent area and floor area.

A teacher in one of the classrooms of the affected area of the facility was complaining of mold odors, headaches, fatigue, and flue like symptoms. Discussions with the teacher indicated that similar complaints were also common among the students. Further discussions indicated that complaints had been registered for several months with no action resulting. It appeared that no record of complaints had been kept.

Investigation and Testing

Upon entering the classroom which the affected teacher and students occupied, visible deterioration of plaster and baseboard surfaces were observed along interior and exterior walls. The deterioration was most intense at the baseboard level, and decreased in intensity with height. Paint had peeled from the plaster at many locations. Water markings were observed on the plaster surfaces. The plaster was soft to the touch and disintegrated when probed. When the plastic covering over the wood baseboard trim was removed, noticeable musty odors were encountered. The wood was soft and "punky". Significant decay of the wood was observed. When the wood baseboard was pulled away from the wall, the intensity of the musty odors increased significantly.

Visual observations revealed a joint between the concrete floor slab and the masonry perimeter wall. Other joints were observed in the concrete floor slab at the interior concrete foundation walls. Smoke pencil testing indicated substantial air flow between the crawl space and the classroom through these exposed joints. Readings taken with a digital micromanometer indicated that the classroom was operating at 4 Pascal’s negative with respect to the crawl space (Figure 1). Furthermore, interior wall cavities were found to operate at 1 Pascal negative with respect to the classroom.

Removal of deteriorated plaster revealed the wall construction. Interior plaster was installed over wood furring strips creating an air space (or channels) between the plaster and the masonry wall. Removal of ceiling tiles indicated that the plaster finish extended just above the dropped ceiling level and that the air space (or channels) between the plaster and the masonry wall was open at the top and connected to the air space above the dropped ceiling. This wall geometry created "chimneys" which extended from the crawl space to the air space above the dropped ceiling.

The air space above the dropped ceiling was used as a return air plenum that operated under a negative air pressure relative to the classroom due to the operation of air handling units within the dropped ceiling (Figure 2). Additionally, each classroom was equipped with a roof top exhaust fan that extracted air from the dropped ceiling depressurizing both the dropped ceiling and the classroom relative to the exterior. When the roof top exhaust fan was shut down, the negative air pressure difference between the classroom and the crawl space was reduced to less than 1 Pascal.

Discussion with school district staff, and photographs indicated that no ground cover was present in the crawl space. According to staff, the top surface of the soil appeared dry. In addition, many of the steam lines in the crawl space were reported to be uninsulated due to ongoing asbestos mitigation work. Crawl space temperatures in excess of 30 degrees C. were typical according to staff.

Crawl space vents were sealed and an exhaust fan was installed in the crawl space exhausting air to the exterior. The access opening connecting the affected crawl space and the adjacent crawl space was also sealed. Air pressure differentials between the affected classroom and the crawl space were monitored. Extracting approximately 325 L/s of air from the crawl space by means of an exhaust fan depressurized the crawl space 4 Pascal’s with respect to the classroom area. This was shown to result in a flow reversal of air between the crawl space and the classroom area. Using a smoke pencil air could be shown to flow from the classroom area into the crawl space when the exhaust fan was operating rather than from the crawl space into the classroom. . .

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