The Impact of Airtightness On System Design

Air pressure acting on building envelopes can wreak havoc with building performance if not properly understood and adequately designed for. Uncontrolled air pressure across the building envelope and within the building itself can cause infiltration and exfiltration that overpower HVAC systems. By disrupting the HVAC design, pressures can cause discomfort and create infection control and indoor air quality problems. In heating climates, exfiltrating air carries with it water vapor and lost energy. The water vapor condenses and causes many problems, from wetting to bacterial growth and deterioration of the building envelope. In cooling climates, water vapor is carried in with the infiltrating air, causing condensation, mold, and bacterial growth. Moisture-laden air can travel within interstitial spaces due to negative pressures inside those spaces. The moisture can condense in strange places like interior walls and ceilings that are connected to the building envelope.