Multifamily buildings usually have relatively tighter enclosures than single-family homes. They also have less exterior surface area per unit of floor area. Furthermore, the multifamily-building exterior is often quite weather-tight to resist rain and wind.
Pressures from the wind and stack effect affect the HVAC ducts and ventilation ducts so that these ducts may deliver air at an unpredictable rate. Air leaks can create large airflows because of high pressures toward the building’s top and bottom.
Wind and stack effect
Wind is stronger at higher elevations above the ground compared to at street level. The higher the building the stronger the stack effect, which causes the building to act as a chimney.
Thus, multifamily buildings experience greater pressures than the single-story buildings. This greater pressure leads to both air leakage and internal convection. This airflow wastes energy, redistributes conditioned air randomly, reduces comfort, and distributes pollutants. For example, higher floors may experience uncomfortably high temperatures because the heated air migrates from lower floors. Pollutants from the basement can spread throughout the building through leaks in ducts and in the internal structure.
Pressure and airflow sometimes overpowers HVAC systems, creating energy, comfort, and indoor-air-quality problems.
Vertical shafts and horizontal cavities circulate air within them and frequently have leaks to the outdoors. Air seals for multifamily-building cavities must be structurally strong to resist strong pressures. Stack effect combined with wind can send blasts of air howling through vertical shafts, blowing lightweight air seals out of their holes.
Ceiling and floor cavities
Horizontal cavities in multifamily buildings may be several feet deep, giving air convection and intruding outdoor air a large pathway. The same cavities may function as return-air ducts for large air handlers that heat, cool, and ventilate the building. Indoor air enters cavities through ceiling joints, recessed light fixtures, plumbing penetrations, and other openings in ceilings and floors. These cavities are sometimes connected to the outdoors through joints in balconies, bay windows, or overhangs. If floor and ceiling cavities contain outdoor air, that air can mix with the cavity’s indoor air and also enter the conditioned spaces.
Vertical shafts
Most larger buildings have vertical shafts for elevators, stairs, pipes, wires, and structural columns. Vertical shafts also include laundry chutes, garbage chutes, ventilation shafts, and incinerator shafts. Openings between indoor spaces and these shafts cause major air leaks. Openings at the shafts’ bottoms and tops establish pressures that pull air into the shafts and push air out of them.
Energy auditors usually consider these shafts outside the conditioned space, with the exception of open stairways. Contractors often renovate many formerly open stairways with fire walls and fire-rated doors to prevent a fire from spreading rapidly through the open stairways.
Sealing air leaks
Air-sealing materials installed between floors and through shafts of large multifamily buildings should also be fire-resistant. Fire-resistant air seals prevent vertical shafts and horizontal cavities from becoming an air source or pathway for a spreading fire. Among the fire-resistant materials used are drywall, masonry block, mortar, non-combustible caulk, fire-resistant foam, and metal sheets.
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