Building Ventilation

Air movement indoors

Air will normally rise (enter low and exit high) in a building from lower spaces into the spaces above, because of the “stack effect” of warm air rising, especially in cold weather.  Air enters from below and beside through openings such as cracks around windows, doors, light fixtures, electrical outlets, and other accidental leaks.  Enough “makeup air” must enter to replace any air exiting.  Air rises through many penetrations in floors and walls, such as for stairways, air ducts, electrical wiring, and plumbing.  Air exits through various openings — much faster when using an exhaust fan or clothes dryer or fireplace.

  • Driving forces of negative pressure drawing air indoors are the weak “stack effect” of warm air rising, and the strong mechanical exhaust fans.
  • Pathways for makeup air to enter are mostly from below and beside, in through many accidental penetrations, or intentional openings.

Ventilation questions

  • Do ventilation system (HVAC) fans/blowers recirculate room air well?
  • Is there strong airflow to all areas?  (Measuring by hand or a tissue flag is good enough.)
  • Is outdoor air added from intakes well away from street traffic or other pollutant sources?
  • Does the system blow continuously daytimes? (Is the fan switched to “ON”?, not to “AUTO”, which only runs when required to cool or heat?)
  • Do carbon dioxide (CO2) concentration levels stay below 700 ppm (parts per million parts of air) indicating abundant outdoor air?
  • Are filters pleated high efficiency, and fit well, and changed often?
  • Are heating/cooling coils and drip trays clean?
  • Is good airflow supplied to the rooms through ducts?, variable air volume (VAV) boxes?, ceiling diffusers?; and returned through ceiling grilles/slots to the plenum space above the ceiling?, and ducts?
  • Is ventilation was made less effective by air not supplied to some areas? not well directed down toward some people who want more?, crowded desks and dividing partitions restricting air flow?
  • Is there negative pressure indoors that can cause back-drafting from combustion appliances (fuel-burning stoves, heaters, or furnaces)?  Are they properly vented to outdoors?  (Strong exhaust fans can suck combustion gases back down flues.)

Ventilation recommendations

To maintain good fresh air supply, distribution, and circulation to minimize accumulation of moisture or emissions and assist dust filtration, have your building ventilation engineers check airflow pathways and driving forces, and design and specify improvements, such as:

  • Monitor carbon dioxide (CO2) when fully occupied to insure adequate fresh air.


  • Open windows for more fresh air as needed to avoid moisture condensation.
  • Provide an easy pathway for outdoor makeup air to enter (for exhaust fans, clothes dryer, vacuum system, and furnace combustion), instead of through leaks (unless your building engineers provided such pathways) (to avoid depressurizing and drawing air and odors from neighboring sources), such as:
  • Install fresh air entry vents from outdoors into the furnace room, or cold air return duct, or bedroom closets for makeup air to enter for exhaust fans, instead of through the walls or floor.
  • Use a horizontal flow air cleaner with high efficiency filters and charcoal blowing cleaned outdoor air in through a window, with the opening around the fan blocked with clear rigid plastic (will look better than plywood).  Air will flow through, and out any room with a window slightly open, or out exhaust fan ducts.


  • Replace the air supply grilles with good diffusers with adjustable directional vanes.
  • Continue to adjust air supply diffuser directions (4-way movable louvers) and speed to blow more toward areas wanting more clean air, and less toward areas too drafty, as the occupants and furniture and seasons change.
  • Circulate room air with portable fans blowing into corners and open cabinets, and under furniture, to speed outgassing from materials and finishes.
  • Balance air flows to positively pressurize the building and blow air out through the doors and the exhaust fans, etc.

Moisture ventilation

 To remove humidity, to avoid moisture condensation and mold growth:


  • Cut a vent opening through the gable end or eaves or a roof jack towards downwind from mold growth (usually toward the lower N side and the NE corner, away from the sun, prevailing SW winds in Seattle, and the warmer center).
  • Install a continuous roof ridge vent.


  • Maintain ventilation; use more if you ever see moisture condensation or mold.
  • Open windows enough to avoid condensation, especially in closed bedrooms at night.
  • Circulate room air with portable fans blowing into corners and under furniture.


  • Use exhaust fans more often to remove steam from showering or cooking (especially with gas) or laundry.
  • Install an exhaust fan in the bathroom ceiling near the back corner.
  • Wire the bathroom exhaust fan to run more often, such as with:
  1. a push-button shut-off timer switch (turn on for 10 to 60 minutes) to encourage people to use it; even better wired in parallel (turns on by either switch) with also:
  2. an automatic dehumidistat switch (turns on when the air is damp, off when dry) on the wall near the ceiling or built into the fan assembly (to be accessed only for occasional adjusting), or
  3. running continuously (fan can be smaller and quieter, or have low speed except faster for showers).
  • Install also a fresh air entry vent high in a bedroom closet wall for makeup air to enter when exhaust fans operate.
  • Verify that the exhaust fan outlet louvers and one-way flapper valves operate (open and close) properly.


  • Run the exhaust vent ducts through the attic and roof to discharge outdoors.
  • Remove the lint blocking the clothes dryer exhaust vent outlet outdoors.