Natural Ventilation Guidelines

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Natural Ventilation Guidelines


  • Provide cooling and air movement when outside air conditions are favorable compared to indoor conditions and GG Inside Design Criteria.
  • Prevent condensation from too much humidity in spaces.
  • Connect people to indoor air quality and energy use.
  • Provide a safe and easy way to unlock and lock windows, open upstairs back windows.

Decisions to be Made

  • When can windows be open.
  • When should windows be closed.
  • Should ventilation and/or radiant floor cooling system also run when windows are open in open office area (hybrid operation mode).
  • What are the different procedures in the enclosed offices and restrooms.
  • Should all windows be closed and locked after 5 pm.

Expected Outcomes

  • Energy savings from not running fans at times.
  • Improved indoor air quality by fresh air and air movement in space.
  • Comfortable occupants.

Guideline Development


  • Compare indoor air conditions to outdoor air conditions relative to desired indoor design conditions and determine if windows are "enabled" to be opened if desired by occupants.
  • If windows are "enabled" to be opened, indicate so to occupants so they know they can open windows, if desired. Otherwise, windows "disabled".
    • Put up a "big ass" pinwheel in the central office space to indicate that conditions are favorable for opening windows.
    • Communicate that it is not appropriate to open windows if the pinwheel is not up.
    • Include a line on the building operations opening checklist to check for NV conditions and raise or lower pinwheel.
    • Develop and communicate special policy related to opening windows in the back conference rooms.
    • If windows in these rooms are open, door to central office space should be closed so as to minimize impact on the rest of the building systems and occupants.
  • Determine when windows should be closed once open and indicate to occupants.
  • Determine systems mode; natural ventilation (NV) only, NV plus mechanical ventilation (low or high speed), NV plus radiant floor, NV plus mech ventil and radiant floor.

Process Measurements

  • Outdoor air temperature, dewpoint, wind speed and direction.
  • Forecast for GG area, short term and long term (immediate control and anticipation of where weather is heading to pre-condition building/systems).
  • Indoor air temperature, humidity, dewpoint.
  • Occupant perception of comfort.
  • Coldest typical surface temperatures in space.

Resources Required

  • NV Daily Worksheet for building daily operations
  • Most reliable forecast - Intellicast
  • List of measured parameters and acceptable range (max-min) where windows enabled
  • Means of communicating windows enabled - Big Ass Pinwheel


Outdoor/Indoor Comparison

  • Indoor Design Conditions: maximum 78 deg F / 60% RH (Cooling Season)
  • Review daily forecast / hourly conditions predicted: temperature, dewpoint, wind speed & direction
  • Use the following conditions as starting point for when windows may be opened in cooling season:
    • Dewpoint equal to or below 55 deg F
    • Outside air temperature at least 3 deg F below 78 deg F
    • Relative humidity accounted for in dewpoint parameter (changes with temperature)
    • Look at wind speed and direction in relation to temperature and dewpoint to determine how effective NV might be. May help decision making after observing and trending for a period of time. Higher wind speed that enters the building may make temps closer to indoor design more effective by creating air movement if from a direction that enters the building more readily.
  • Initial conditions to use for when windows should be closed:
    • Check conditions at 10:00 a.m. and close if
      • Dewpoint greater than 55 deg F
      • Outside air temperature equal to or above 78 deg F
      • No apparent air movement from having windows open observed
    • Otherwise close windows at noon regardless of conditions

Systems Mode

  • Run ERV system with wheel off to augment air movement, especially in rooms without windows or not on windward/leeward side of building.


  • Use the 10-day forecast to determine if building is generally in the heating or cooling mode. If in heating mode, may not want to open windows unless "Indian Summer" day is forecast.
  • Use hourly forecast to determine when windows should be closed to hold cooling in and prevent heat from entering (in the summer time, this may be around 10:00 a.m.).


  • "Big Ass" Pinwheel up when windows may be opened (Joe = Big Ass Pinwheel).
  • If Pinwheel not visible, windows should be closed.



  • Dirt?
  • Noise?
  • Wind blowing papers?
  • Security?


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