Should Mechanical or Natural Ventilation Be Used for Multi-Family Projects?




This blog applies to the dwelling unit ventilation strategy for multi-family buildings that are four stories or greater in height.

What is ventilation?

All occupied spaces, including dwelling units, require ventilation according to the International Mechanical Code (IMC). Ventilation can be either:

  • mechanically ventilated with fresh air ducted into the mechanical room or
  • naturally ventilated with operable windows and doors.

Why is ventilation important?

Ventilation intends to provide an indoor atmosphere that protects the health and well-being of building occupants.

How do you determine whether to use mechanical or natural ventilation?

Proficient would recommend mechanical ventilation to insure adequate fresh air will be provided to the occupants. If natural ventilation is preferred by developers, Proficient recommends the following steps.

#1 – Do the unit plans meet the minimal openable area per IMC 402 (see below)?

#2 –Will a blower door test be required per IECC R402.4.1.2 (see below)? If so, the test must prove more than 5 air changes per hour (loose construction) for natural ventilation. Contrarily, per the same test, the envelope shall prove less than 5 air changes per hour (tight construction) per the energy code. Thus, if a blower door test is required, mechanical ventilation should be used.


Every project needs to be evaluated thoroughly to determine the best ventilation strategy. Natural ventilation can be a cost-effective ventilation strategy that has been used for years on multi-family projects and it can be helpful if the owner/contractor wants to value engineer (VE) the project. However, as energy codes continue to drive tighter construction, mechanical ventilation will often become the most suitable solution. Due to the various considerations involved with natural ventilation, Proficient recommends having mechanical ventilation in early MEP narratives and/or pricing sets. If mechanical ventilation was considered early in design and natural ventilation is not allowed, it can be added later without significant additional costs to the project.

    Related Code References (confirm latest codes

    Below is the requirement from the IMC:

    401.2 Ventilation required.

    Every occupied space shall be ventilated by natural means in accordance with Section 402 or by mechanical means in accordance with Section 403. Where the air infiltration rate in a dwelling unit is less than 5 air changes per hour when tested with a blower door at a pressure of 0.2-inch water column (50 Pa) in accordance with Section R402.4.1.2 of the International Energy Conservation Code, the dwelling unit shall be ventilated by mechanical means in accordance with Section 403. 

    402.1 Natural ventilation. 

    Natural ventilation of an occupied space shall be through windows, doors, louvers or other openings to the outdoors. The operating mechanism for such openings shall be provided with ready access so that the openings are readily controllable by the building occupants.

    402.2 Ventilation area required.

    The minimum openable area to the outdoors shall be 4 percent of the floor area being ventilated. 

    402.3 Adjoining spaces. 

    Where rooms and spaces without openings to the outdoors are ventilated through an adjoining room, the opening to the adjoining rooms shall be unobstructed and shall have an area not less than 8 percent of the floor area of the interior room or space, but not less than 25 square feet (2.3 square meters). The minimum openable area to the outdoors shall be based on the total floor area being ventilated.

    Exception: Exterior openings required for ventilation shall be permitted to open into a thermally isolated sun-room addition or patio cover, provided that the openable area between the sunroom addition or patio cover and the interior room has an area not less than 8 percent of the floor area of the interior room or space, but not less than 20 square feet (1.86 square meters). The minimum openable area to the outdoors shall be based on the total floor area being ventilated.

    Below is referenced section R402.4.1.2 from the 2015 IECC: 

    R402.4.1.2 Testing.

    The building or dwelling unit shall be tested and verified as having as air leakage rate of not exceeding five air changes per hour in Climate Zones 1 and 2, and three air changes per hour in Climate Zones 3 through 8. Testing shall be conducted in accordance with ASTM E 779 or ASTM E 1827 and reported at a pressure of 0.2 inches w.g. (50 Pascals). Where required by the code official, testing shall be conducted by an approved third party. A written report of the results of the test shall be signed by the party conducting the test and provided to the code official. Testing shall be performed at any time after creation of all penetrations of the building thermal envelope.

    During testing:

    1. Exterior windows and doors, fireplace and stove doors shall be closed, but not sealed, beyond the intended weather stripping or other infiltration control measures.
    2. Dampers including exhaust, intake, makeup air, backdraft and flue dampers shall be closed, but not sealed beyond intended infiltration control measures.
    3. Interior doors, if installed at the time of the test, shall be open.
    4. Exterior doors for continuous ventilation systems and heat recovery ventilators shall be closed and sealed.
    5. Heating and cooling systems, if installed at the time of the test, shall be turned off.
    6. Supply and return registers, if installed at the time of the test, shall be fully open.