Code Required Ventilation

By Nate Rogers, E.I.T. | January 16, 2021

It is more important than ever to provide fresh and clean air to indoor spaces. Providing ventilation to protect occupant health and wellbeing in a building is not only best practice but also required by code. Chapter 4 of the International Mechanical Code dictates the ventilation requirements for both residential and commercial buildings. There are two acceptable methods to provide ventilation.

Natural Ventilation

Natural ventilation is a passive strategy that simply means providing openable doors, windows or louvers. These openings need to be easily controllable by building occupants to control the amount of ventilation. When using this method, the minimum openable area to the outdoors shall be 4 percent of the floor area being ventilated. For example, a 100 square foot room would need a 4 square foot window that could be completely opened. Natural ventilation is the easiest option in residential buildings and does not required any additional HVAC equipment. However, this method also puts all the responsibility for providing ventilation on the occupants.

Mechanical Ventilation

Mechanical ventilation systems can take a variety of forms using make-up air units, energy recovery ventilators, and air handling units in nearly unlimited combinations and configurations. A critical part of the design is determining how much fresh air is required per the code. The volume of required fresh air should be determined room to room and is dependent on three variables: occupancy classification, number of occupants, and area. Certain occupancies also require a specified exhaust airflow. The table below is a recreation of a small portion of the table found in Chapter 4 of the IMC.

After determining the required cubic feet per minute (CFM) of fresh air required, we can start sizing the ventilation equipment and designing the ventilation system. One of the most important considerations is efficiency. Bringing very cold or very hot air into a building can put a huge strain on the HVAC system but good design, proper sizing, and use of equipment such as energy recovery ventilators can greatly improve both efficiency and comfort of interior spaces.

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