Behind the Intent section, add the following after the second paragraph:
Different kinds of occupants, activities, and equipment in a building will make for different IAQ parameters, so requirements vary both by space type in a building and by project type. For example, residential projects must meet additional prescriptive requirements that protect occupants from indoor contaminants, such as combustion byproducts and radon.
When installing combustion equipment, the best choice is sealed-combustion, direct-vent gas-fired units. For woodburning or open gas-fired equipment, operable doors can reduce backdrafting. Regardless of the type of equipment installed, carbon monoxide detectors are an effective way to limit the risk of exposure to carbon monoxide from leaks, equipment failures, and unanticipated behavior by occupants.

Ventless fuel-fired equipment, such as ventless fireplace and space heaters, can expose occupants to high levels of moisture carbon monoxide, and combustion byproducts; and therefore are not allowed inside the building envelope.

Under Step-by-Step Guidance, above step 1, add subheader "EBOM, SCHOOLS, RETAIL, DATA CENTERS, HOSPITALITY, WAREHOUSES & DISTRIBUTION CENTERS"
At the end of the section, add a second subheader "Multifamily", followed by the following:

STEP 1: CONFIRM COMPLIANCE FOR THE COMMON AREAS
All common areas must be either mechanically or naturally ventilated.
• For mechanically ventilated common areas, follow Step-by-Step Guidance. Mechanical Ventilation Spaces (and Mixed-Mode Spaces When Mechanical Ventilation is Active) Steps 1-7.
• For naturally ventilated common areas, follow Step-by-Step Guidance. Naturally Ventilated Spaces (and Mixed-Mode Spaces When Mechanical Ventilation is Inactivated) Steps 1-6.

STEP 2: CONFIRM COMPLIANCE FOR EACH RESIDENTIAL UNIT
Each residential unit must be either mechanically ventilated or naturally ventilated.

For mechanically ventilated units,
• Follow Step-by-Step Guidance. Mechanical Ventilation Spaces (and Mixed-Mode Spaces When Mechanical Ventilation is Active) Steps 1-7 OR;
• Demonstrate compliance with ASHRAE 62.2-2010 whole-building ventilation.

For naturally ventilated units,
• Collect the following information for each bedroom in the building:
• Total openable window area. If the window is covered with louvers, insect screens, or otherwise obstructed, the openable area must be based on the free unobstructed area through the opening.
• Net occupiable floor area (The net occupiable floor area is measured from the inside surfaces of the walls, excluding permanently enclosed obstructions such as shafts and columns).
• Calculate the openable window area as a percentage of the net occupiable floor area. Alternatively, confirm that the openable window area is capable of providing at least 10 cubic feet per minute (5 liters per second) of outdoor air per person.

• Collect the following information for each full bathroom in the building:
o Is there an exhaust fan that vents directly to the outdoors?
o Is there an operable window?

Evaluate compliance and make adjustments if necessary
• For each residential unit, compare the information collected with the credit requirements. Conduct this assessment early in the project timeline, since capital- or time-intensive improvements may be needed to bring the building into compliance with the natural ventilation requirements. If the upgrades required to comply with the natural ventilation requirements are not feasible, project teams may be able to achieve the prerequisite through mechanical ventilation.

STEP 3: CONFIRM COMPLIANCE WITH COMBUSTION-RELATED REQUIREMENTS [E] [P]
In addition to meeting the ventilation requirements for the common areas and each residential unit, all requirements for combustion appliances and radon-resistance must be met.
• Determine whether carbon monoxide monitors must be installed on each floor. While carbon monoxide monitors are recommended to be installed in all residential units, they are required if the unit has gas-fired ovens and ranges that are not vented to the outdoors, or combustion equipment that is not direct-vented or power-vented.
• Confirm that there are no unvented combustion appliances, such as decorative logs, in the residential units.
• If there are indoor fireplaces and/or woodstoves present, confirm they have solid glass enclosures or doors that seal when closed and determine whether they are close-combustion, power-vented, or natural-draft (see Figure 1 and Figure 2].
o If the fireplace or woodstove is natural-draft perform a backdraft potential test to ensure minimal risk of backdrafting.
o A qualified energy rater will be capable of performing BPI or RESNET combustion safety testing protocols to ensure that the depressurization of the combustion appliance zone (CAZ) is less than 5 Pa. The energy rater will turn on all exhaust fans and air handlers in an attempt to backdraft the fireplace or stove and determine the risk that smoke will be sucked into the house.
o If the fireplace or woodstove CAZ test identifies pressure above 5 Pa, the project does not meet the requirements of the prerequisite. Corrections may include replacing the natural-draft unit with a sealed-combustion fireplace, providing more outside makeup air, or reducing the depressurization level by installing smaller exhaust fans.
• Determine whether the project is located in a high risk radon area.
• If the building is in EPA zone 1 or international equivalent, or the radon risk is unknown, test the building for radon.
o The radon test must be performed in the lowest level of the building possible, such as the basement.
o If the results are greater than 4 pCi/L, install an active ventilation system. If results are less than 4 pCi/L, no radon-resistant construction techniques are required.
The presence of radon can change over time, so a negative radon test does not ensure the absence of radon in the future. EPA recommends regular testing of radon, especially in high risk areas. For LEED, testing must be performed every five years.
Put radon test results in the operations manual, and provide information on the importance of regularly testing the building for radon.
Figure 1. Closed combustion fireplace. Illustration by Terrel Broiles.
Figure 2. Power vented fireplace. Illustration by Terrel Broiles.

Add second Required Documentation table:
Multifamily
(E) Calculations for common area ventilation
(E) For residential units with operable windows, documentation confirming operable windows in each bedroom and exhaust fans or operable windows in each bedroom.
(E) For residential units with operable windows, calculations for openable window area in each bedroom.
(E) For mechanically ventilated residential units, calculations for mechanical ventilation.
(E) Confirmation that each dwelling unit meets the rating system requirements for gas-fired ovens and ranges, carbon monoxide monitors, unvented combustion appliances, and indoor fireplaces and woodstoves.
(E) If applicable, radon test report.
(P) Measured outdoor airflow rates for mechanically ventilated spaces and description of method or protocol used to take measurements.
(P) Ventilation maintenance program.

5. In the Definitions section, add:
Add the following definition: openable window area - the free unobstructed area through the opening.