According to the LEED v4: Building Design + Construction Guide, the location of CO2 sensors in

naturally ventilated spaces to comply with the Indoor Environmental Quality Prerequisite, Minimum

Indoor Air Quality Performance is between 3 ft. and 6 ft. (1 m and 2 m) above the floor. This is one of

the requirements for Option 2. Naturally Ventilated Spaces, which applies to projects that rely on

natural ventilation for all or part of the occupied spaces. The CO2 sensors must be located in each

zone with natural ventilation openings, and must be capable of generating an alarm when the CO2

concentration exceeds the design value1.

The other choices are not correct, because:

At least 6 ft. (2 m) above the floor is the location of CO2 sensors in mechanically ventilated spaces,

not naturally ventilated spaces1.

At least 3 ft. (1 m) away from windows is a general guideline for locating CO2 sensors, but it does not

specify the height above the floor2.

On the ceiling 3 ft. (1 m) away from adjacent walls is not a recommended location for CO2 sensors, as

it may not reflect the actual CO2 concentration at the breathing zone of the occupants2.

Reference: LEED v4: Building Design + Construction Guide, Indoor Environmental Quality

Prerequisite, Minimum Indoor Air Quality Performance, Option 2. Naturally Ventilated Spaces,

Requirements1; ASHRAE Standard 62.1-2016, User’s Manual, Chapter 6, Section 6.2.72