Although the molecular weights of these gases differ, convection and not density differences dominate the distribution of CO, methane etc. The release of gases associated with combustion tend to rise because they are hotter than ambient. For this reason smoke detectors and CO detectors are usually placed high on the walls of hallways etc
To answer the OP's question: Most local codes incorporate NFPA 72, the "National Fire Alarm and Signaling Code."
Home Inspections, Infrared (Thermal Imaging) Leak Identification and Inspection Services, Roof, Attic, Building, Basement and Foundation Moisture Intrusion and Water Leak Inspections, Troubled Building Consultations - Serving Chicago and Suburbs http://paragoninspects.com/