Define the question before measuring
Indoor carbon dioxide can indicate occupancy-related ventilation performance but is not a complete indoor-air-quality score.
Capture a useful baseline
Record the exact location, date, current weather or equipment state, recent activities and the first visible, audible, sensory or measurable change. Keep one unaffected reference point.
Safe observations to repeat
- Log occupancy and door state
- Compare overnight trends
- Place sensors away from breathing zones and direct airflow
Compare supporting and weakening evidence
Supports the pathway
- CO2 rises with occupancy and falls when ventilation increases
- Patterns repeat by room and time
- Outdoor reference is known
Weakens the pathway
- Combustion or sensor error is suspected
- The reading is isolated and unrepeatable
- Occupancy timing does not fit
Interpret the result conservatively
Look for a repeatable relationship between trigger, location, timing and change. Treat exceptions as useful evidence rather than forcing every event into one explanation.
Escalation boundary
Very high readings, combustion concerns or illness symptoms require ventilation and safety assessment rather than DIY interpretation alone.
Close the loop
If work is performed, repeat the same observation under a comparable trigger and through the original lag window. Record both recurrence and non-recurrence events.
Primary references for this topic family
These references ground the general building-science or safety context. They do not diagnose a specific house, replace local requirements or certify a repair.
