Airtightness of a home using NDIR CO2 detectors

Measuring the CO₂ decay gradient using two Aranet4 NDIR sensors (one upstairs and one downstairs) can give a useful, though indirect, indication of your home's airtightness. Here’s how and what you might learn:


1. The Principle Behind Using CO₂ Decay to Estimate Airtightness:

  • CO₂ levels inside a house rise above outdoor concentrations (~420 ppm) due to occupant respiration and combustion sources. When the occupants leave and all internal sources stop, CO₂ gradually decreases towards outdoor levels through air exchange (infiltration and exfiltration).

  • In the absence of ventilation or HVAC operation, the rate at which CO₂ concentration returns to baseline primarily reflects natural infiltration rates (air leaks).


2. How to Perform and Analyze the Test:

Setup:

  • Pick the right day. One that's not windy and when you'd not normally run heating or cooling. Windy and hot or cold days (winter and summer) are likely to yield higher air changes per hour (ACH) 

  • Place two CO₂ sensors centrally—one upstairs, one downstairs—to capture stratification and differences between floors.

  • Raise indoor CO₂ levels through normal occupation (typically >1000 ppm).

  • Leave the home empty (no occupants, no pets) with:

    • HVAC off

    • All external doors/windows closed

    • All internal doors open (allowing air mixing throughout the home)

  • Record data continuously for several hours (ideally at least 3–6 hours or overnight).

Analysis:

  • Average the CO₂ concentrations at each time point from the upstairs and downstairs sensors to create a single representative indoor concentration value

  • Plot that aveerage sensor data over time after occupancy ends.

  • Determine the rate at which CO₂ decreases (ppm/hour).

  • Fit a simple exponential decay curve, if possible, to calculate an approximate air change rate.

The decay of CO₂ typically follows an exponential form:

C(t)=Coutdoor+(CinitialCoutdoor)eACHtC(t) = C_\text{outdoor} + (C_\text{initial} - C_\text{outdoor}) e^{-ACH \cdot t}

Where:

  • C(t)C(t) is CO₂ concentration at time tt,

  • CoutdoorC_\text{outdoor} ≈ 400 ppm,

  • CinitialC_\text{initial} is initial indoor CO₂ level,

  • ACHACH = Air Changes per Hour.

From measured data, you can estimate the ACH, which is directly related to airtightness.


3. What the Gradient (Decay Rate) Tells You:

  • A fast CO₂ decay rate (steep gradient) indicates:

    • Higher natural infiltration (less airtight).

    • Likely presence of leaks and gaps allowing more rapid air exchange.

  • A slow CO₂ decay rate (shallow gradient) suggests:

    • Lower infiltration rate (more airtight home).

    • Better sealing, improved thermal efficiency, and comfort, but possibly poorer indoor air quality if no ventilation is provided.

  • Upstairs vs. Downstairs: Differences in decay rates between floors may indicate air stratification or differential leakage. Similar decay rates suggest good internal mixing and more uniform airtightness.


4. Example Interpretation of Decay Rates:

ACH (at 50 Pa)ClassificationDescription/Recommendation
<1.5 ACHExcellent (Passive House standard)Highly energy-efficient, tightly sealed.
1.5–3.0 ACHGoodEfficient home, typical of newer homes.
3.0–5.0 ACHModerateAverage older home, potential improvements
>5.0 ACHPoorSignificant air leakage, requires sealing.




5. Caveats & Limitations:

  • Averaging two sensors reduces the impact of local variations, like stratification or small temperature-driven airflow differences, ensuring the decay curve accurately represents the house’s average airtightness.  A couple of fans in the hallways might force a more uniform mixing.

  • Outdoor conditions: Windy or temperature-variable conditions outside strongly influence infiltration rates.

  • Initial CO₂ level: Higher initial differences produce clearer decay curves.

  • Sensor accuracy: Although Aranet4 sensors are highly accurate (±30–50 ppm), very low decay rates could approach detection limits.

  • No exact ACH: This method provides approximate ACH; for precise measurement, a blower door test remains ideal.


6. Recommendations for Best Results:

  • Start high: Ensure initial indoor CO₂ is well above outdoor levels (ideally 1000–2000 ppm).

  • Long measurement periods: Collect data for at least several hours to clearly observe decay.

  • Repeat tests: Conducting multiple tests under varying outdoor conditions (windy vs calm days) provides better insight into overall airtightness.


In summary:

The CO₂ decay gradient measured by your two Aranet4 devices offers a practical, non-invasive way to estimate whole-house airtightness. Faster decay signals more leakage and lower airtightness, while slower decay indicates a tighter envelope. It’s a useful complementary method to professional blower door testing for understanding your home's performance or find the actual weakpoints in airtightness.

Note: Very windy days and days with a large difference between the home's indoor and outdoor air temperature would force a higher ACH.


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