Oct 27, 2022UPD Oct 27, 2022
Using my own simulator, I simulated the relationship between ventilation airflow and CO2 concentration.
The graph below is the simulation result for a typical 120 m² (36 tsubo) house. Airflow is 120 m³/h, which corresponds to roughly 0.5 ACH. Occupants are a three-person family on a weekday.
Environmental engineering recommends keeping CO2 below 1000 ppm. The result stays in the 800–900 ppm range — a good environment is maintained.
Common conditions: the husband leaves at 8 am and is out until 19:00; the wife and child stay home; for 2 hours from 8 am, the wife does housework indoors.
Next, the result with airflow reduced to 90 m³/h.
At the moment the husband returns, CO2 is at its lowest, 816 ppm. It rises gently overnight and stabilizes around 994 ppm by 7 am. Then with just the wife and child, housework drives CO2 to 1089 ppm over 2 hours.
90 m³/h works out to 30 m³/h per person — matching the recommended airflow in environmental engineering. As long as you don't exercise indoors, CO2 doesn't exceed 1000 ppm.
Finally, the result with airflow reduced to 60 m³/h.
Peak CO2 reaches 1388 ppm, but the average is around 1200 ppm, which isn't particularly problematic.
The conclusion across the three simulations: a three-person family can drop the ventilation rate to about 0.3 ACH without real issues. However, Article 20-8 of the Building Standards Act Enforcement Order — somewhat meddlesomely — mandates a 24-hour mechanical ventilation system of at least 0.5 ACH, and we architects must comply.
There are passive ventilation systems without electric fans, and demand-controlled systems that vary airflow by CO2 — but as long as this clause exists, 0.5 ACH via 24-hour fans is mandatory. The penalties are...
Personally, I think families with growing children or students preparing for entrance exams should secure about 0.5 ACH.
A little exercise quickly raises CO2 — and once raised, it doesn't drop easily.
I wouldn't want kids using "I can't concentrate because the house ventilation is bad" as an excuse. Then again, a kid who can come up with that excuse is probably plenty smart already.