Oh, and I realize that the CO2 concentration in a "normal" house will depend on:
- how tightly insulated the house is (older houses "breath" more than newer ones)
- concentration of people/animals: how many people/animals are in the house as compared to how big the house is
- an all-electric home vs. one with gas pilot lights, gas cooking, gas water heater, or gas "space heaters" in use
- lots of opening/closing of doors, and if a window is open
- any other influencing factors you can think of?

So, what were YOUR house's conditions when you made a CO2 measurement in your "normal closed-up house"?