What's "normal" CO2 PPM for a closed-up house?

If my grow closet is recirculating air from the house, what CO2 concentration is "normal" for a closed-up house (the house is not vented: windows are closed, and using a central air conditioner).

Perhaps some folks who have CO2 monitor/controllers have walked around their house and taken measurements.

Could CO2 concentrations in a closed-up house rise above the "safe" maximum of 2500 PPM?

Almost a year ago, extinctx11 posted a similar question (but got no responses):
http://boards.cannabis.com/closet-ca...ml#post1569936

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"?

When you bring in air from your living space it will generally have higher CO2 levels than outside air, but as you said it depends on how much CO2 is being produced by people, pets etc. I find about 400 to 450ppm in my room.

"Normal" is gonna range between 300-600 ppm in an enclosed structure occupied by mammals. Much higher when the same enclosed space is occupied by mushrooms.