Our house was first built as a log cabin in 1890, then extended in the 1950s, and again extended and completely renovated in 2000. It’s well insulated, and has no drafts — it seems to be relatively airtight.
Great for keeping the heat in, not so great for the air quality. Add the fact that we didn’t have a single fresh air vent, only those tiny trickle vents above the windows. What you are left with is a house with bad air quality.
So we installed a balanced ventilation system 🙂