To give you some perspective on how air tightness has improved here is a look at the air standards of various building standards

• An unrenovated pre - 1960's 4 square brick home, could have an ACH of 10-20.
• Pre 1990's would more likely be around 5 - 10 ACH.

Today's current building code does not require a maximum ACH for performance, the only situation is if a trade off is required (ie. if due to circumstances you weren't able to achieve a certain insulation level you could compensate with a lower ACH.) in which case 3.0 ACH for a detached house and 3.5 ACH for attached.)

(source: Homesol Building Solutions)