I think my single-family house built in 1955 is at or above average of those built in the USA since WWII, in terms of tightness and energy efficiency. It was very well built, and occupied, by talented builder, my Uncle Russell Richardson, who left a substantial mark upon my Portland, Oregon neighborhood.
I have one blower door reading for my house, 1330 CFM50 , recorded before I sold and renounced the blower. I am quite content to estimate reduction to about 800 CFM50, from subsequent actions. This corresponds to annual savings of about $40 per year, a small amount versus the cost of another blower door test. With living space volume of 7900 cubic ft, 800 CFM50 is about 6 air changes per hour, ACH50, at minus 50 pascal test conditions. Divide by about twenty for the fresh air exchange at natural conditions, 0.3 ACHnat. Internationally, the accepted range for energy efficient housing is between 0.3 and 0.5 ACHnat.
The builder of an ultra-tight new home might take aim at PassivHaus standards, of 0.6 ACH50, 0.03 ACHnat. Whatever for? For me, this would be a ridiculously-small four or five cubic ft per minute at natural pressures, quickly overwhelmed by the routine and necessary running of bath and kitchen exhaust fans.
I continue to tighten my house at every opportunity, in drastic measures. I have taken down most of the exterior wall drywall, in replacing windows and insulating exterior walls, with small consequence to tightness, since I am smart enough to not drill and blow through my wonderful old-growth cedar siding and airtight Celotex sheathing. I will yet let in some fresh air through windows almost year-round. I aim blindly at 0.25 ACHnat, about 5 ACH50, 660 CFM50. My Insulation Math tells me that this costs, 0.074*CFM50, about a $49 per year investment in my good health, vs. having no fresh air. $50 per year is not nothing, but my opportunity of control in extreme measures, is by not more than 20%, $10 per year. Please, let's not talk about it.
For most of us, a push toward PassivHaus being-green, is absurd. I could never get there. I won't try. I can't and won't invest in some extreme new house. I am doing just fine, in what I can afford. And, I am proud of this old house.