Or they buy new high efficiency windows and doors, and that 'tightens up' the house, but they don't also install an HRV (Heat Recovery Ventilator) to exchange outside air.
Having the foam crew do a blower door test for free if we go with them, so I guess I'll see. I live in climate zone 5a (cool, humid) so historically natural ventilation was used via induced pressure differentials generated by mechanical means (dryer, furnace, bathroom and kitchen vents). When we get new furnace and water heater that pull combustion air from outside, we're left with mostly manual means for generating negative pressure that drives infiltration. (But also wind, and pressure differentials caused by temperatures in winter)
In our case, we're definitely considering either an HRV or ERV to supply fresh outdoor air that we control (input and output CFM, MERV #). Our indoor air quality is poor enough to the point where we live with the tangible symptoms (Rhinitis), not just numbers on a sensor.
For those with interest in these topics, I suggest skimming a copy of one of the ASHRAE fundamentals handbooks[1]. It reads as a textbook but has incredible depth into the physical sciences that relate to building science, heating, cooling, and ventilation.
[1] https://archive.org/details/ashraehandbook2017hvacfundamenta...
And even if the installer air sealed your doors and windows, there are plenty more places for air to leak in.
You really need to run a blower door test to see if you need an HRV/ERV.