4B, 2-story home with an unconditioned attic and 2 different HVAC systems (1st and 2nd floor)

I’ve been improving the insulation in the attic (I now have R-60 with cellulose instead of R30 of fibreglass), I also air sealed the attic floor, as well as HVAC ducts with CCSF (2 inches), I’m happy to report that my HVAC runtimes during the hottest months are 40% less than the last 3 years. (Checked the historical averages from my tstat).

However, I have another issue that I am trying to solve: Fresh Air / Exhaust ventilation. (Co2 levels are elevated now, possibly due to HVAC system upstairs not running as often, plus it’s not actually pulling in ‘fresh air’ / exhausting from the attic via leaky ducts as much?) 

I have 2 bathrooms on the upper floors, 1 is the master and another one is in the hallway. Both have Broan bathroom exhaust fans. I’d like to put fresh air intake, or even a ERV somewhere upstairs, but I’m limited on where I could do an ERV because I do not believe there are ERVs that can be installed in unconditioned attics. I could install one in the ceiling upstairs, somehow. 

Some thoughts:

– A damper in the attic A/H to exhaust air (negative pressure) from the return box, and a 1st floor positive pressure fresh air intake).
– Or, I could just do a fresh air intake in the attic, attach it to the return box, theroritally, the positive pressure from the fresh air intake would exhaust from the bathroom exhaust fans? (the dampers on the Broans do not close all the way when the unit is off). 
– Or, a 1st floor ERV, with a dedicated damper in the attic to run at x? 

What do you think?