HRV (heat recovery ventillators) are the thing you want to consider if you have a truly tight house. They run the air by fancy heat/vapor exchangers so the incoming air is warmed by the exhausting air (or vice-versa in the summer) and helps to maintain the relative humidity levels in the process. These are much more common in Canada, which has mandated much stricter house building regulations than the USA. They also help to maintain a balance in the house pressure to account for both over and under pressurizing the house which will affect inflitration and the drafting of burner equipped appliances, and things like bathroom and kitchen exhaust fans.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013
In terms of foam R/$ you're usually better off with low-density EPS rather than iso.
On foundation sills and band-joists the ignition barrier requirements are eliminated in local codes, per the IRC, so long as:
* The maximum thickness of the foam plastic shall be 3-1/4 inches
* The density of the foam is between 0.5 & 2.0lbs per cubic foot
* has flame spread index ≤25
* has a smoke developed index ≤450
(Which would be true of most rigid foam products.)