The big problem with the oft repeated "Let the house breath!" is that if you don't bother to control air leakage you're letting that unfiltered, unconditioned a

The big problem with the oft repeated "Let the house breath!" is that if you don't bother to control air leakage you're letting that unfiltered, unconditioned air percolate through the insulation which can be a good media for mold and mildew to grow in. Not to mention the fact that you're throwing away energy, even ignoring air conditioning surely you're still heating during the winter. Ideally you'd have an airtight house and bring in fresh air through an ERV or HRV to save energy and filter it but even if it's just fresh untreated outside air brought in straight from a vent you'd still eliminate the moist air moving through fiberglass batt insulation and the occasional condensation on the paper backing of the drywall. One thing I really like about spray foam insulation is that your insulation is your air barrier and thermal barrier and you don't have to deal with air passing through your thermal barrier and potentially causing condensation. In southern climates the air barrier is always on the outside of the insulation and in northern climates from what I understand they put the air barrier on the inside. Spray foam effectively puts it on both sides so you're not compromising based on what location is more often going to prevent condensation. Think about what an air filter looks like after a few months. That's what happens inside wall cavities in leaky homes only there's no way to change them and it builds up slowly over decades. There's a lot more surface area, but that only delays the problem, if you look at any renovation photos when a wall is opened up after a few decades they're typically filled with dust and grime. Even if you just had a 6 inch duct with nothing but some louvres separating inside from outside you've still eliminated most of the potential buildup.