This question comes up a lot and is often misunderstood.  Inert building materials don’t really need to breathe but occupants in the home do!  Seal tight and ventilate right!  Rather than let a leaky home bring in unregulated amounts of outside air along with pollen, dust, odors, moisture, pollutants and other contaminants, seal up the house with foam and let an air exchanger ventilate the house.  With an air exchanger, you are also allowed to recover your heat or cooling from the stale air that is exhausted from the interior and pre-heat or pre-cool that fresh air that is coming in.

What about the roof, it needs to breathe, it needs to be vented, right?  The fact is, we started to vent roofs because the typical fiberglass insulation didn’t do what it was supposed to do.  It doesn’t stop warm, moist air from passing through it.  Roofs are normally vented to release this warm, moist air that passes freely through the fiberglass to the outside before it has a chance to condense to the underside of the cold roof sheathing (in our northern climate).  With closed cell foam being both an air barrier and a class II vapor retarder, there is no chance of warm moist air passing through the foam and ever contacting the cold sheathing!  CC foam is an air barrier.  In these volatile northern climates, it is recommended to seal a home up to stop drafts and then to install a heat recovery air exchanger which will remove stale, dusty, and humid air outside and bring in cleaner and fresher air while removing excess moisture.