I would be concerned with pathogens that are under your house. Is this a mobile home, or is the 'crawlspace' actually a shallow cellar with concrete floor and walls?
Bringing in outside air is a good way to introduce pests and disease if it is not filtered correctly. Usually, the air within your house is preferable, as it is at the correct temperature already, and as a bonus, has elevated levels of CO2 from the human inhabitants...