As with commercial chicken houses, ventilation can be either positive pressure or negative pressure (just on a smaller scale). So the fan can be blowing in or out.
On commercial farms, another big concern with ventilation is biosecurity and keeping pathogens out of the building. In any event they use major filtration. The primary purpose for both large scale and backyard operations is to provide the freshest/cleanest air possible for the birds. Many large scale operations are climate controlled. Small scale are usually not but it may not be a bad idea if people insist on having fragile breeds in frigid climates. I think it best to keep breeds suited to one's climate and temperature isn't nearly as big of a concern as insuring a constant supply of fresh air.
