The more ventilation in a barn or stable, the better. Many builders like to seal up a barn like they would a house, but that simply is not necessary. Horses need plenty of fresh air to avoid respiratory ailments that can occur by breathing the same air over and over. They can take the wind and cold better than they can deal with stale air. A good idea is to keep the top board of siding off on what would be the outside wall closest to the gutter, given the typical barn layout. Combine this with roof ventilation such as a cupola and gable end vents, and keep the aisle doors and stall windows open except in extreme weather. This also helps keep the hay up in the loft from getting moldy.