There should, at all times, be good ventilation. The average horse respirates about two gallons of moisture into the air each day. Naturally, there will be condensation from the difference between indoor and outdoor temperatures. If you do not have good ventilation, moisture will collect on the roof and create a damp environment. This dampness contributes to respiratory ailments and bacterial and fungal growth. Even in wet weather, moving air will remove some of the condensation and will replace stagnant air with fresh.
Heat in summer is another factor to consider. Venting your shed row barn with a ridge vent or gable vents will allow the warm air to escape. Warm air rises; therefore, even though you can always open windows and the top halves of the Dutch doors in your barn, the heat would escape best through the ridge or gable vents.