Generally, cold is not an issue for chickens unless its really, really, really cold. OTOH, cold breezes on chickens are a problem, and inadequate ventilation is a problem year round.
Based on pictures, I would both lower the roosts and either replace the shutters with louvers or similar, or change the shutter into a slide like a typical chicken door, so you can leave the bottom partially covered as needed against inclement weather. You may also want to consider adding ventilation at the eaves, or, if design allows it, under soffit and ridge venting for time.
How big is the space, 4'x6'? and how many chickens inside??? You don't have a location listed so I am simply guessing at your climate.