Window Blinds
Window blinds—vertical or horizontal slat-type—are more effective at reducing summer heat gain than winter heat loss.
Interior Blinds
Because of the numerous openings between the slats, it's difficult to control heat loss through interior window blinds, but the slats offer flexibility in the summer. Unlike shades, you can adjust the slats to control light and ventilation. For example, when completely closed and lowered on a sunny window, highly reflective blinds can reduce heat gain by around 45%. They can also be adjusted to block and reflect direct sunlight onto a light-colored ceiling. A light-colored ceiling will diffuse the light without much heat or glare.
Exterior Blinds
Exterior roller blinds are usually made of wood, steel, aluminum, or vinyl. They're mounted above the window. Side channels guide them as they're lowered and raised. When you lower these blinds completely, their slats meet and provide shade. If partially raised, the blinds allow some air and daylight to enter through windows.