Window Solar Screens are designed to keep Direct Heat and Radiating Heat from entering you home. If some of your windows don't get direct sunlight (due to trees or patio covers), heat may still transfer through the glass due to Radiating Heat. For maximum energy efficiency, Window Solar Screens should be added to all glass surfaces and windows. However, we do understand if you only want to start off with the windows that get the direct heat first and add other screens later.