Cardboard on windows, without any additional insulation like aluminum foil, can provide a certain degree of thermal insulation and help reduce heat transfer, but its effectiveness may be limited compared to other insulation materials specifically designed for this purpose.
Cardboard has some insulating properties due to its composition, which includes trapped air pockets. Air is a poor conductor of heat, so these pockets can help slow down heat transfer to some extent. Additionally, cardboard can block a certain amount of direct sunlight, which can contribute to heat gain.
However, it's important to note that cardboard is not specifically engineered as a thermal insulator like materials such as insulation foam, double-glazed windows, or reflective barriers. These specialized materials often offer better insulation properties and are more effective at reducing heat transfer.
If you're looking to keep heat out of a room or improve energy efficiency, it's recommended to use proper window treatments designed for insulation, such as blinds, curtains, or window films. These options are specifically designed to reduce heat transfer, block sunlight, and improve insulation, providing more effective results than cardboard alone.