They’re designed to block up to 100% of light, which also blocks heat. Of course, this does mean sitting in the dark, but it’s a small sacrifice if your home is too hot.
Foil reflects heat, which is generally why we use it in cooking. This also makes it ideal for covering your windows.
Your outdoor area could be making your heat problem worse. So, one of the best cheap ways to block heat from windows is to use a shade sail. You could buy one or make it yourself.
The little air pockets trap heat and stop it from transferring through the material. Again, it won’t look the best, but it can reduce heat transfer by up to 50%.
Like bubble wrap, these insulate and so reduce heat transfer. In fact, cellular shades can reduce heat transfer by up to 60%.