Window units are by far the most popular. Models can be installed in most single or double-hung windows and typically come with accordion panels that allow for a secure fit. Some models can fit in sliding windows. They do require a support of some kind, such as a wooden plank or a specially designed shelf that can be attached to the exterior of your home for dedicated reinforcement. Installing a window unit is an easy DIY project. Make sure you measure the inside window dimensions before you shop.