There are a variety of advantages and drawbacks to the use of thick curtains, and some of these will depend on the climate a person lives in and his or her own preferences. Curtains that are fairly thick can help keep sunlight from passing through the curtains and entering a room, keeping the room darker than lighter curtains typically will. Thick curtains can also help keep rooms warmer during the winter, and can be somewhat useful for keeping rooms cooler in the summer as well.
Sometimes also called blackout curtains, thick curtains are designed to be hung over windows using fairly heavy fabric or materials. There are advantages and disadvantages inherent to using these sorts of curtains, though some advantages can also be drawbacks depending on the preferences of a person. One of the major advantages of thick curtains is that they prevent much sunlight from entering a room through windows they cover. This is ideal for rooms that have televisions or similar devices in locations that could easily be struck by sunlight and create glare on the screen.
In some rooms, the darker conditions created by thick curtains are clearly an advantage, though in other situations they can be a drawback. Some people prefer curtains that block out some light while still keeping a room well lit, and so curtains that are too thick would likely block out too much light for such people. The use of such thick curtains could also require that other forms of light be used more often, which could potentially increase electricity costs.
One of the major advantages of thick curtains is the ability for such curtains to help maintain comfortable temperatures in a room. How well this works, however, depends a great deal on other factors. In general, curtains are typically better at keeping a room warm when outside conditions are cold than in keeping a room cool on a hot day.
Air in a room will typically come into contact with the glass of a window, which then affects the temperature of the air. Hot windows and sunlight entering through such windows will typically heat a room regardless of curtains. Preventing sunlight from reaching the windows, through external blinds or coverings, is often more effective for this purpose.
Warm air can also be cooled by coming into contact with cold windows, but thick curtains can prevent this more effectively than blocking out heat. Curtains used to keep cold air out, however, should have a pelmet at the top of the curtains. Without a pelmet, thermal convection will draw warm air down from the top of a room into the space between a window and curtains, cooling the air and undoing much of the warmth afforded by the curtains.