A horse's normal winter hair coat is far more insulating than a man-made blanket. As a general rule, the horse that has been allowed to grow long winter hair and become gradually accustomed to the onset of colder weather will be better off without a blanket.
Long winter hair traps a layer of body heat between the skin and the cold air. When it's cold, tiny muscles in the skin make the hair stand up, increasing this insulating effect. Adding a heavy blanket or piling on several layers of lightweight blankets can actually make a horse colder, since the hair is flattened and its insulating effect lost. Blanketing is necessary, however, for a clipped horse or one moved into a colder climate during winter months. It's also necessary for a horse that is outside in winter storms without a windbreak.
Blanketing a horse that's not used to being covered may actually put him at risk. He may become overheated and start sweating, then chill afterward because he is damp. This sets him up for a respiratory infection or pneumonia. Don't blanket a horse for turnout in the cool of the morning if you're not going to be there to take the blanket off before the warmth of the afternoon.
Choose your blanket according to how much weather protection the horse actually needs. Many blankets available today are warmer and more weather-resistant than the traditional wool blankets. Some use inner and outer shells surrounding an insulating fiber. The outer shell, made from a durable synthetic fiber, is water repellent and windproof. The filler inside provides warmth without bulk or heaviness, and the inner shell is smooth and non-abrasive so it won't rub or chafe the horse's skin. These lighter blankets are more durable than cotton or wool, and light enough not to press down on a horse's hair.
The nylon fiber used in many outer shells is strong and nearly indestructible; it doesn't snag, tear, rot or mildew. Many have a waterproof finish that keeps moisture out and holds heat in, although the coating may crack in extreme cold. The inner filler is usually made of foam particles or fiberfill, which is extremely lightweight but still has good insulating qualities.
Regardless of the blanket used, proper fit is important. A blanket that is too tight can rub and irritate the skin in several places; a too-large, loose blanket can slip under the horse's belly. If his legs become entangled in the straps, the horse may be seriously injured during his struggle to free himself. When you put the blanket on, place it well forward, then pull it back into place so it won't ruffle the hair the wrong way.
Don't use the same blanket on different horses or you may spread skin problems. Any horse with ringworm, girth itch or other fungal infections or skin irritations should have its own blanket that is never used on any other horse. Even if you have the skin problem under control, fungal spores can be transmitted to others using the same blanket. Wash blankets in cold water at least twice during the winter if they are used often. Use soap and a disinfectant, and rinse them well to make sure there is no soap residue that might irritate the skin.
Dry cleaning is not a good idea; it does not remove odors, the solvents may disintegrate the waterproof coating, and the heat will shrink the bindings.