The haircoat has the same effect as wearing a warm overcoat or long underwear. If the horse is exposed to cold, windy weather, most of the body heat trapped in the haircoat is removed. So horses need some type of windbreak when it's cold and windy. Put your horses in stalls, especially at night, or allow them access to open stalls or run-in sheds in winter. A stand of trees in a low area below the crest of a hill will provide a natural windbreak.
Older horses suffer more in cold conditions.
If they're thin, they won't have the insulation of body fat. Teeth problems may keep them from chewing coarse, fibrous hays. So put older horses in stalls during extremely cold weather. Feed them all the good-quality hay they'll eat. Provide them water from the hot-water faucet.