Fresh air and good ventilation are important requirements for horses.  Judging from their own perception of what's comfortable, humans tend to close barns up too tightly.  Horses can tolerate considerable cold if they can move around and are dry.

Avoid drafts, but allow sufficient air exchange to move stale air, humidity and ammonia out of the horse's environment.

Horses should be allowed outside where they have an area for adequate exercise and access to a run-in shed, free-choice quality hay, free choice salt and minerals, and water heated to temperatures of 60° F.

Several studies have shown that warming water to at least 60° F will increase water consumption to 40-100%.  Dehydration (lack of water) is the number one cause of impaction colic in horses.  Reduced water consumption due to due to cold weather combined with a diet of dry feed puts horses at increased risk of impaction colic.  Warming the water is much more effective than feeding bran, linseed meal, or other so called laxative diets.

Most horses will not require grain if the hay is of good quality.  Hay generates more heat than grain does during digestion and thus is more helpful in maintaining good body heat during cold weather.