So blanketing is not always necessary, even in the harshest of winter conditions, but you need to consider several factors to make the best decision for your horse.
??How well is he adapted to the cold? Seasons change gradually in many parts of the United States, so horses have time to adjust to dropping temperatures. However, in some areas abrupt changes occur that don't allow the horse time to completely adapt. In those conditions, you may need to change blankets daily in response to day-and-night temperature fluctuations during the spring and fall.
? Is he staying dry? Whether from rain or sweat, too much moisture will weigh down a horse's winter coat, destroying the loft effect you mentioned. When this happens, the horse begins losing heat and is at risk for getting too cold. (Do not, however, put a blanket on a horse with a wet coat. You will only trap the moisture against his skin. Instead, put on a cooler, which will keep him warm while wicking dampness away. Once he is dry, then you can blanket him.)