Down is the delicate, fluffy undercoat of a duck or a goose -- the natural insulation that keeps the bird warm. A down comforter is light, breathable and the best insulation for sleeping in cold weather. Alternative-down comforters are made with synthetic fill fibers, cotton, wool or silk. Some allergy sufferers prefer them, and many of the comforters are designed to toss in a washing machine and dryer. The loft in a down comforter is featherweight, compared to a heavier alternative down, which can be built with many layers of filling to provide sufficient warmth. Down now can be treated and cleaned to minimize allergic reactions, so it's worth exploring your options when choosing a comforter.
When choosing a comforter -- down or hypoallergenic -- select for year-round, seasonal or winter chill. Down comforters are rated by fill power, the amount of space 1 ounce of down will loft to, or expand to. Larger clusters of down, such as those from wild geese, have higher fill power and give greater warmth. A fill power of 600 to 800 will keep you warm on the coldest nights. Summer comforters might be 500 fill power or lower. Choose baffle-stitch construction to hold shifting loft in place and prevent cold spots. Alternative-down comforters come adapted to the average warmth of your bedroom. A cool 55 degrees Fahrenheit means a heavier, warmer cover; a bedroom kept at 70 degrees or higher is should be cozy enough with a summer-weight comforter.