Insulation is essential to keeping warm in the outdoors, and is usually worn as a mid-layer, or as the outer layer in dry, cold climates. It works by trapping the warmth that our bodies create to maintain our body temperature, but this can be achieved in a variety of ways.
Down insulation is obtained from geese and ducks: the smallest feathers found next to the skin. Goose down generally has a higher warmth-to-weight ratio and packs down smaller than duck. The warmth of a down jacket, or the 'loft', is measured by its fill power. The higher the fill power rating and the ‘loft’, the more warm air the down can trap, and the warmer and more packable the jacket.
The main drawback of down insulation is that it loses its ability to retain heat if it gets wet, because this causes the down to stick together. Therefore, down jackets only perform well in dry conditions. Down jackets which have a hydrophobic (water resistant) coating are available, however, they still don't perform as well as synthetic insulation in wet conditions.
Remember, many jackets come with a DWR (durable water-repellent) coating, so will be able to handle quick showers, but not much more.
In synthetic fill jackets, man-made polyester strands are used to imitate the air-trapping down filaments to create pockets of warm air. These fibres are moisture-resistant, and retain their insulating properties when wet. If you plan to use your jacket in wet conditions or when you're highly active, synthetic insulation is the best choice.
Synthetic insulation is measured in grams per square metre - not to be confused with the weight of the whole jacket. As a rule of thumb, 50g-100g of insulation is a good choice for spring or autumn, or as a technical mid-layer, whilst jackets with 100-200g of insulation are ideal for colder conditions.
Less common and highly technical, these jackets draw on the strengths of down and synthetic fills to maximise functionality and weight savings. They commonly use synthetic fill in the shoulders and arms of the jacket, and down in the core body area. This is because synthetic fill retains its insulating characteristics better than down when compressed, for example by a rucksack on the shoulders, and if wetted out, which normally occurs first on the arms and shoulders. The use of down in the main body enables the jacket to remain highly packable and gives excellent warmth in the core.
Baffles are the containing sections of insulation. Their purpose is to prevent the material from gathering in the bottom of your jacket and evenly distribute the insulation across the jacket. The way the baffle is constructed can affect how well your jacket performs.
Stitch-through baffles keep the insulation evenly distributed. However, the stitched areas can cause heat to be lost.
Box wall baffles allow the insulation to expand to its maximum loft, and the stitching is designed to reduce heat loss.
Wide baffles are the warmest option as they contain more grams of down and can be constructed such that there are fewer seams where your body heat can escape. They can, however, feel bulky and do not layer well or compress down as small in your pack.
Narrow baffle or micro baffle jackets, as well as looking fantastic, work well underneath a waterproof in cold and wet environments and compress down small enough to stuff easily in your pack ready for you to pull out when you stop moving.
If you’re going to be using your jacket in very cold, low-activity environments then a jacket with an insulated hood may benefit you. Conversely, if you’re looking for a packable, lightweight garment to keep your core warm, then a hood may just add bulk and weight.