How does insulation work?

The most effective insulator is dry air. Insulated jackets work by trapping air around your body, reducing heat loss. However, moisture from sweat or rain can reduce insulation's effectiveness, so it's important to consider your entire layering system when choosing an insulated jacket.

Down or synthetic insulation?

The two main types of insulation are down and synthetic, and each has its advantages.

Down offers an excellent warmth-to-weight ratio and is highly compressible, making it ideal when weight and pack size are important. The quality of down is measured by its fill power – the higher the fill power, the more air the down can trap and the greater its loft.

Down performs best when dry, although many modern jackets use hydrophobic down treatments to improve its resistance to moisture. However, treated down is not waterproof and can still lose performance when thoroughly wet.

Synthetic insulation is generally made from polyester and is easier to care for than down. It retains much of its insulating ability when damp, making it particularly useful for the UK's cold and wet conditions. Synthetic insulation is also available in recycled forms, making it a popular choice for those looking for more sustainable options.

Types of synthetic insulation

Synthetic insulation is available in several constructions. Sheet insulation is generally durable, weather resistant and suitable for cold, wet conditions, although it can be heavier and bulkier.

Blown synthetic insulation uses clusters of fibres to mimic the loft of down, offering a lightweight and compressible design.

Active synthetic insulation is designed to be more breathable, making it particularly useful for high-output activities where you're likely to generate more heat.

How warm will an insulated jacket be?

A jacket doesn't generate heat – your body does. The insulation traps the warm air your body produces to reduce heat loss.

For down jackets, consider both fill power and the amount of down used. A high fill power indicates quality, but the total quantity of down is also important when comparing warmth.

For synthetic jackets, insulation weight is usually given in grams per square metre. Generally, a higher insulation weight means a warmer garment.

The fit is equally important. A jacket that's too loose can allow warm air to escape, while one that's too tight can compress the insulation and reduce its effectiveness. If you plan to wear layers underneath, make sure there's enough room without the jacket becoming excessively loose.

What about baffles?

Baffles keep the insulation evenly distributed throughout the jacket. Sewn-through baffles are common on lightweight jackets, while box-wall construction provides a more continuous layer of insulation and is generally found on warmer jackets designed for colder conditions.

Do I need a waterproof insulated jacket?

Most insulated jackets aren't fully waterproof. Waterproof construction requires taped seams and can add weight while reducing breathability.

Instead, an insulated jacket can be used as part of a layering system, with a waterproof shell worn over the top when conditions turn wet. For dry, cold weather, an insulated jacket with a wind-resistant outer fabric can be worn on its own.

Don't overlook the hood

A well-designed hood can make a significant difference in cold conditions by helping retain heat around your head and neck. If you're buying an insulated jacket for climbing or mountaineering, make sure the hood fits comfortably over your helmet and can be adjusted securely.

Ethical and recycled insulation

If buying down, look for recognised responsible sourcing standards such as RDS (Responsible Down Standard). Recycled down is another option, while many manufacturers now use recycled synthetic insulation and recycled outer fabrics.

Caring for your insulated jacket

Regular washing and proper maintenance can extend the life and performance of your jacket. Follow the manufacturer's instructions and use a technical wash where recommended.

Synthetic jackets are generally straightforward to machine wash. Down jackets require more care, particularly during drying. Down must be thoroughly dried to restore its loft and prevent the insulation from clumping.

Key buying tips