Down vs. synthetic: how to choose mountain insulation

Down and synthetic insulation pursue the same goal: trapping stable air around the body to reduce heat loss. The difference lies in how they achieve this and the conditions in which each solution performs best.

How insulation works

Neither down nor synthetic fibers "generate" heat. Their function is to create a structure capable of retaining air. The more effective that structure is at holding still air with little weight, the better the ratio between insulation, volume, and weight will be.

Down: maximum warmth-to-weight ratio

Down is made up of small, three-dimensional clusters capable of expanding and trapping a large amount of air with very little weight. That is why it remains the benchmark when the priority is maximum warmth with minimum weight and pack volume.

What is fill power

Fill power measures the expansion capacity of down. A higher value indicates down that achieves more loft for the same weight.

It is important not to confuse down quality with amount of insulation: a 900FP jacket is not necessarily warmer than an 800FP one if it uses much less fill. To understand thermal capacity, one must consider fill power, amount of down, construction, and garment design together.

Hydrophobic down

Rab uses down with a hydrophobic treatment in many products to help it better maintain loft in humid environments and dry faster. Even so, hydrophobic down does not mean a waterproof jacket.

Synthetic insulation: stable performance in wet conditions

Synthetic insulation is usually made with polyester fibers interwoven to form air chambers. Rab currently distinguishes between several types of construction:

• Loose fill: seeks to imitate the loft and feel of down.
• Static sheet: prioritizes stable insulation and compression resistance.
• Active sheet: designed for stop-and-go activities or intense ascents, combining with highly breathable fabrics.

Main advantages of down

• Excellent warmth-to-weight ratio.
• Very high compressibility.
• Ideal when every gram and every liter in your pack counts.
• Very suitable for dry cold, bivouacking, mountaineering, and activities with static periods.

Main advantages of synthetic

• Quick drying.
• Maintains performance better when wet.
• Good resistance to compression and repeated use.
• Active constructions can offer great breathability during intense movement.
• Very useful in changeable weather and stop-start activities.

Down or synthetic?

Choose down when your priority is to minimize weight and volume and you need a high warmth-to-weight ratio.

Choose synthetic when you expect moisture, intense activity, or frequent changes in pace and want insulation that dries quickly and maintains stable performance.

In many activities, both solutions are complementary: an active synthetic layer during movement and a warmer down garment for stops or intense cold.

Explore Rab insulation at Mountain Group

Men's down jackets · Women's down jackets · Men's synthetic jackets · Women's synthetic jackets

Content reviewed by Mountain Group based on current Rab technical documentation regarding down, fill power, and synthetic insulation.