Key takeaways
- Loft is the thickness/fluffiness of insulation when fully expanded.
- More loft = more trapped still air = more warmth.
- It applies to down and synthetic insulation in bags, jackets, and quilts.
- Compression, moisture, dirt, and age reduce loft (and warmth) — store insulation uncompressed and keep it clean and dry.
What loft is
Loft is the thickness, height, or ‘fluffiness’ of insulation when it expands fully — what you see when a down jacket or sleeping bag puffs up after being unpacked. It’s the key to warmth, because insulation works by trapping a layer of still air, and more loft means more trapped air.
Why loft determines warmth
Insulation doesn’t warm you directly — it traps still air, and that trapped air slows heat loss to keep you warm. A thicker, loftier layer traps more air, so it’s warmer. This is exactly why insulation that’s gone flat, compressed, or wet loses much of its warmth even though the material is unchanged — it has lost its loft.
A camper pulls their down sleeping bag from its stuff sack and shakes it out an hour before bed, letting it fully loft into a thick, fluffy layer — restoring the trapped-air thickness that makes it warm, where a bag used straight from compression would sleep cold.
Preserving loft
Protect loft to protect warmth: store insulation uncompressed (a large loose sack, not stuffed tight long-term), and keep it clean and dry — moisture, body oils, dirt, and age all reduce loft. This applies to both down (whose quality is rated by fill power) and synthetic insulation.
The bottom line
Loft is the fluffy thickness of insulation when fully expanded — and it's what makes insulation warm, since the trapped still air does the insulating and more loft means more trapped air. Protect it: store sleeping bags and down jackets uncompressed and keep them clean and dry, because lost loft (from compression, moisture, dirt, or age) means lost warmth.
Frequently asked questions
What is loft?
Loft is the thickness, height, or 'fluffiness' of insulation when it expands fully. It's the key to how warm insulation is, because insulation works by trapping a layer of still air, and the more loft (thickness of fluffy material), the more air it traps and the warmer it is. You see loft when a down jacket or sleeping bag 'puffs up' after being unpacked.
Why does loft determine warmth?
Because insulation doesn't warm you directly — it traps a layer of still air, and that trapped air is what slows heat loss and keeps you warm. More loft means a thicker layer of trapped air, hence more warmth. This is why a flat, compressed, or wet insulation that has lost its loft also loses much of its warmth, even if the material is the same.
How do you preserve loft?
Store insulation uncompressed — keep sleeping bags and down jackets in a large, loose storage sack rather than stuffed tight long-term, since prolonged compression degrades loft. Keep insulation clean and dry, because moisture, body oils, and dirt all reduce loft (and warmth). Letting a bag or jacket fully loft before use, and washing it properly when needed, helps restore and maintain its loft.
Sources
- Insulation & sleep systems — The Mountaineers
- Gear care — American Hiking Society