| What it is | Thickness of lofted insulation |
| Why it warms | Traps a layer of still air |
| Lost when | Compressed, wet, or dirty |
| Difficulty | Beginner concept |
Loft is the thickness and fluffiness of insulation when it expands and traps air — the property that actually keeps you warm. Down and synthetic insulation work by lofting to hold a layer of still air against the cold; compressed, dirty, or wet insulation loses loft and warmth, so more loft generally means more warmth.
Why air, not fabric
The trapped still air does the insulating — which is why crushed or wet down fails. Down quality is rated by fill power.
Keep the loft
Store gear uncompressed, clean, and dry to preserve loft in a sleeping bag.
Frequently asked questions
What is loft?
Loft is how thick and fluffy insulation is when it expands fully. Down and synthetic fills keep you warm by lofting up to trap a layer of still air, which slows heat loss. The more an insulation lofts (within reason), the more air it traps and the warmer it is.
Why does loft matter for warmth?
Because it's the trapped air, not the material itself, that insulates. Anything that flattens the insulation — body weight, compression, moisture, dirt, or worn-out fill — reduces loft and therefore warmth. That's why the crushed underside of a sleeping bag doesn't insulate.
How do you restore loft?
Store insulation uncompressed (loose in a large sack, not stuffed), keep it clean and dry, and wash down periodically with proper down wash to revive its loft. Letting a bag or jacket fully expand and fluffing it before use also helps it reach full loft.
Sources
- How insulation works — The Mountaineers