A sleeping bag liner is the single most underrated piece of camping gear. It costs $20-70, weighs 3-14 ounces, packs down to the size of a grapefruit, and does three genuinely useful things: adds warmth to your sleeping bag, keeps your bag clean (extending its life), and works as a standalone lightweight cover on hot summer nights. If you've never used one, you're probably carrying more sleeping bag than you need to.
Four materials dominate the liner market. Each has a distinct profile for warmth, weight, and feel. Here's how they compare on paper:
| Material | Temperature Boost | Weight (Typical) | Pack Size | Feel | Durability | Price Range |
|---|---|---|---|---|---|---|
| Silk (mulberry) | +5°F to +8°F | 3.5-5.5 oz | Softball | Smooth, slippery, breathable | Good — snag-prone but holds up with care | $50-$80 |
| Coolmax/Thermolite | +15°F to +25°F | 8-14 oz | Grapefruit | Soft fleece-like, wicking | Excellent — machine washable, no pilling | $55-$70 |
| Fleece (microfleece) | +12°F to +18°F | 10-16 oz | Large grapefruit | Plush, warm, slightly bulky | Very good — resists abrasion | $25-$60 |
| Cotton | +3°F to +5°F | 8-14 oz | Cantaloupe | Soft, familiar, absorbs moisture | Moderate — dries slowly, heavy when wet | $15-$30 |
Cotton is the worst choice for camping. It absorbs moisture (including body vapor overnight), dries slowly, and provides minimal warmth. Cotton liners are fine for hostel travel or guest beds, but they have no place in a camping pack. Silk, fleece, and Thermolite are the three materials you should actually consider.
Manufacturers quote temperature boosts, but these aren't additive in a simple way. A liner rated for +15°F doesn't turn a 30°F sleeping bag into a 15°F bag. The actual boost depends on how snug the liner fits inside your bag, your metabolism, and what you're wearing. Think of a liner as shifting your bag's comfort limit by roughly half the quoted boost — a Thermolite liner claiming +15°F realistically extends comfort by 7-10°F, which is still meaningful.
More importantly, the liner prevents convective heat loss. Your sleeping bag's insulation works by trapping body-warmed air. A liner adds another layer of trapped air near your skin and seals the micro-gaps around your shoulders and neck.
| Model | Material | Claimed Boost | Weight | Pack Size | Price | Best For |
|---|---|---|---|---|---|---|
| Sea to Summit Reactor Extreme | Thermolite Pro | +25°F | 14 oz | 5" x 4" | ~$65 | Cold sleepers, shoulder season |
| Sea to Summit Reactor (standard) | Thermolite | +15°F | 8.7 oz | 4" x 3" | ~$55 | Best overall warmth-to-weight |
| Cocoon Silk Mummy Liner | 100% mulberry silk | +5.8°F to +8.6°F | 5.5 oz | 4" x 3" | ~$65 | Summer weight, ultralight travelers |
| Mountain Laurel Designs (MLD) Silk Liner | Ripstop silk | +5°F | 3.8 oz | Softball | ~$70 | Ultralight gram-counters |
| Rab Silk Ascent Liner | Silk | +5°F to +7°F | 5.3 oz | 4" x 3" | ~$55 | General purpose, good value silk |
Sea to Summit Reactor Extreme: This is the warmest liner on the market and genuinely extends your sleeping bag's range by a noticeable margin. The Thermolite Pro fabric is a hollow-core polyester that traps more air than standard Thermolite. At 14 oz, it's heavier than silk alternatives, but for shoulder-season camping where you'd rather add a liner than buy a whole second sleeping bag, it's cost-effective. Check price on Amazon.
Cocoon Silk Mummy Liner: The Cocoon is the gold standard for silk liners. It uses a higher momme weight (silk density measurement) than most competitors, which means it's less transparent and more durable. The mummy shape with a footbox reduces bunching inside your sleeping bag. Silk's natural temperature regulation means it also works surprisingly well on 80°F nights as a standalone sheet. Check price on Amazon.
MLD Silk Liner: At 3.8 oz, this is the lightest reputable liner you can buy. Made from ripstop silk (not plain weave), it resists tearing better than standard silk liners. The weight savings come from a narrower cut and simpler construction. If every gram counts, this is the pick.
Rab Silk Ascent: Good middle-ground silk liner at a slightly lower price than Cocoon or MLD. The rectangular cut offers more room than mummy-shaped liners if you don't like feeling constrained.
Sleeping bag down and synthetic insulation both degrade from body oils, sweat, and dirt. Washing a sleeping bag is tedious and shortens its lifespan — each wash cycle stresses the baffles and can clump down. A liner absorbs your body's nightly oil and sweat output instead. Liners are machine washable and dry in an hour. This alone justifies the purchase if you own an expensive down bag like a Western Mountaineering or Feathered Friends.
Also consider your sleeping pad's R-value — a liner only adds warmth from above and around; you still lose significant heat through the ground if your pad is insufficient.
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Disclosure: Some links on this page are affiliate links. If you click through and make a purchase, we may earn a small commission at no extra cost to you. We only link to products we've researched and believe are worth your consideration. Temperature boost numbers are manufacturer claims; actual results vary by user, bag type, and conditions.