Ask yourself five key questions before buying a new sack.
1. How warm? Determine the lowest temperature you expect to encounter while camping—then subtract up to 10 degrees for insurance. (See EN ratings, below right.) Be realistic: Do you tend to sleep hot or cold? Is your pad a deluxe, well-insulated mattress or a minimalist model that’s only an inch or so thick?
2. What type of fill? Two choices: down or synthetic. Down offers superior compressibility, warmth-to-weight ratio, and long-term loft, but unless it’s paired with a highly water-resistant shell, down is a bad choice for damp conditions. Synthetic bags insulate better when wet and, especially for entry-level models, usually cost less, but specific performance characteristics are associated with filament type. Better warmth-to-weight ratios, compressibility, and durability add cost to bags of both types.
3. How roomy and how long? A mummy bag’s tapered, snug cut enhances heat-trapping efficiency, but can be uncomfortably tight for large or restless sleepers. A semi-rectangular shape offers more roll-around room, but increases weight and bulk. Beware of a too-tight fit: compressing insulation causes cold spots. Most bags come in several lengths (excess space at your feet means excess weight, but the benefit of added storage space), as well as women’s-specific models (which have different proportions and extra insulation).
4. What features? Zippers vary from full-length (great for venting) to none (great for cutting weight). Do you need a stash pocket for a headlamp and iPod? Brushed linings add a little weight, but are cozy and wicking.
5. Is it comfortable? Use warmth and features as a guide to narrow your list. Then crawl into the finalists and cinch the hood around your face to check for scratchy fabrics and drawcord access.
Lay all the bags you’re considering on the floor, side by side, to compare loft. Wet the bags’ shells with water (a good store will have a spray bottle on hand) to see how well it beads. After crawling in for a fit and comfort test, stow the bags in their included stuffsacks and compare compressed sizes of bags in the same weight and warmth categories.
Buy in the off-season: Summer bags are cheaper in January, while winter bags cost less in July (one editor nabbed a top flight -20°F Sierra Designs bag for under $200 at steepandcheap.com). Save bucks by purchasing a lightly used bag online. Look for sales from gear-rental companies with generous return policies like LowerGear (lowergear.com) in case of a deal-breaking defect.