Three numbers, one useful one
EN/ISO testing produces a comfort rating, a limit rating and an extreme rating. The comfort rating is the temperature at which a standard cold sleeper stays asleep. The limit rating assumes a warm sleeper curled up. The extreme rating is a survival number — it means six hours of shivering and possible frostbite. Shop by comfort, ignore extreme entirely.
Your pad carries half the load
Down compresses to nothing underneath you, so the insulation between your body and the ground is almost entirely the pad's job. R-value is additive: stack a 2.0 foam pad under a 3.5 inflatable and you get 5.5. That's a cheaper and more reliable way to buy warmth than upgrading the bag.
Down fill power vs fill weight
Fill power measures loft quality; fill weight measures how much down is actually in the bag. A 900-fill bag with 300 g of down is colder than a 650-fill bag with 700 g. Brands quote fill power because it's the flattering number — always look for grams of fill.
Practical layering
A 30°F bag plus a good liner and dry base layers covers most three-season trips in most places. Add a hooded puffy worn inside the bag before you buy a warmer bag: it's lighter, it works in camp, and it doesn't cost $400.
