How to stay warm while camping: a complete overnight guide

Warmth while camping is a system. Your sleeping bag, your pad, your layers, and even your dinner all pull in the same direction or against each other. Get the system right and you sleep through the night. Get one piece wrong and a bag rated to 20°F leaves you shivering at 35°F.

How sleeping bag ratings actually work

Every sleeping bag tested to the ISO 23537 standard receives three numbers, not one. The Comfort temperature is where a sleeper lying relaxed on their back is at the threshold of feeling cold. The Lower Limit is where a sleeper curled up is at that same threshold. The Extreme temperature is where hypothermia becomes a risk: an emergency figure, not a target.

The gap between Comfort and Lower Limit grows as bags get warmer. In the ISO 23537-1:2022 table it runs from about 5°F for the lightest bags to about 14°F for the warmest it covers, and for a bag with a 20°F lower limit it is about 11°F.

Model names and labels do not always tell you which number you are looking at. The men's Kelty Cosmic Down 20, for example, is 31°F Comfort and 21°F Lower Limit, while the women's version with the same "20" in its name is 25°F Comfort and 13°F Lower Limit. Look up the ISO Comfort figure and buy to that unless you run genuinely warm and accept some discomfort at the bottom of the range.

The pad is half the system

ISO 23537 bag ratings are measured with a heated manikin lying on a foam mattress with a thermal resistance of 0.85 m²K/W. Converted to the US units pad makers label with, that is roughly R 4.8 (the two tests use different methods, so treat the conversion as approximate). If your sleeping pad is well below that, your bag will sleep colder than its rating. Ground conducts heat away from your body far faster than still air at the same temperature, so a summer pad on cold earth drains warmth your bag cannot replace.

Pad R-values are standardized under ASTM F3340-18, which Therm-a-Rest says was finalized and published in 2019. Therm-a-Rest also notes that R-values are additive: an R 2.0 closed-cell foam pad like the Therm-a-Rest Z Lite Sol stacked under an R 3 inflatable gives a combined R 5. This matters on cold trips where weight limits what you can carry: a thin closed-cell foam pad used as a base layer under your inflatable both adds R-value and protects the inflatable from punctures.

Want a single number for your setup? The sleeping bag warmth calculator combines your bag rating, pad R-value, and clothing into one estimated low.

Eat before you sleep

Your sleeping bag only traps heat; your body has to make it. Digesting food produces some heat, and a snack before bed means you are not starting the coldest hours of the night on empty after a long day outside.

Going to bed hungry and underfed after a long day is a common reason campers wake up cold in the small hours, when temperatures hit their overnight low. The bag and pad have not changed. The heat source has.

Your bag cannot create heat. Your body can, so give it something to work with before bed.

Practical choices: a handful of nuts, a piece of dark chocolate with peanut butter, or a small portion of the calorie-dense camp food you already carry. This does not need to be a full meal. It needs to be enough to keep digestion ticking through the night.

Layering and what to wear inside the bag

The goal inside the bag is dry insulation against dry skin. Moisture on skin accelerates heat loss. Cotton holds moisture and should be left at home.

Sleep in a clean, dry base layer of merino wool (like the Smartwool Classic All-Season Merino Long Sleeve) or moisture-wicking synthetic. If you wore your base layer all day while sweating, change before bed. Damp insulation performs poorly whether it is in your sleeping bag or on your body.

Cover your head. The head and neck are a major convective heat-loss surface. A light merino balaclava or the hood of your sleeping bag cinched down makes a meaningful difference at the lower end of your bag's rated range.

Moisture, condensation, and vapor barriers

Moisture is the part of the warmth system that compounds quietly across a trip. Your body gives off water vapor all night through breathing and skin. In a sleeping bag, that vapor migrates outward through insulation toward the cold shell, where some of it condenses.

Down insulation is particularly sensitive to moisture. Even a small amount of condensation compresses fill, reduces loft, and cuts warmth. Claims of "condensation-proof breathable shells" are marketing hyperbole: condensation forms regardless of shell fabric, and airing the bag is the only reliable way to restore loft on multi-night trips.

The morning routine: open your bag inside-out and drape it in sunlight or on top of your shelter for 20–30 minutes. This evaporates trapped moisture before it compounds on night two.

Ground insulation: the underestimated variable

Ground robs heat faster than still air at the same temperature. This is why R-value matters more than most people expect on their first cold-night camp.

A thin closed-cell foam pad on bare ground outperforms no pad even when placed under a high-R inflatable, because it adds insulation and protects the inflatable from puncture or slow deflation. Deflated sleeping pads provide almost no insulation regardless of their rated R-value.

If you camp on snow or cold rock regularly, treat R 5 as a floor, not a premium; that is the bottom of Therm-a-Rest's winter range. Stack pads rather than carry one impossibly light option.

Never heat the tent with a flame

As nights turn cold, it is tempting to run a propane heater or a stove in the tent or vestibule. Do not. Fuel-burning heaters produce carbon monoxide, which has no smell, and patio and camp heater manuals are blunt about it: never use the appliance in an enclosed space such as a camper, tent, car, or home. The CDC's carbon monoxide guidance likewise says never to use a portable gas camp stove indoors. Get warm with insulation instead: a higher-R pad, a warmer bag or quilt, a tightly sealed bottle of hot water inside the bag, and dry layers.

If you camp near your car and have power, electric options such as a 12V heated pad are covered in our guide to keeping an air mattress warm.

FAQ

Where these numbers come from

The R 4.8 figure is our conversion of the ISO test pad's metric value (0.85 x 5.678). It is approximate because the ISO and ASTM tests differ.

For specific bag picks at each temperature rating, see our guide to the best sleeping bags for camping.

Browse all camp gear guides, or read how we research and rate the products we cover.