How to stay warm in a tent on a cold night

A sleeping pad’s warmth is its R-value: the higher the number, the more it resists heat loss into the ground. Therm-a-Rest describes how pad makers began building a shared test, ASTM F3340, in 2016 so that R-values from different brands could be compared directly. Look for “ASTM” next to the number when you compare pads.

R-values add up when you stack pads. Therm-a-Rest says layering its Z Lite SOL foam pad under another pad adds R-2.0 and protects the top pad from punctures, which is the cheapest way to warm up a summer setup for a cold trip.

No official chart turns an R-value into a safe temperature. Sea to Summit, which tests its mats to the ASTM standard, explains why it does not publish one: the ground can be much colder than the forecast air temperature, damp ground pulls heat faster than dry ground, and air moving inside some inflatable pads carries warmth away in a way the static lab test does not capture.

Its rough guide is to plan by frost instead of by the forecast number.

If you were cold from below on your last trip, raise the R-value before you buy a warmer bag. Our sleep systems hub compares pads on their official ASTM numbers.

1. Pitch out of the wind. The National Weather Service explains that wind draws heat away from exposed skin faster as it picks up, which is why a breezy night feels much colder than the thermometer says. Use the lee side of trees, terrain or your car.
2. Get the pad right. Inflate it fully, and add a foam pad underneath on frozen or damp ground.
3. Use a bag or quilt rated for the low. A quilt saves weight but relies on the pad under you for warmth; our quilt vs sleeping bag guide covers the trade.
4. Change into dry clothes for sleeping. The CDC notes that hypothermia can set in even above 40°F when you are chilled by rain, sweat or cold water, so keep a dry base layer, socks and a hat for the tent. An insulated jacket worn inside the bag adds warmth too; see our insulated hoodie comparison.
5. Eat and drink before bed. In its report on campers who died after bringing fuel-burning heat into tents, the CDC advised packing enough bedding and clothing and eating extra calories and fluids to prevent hypothermia, instead of relying on heaters.
6. Keep the heat source on the outside. Cook and boil water outdoors, then bring the warm meal or a filled bottle in, never the stove.

• Buying a warmer bag to fix a cold pad. If you were cold from below, the number that measures heat loss into the ground is the pad’s R-value, so raise that first.
• Bringing a stove, grill or propane heater inside. The CDC says opening tent flaps, doors or windows is not enough to stop carbon monoxide building up from these devices, and the U.S. Forest Service says unvented propane heaters should never be used inside a personal tent. Read camp stoves, heaters and carbon monoxide before any cold trip.
• Sleeping in the clothes you sweated in. Damp layers chill you through the night, even well above freezing.
• Drinking alcohol to warm up. The CDC lists alcohol use as a hypothermia risk factor and says not to give alcoholic drinks to someone who is cold.
• Trusting the forecast air temperature for the ground. Frost forms because the ground gets colder than the air.

The CDC lists these warning signs in adults: shivering, exhaustion or feeling very tired, confusion, fumbling hands, memory loss, slurred speech and drowsiness. Because it affects thinking, the person may not notice it is happening. If a person’s temperature is below 95°F, the CDC says to get medical attention immediately.

If medical help is not available right away, the CDC advises:

1. Get the person into a warm shelter.
2. Remove wet clothing.
3. Warm the center of the body first (chest, neck, head and groin), using skin-to-skin contact under loose, dry layers of blankets or sleeping bags.
4. Give warm drinks only if the person is conscious, and never alcohol.
5. Once body temperature rises, keep the person dry and wrapped, including the head and neck, and get medical care as soon as possible.

For severe hypothermia, when a person may be unconscious and seem to have no pulse or breathing, the CDC says to handle them gently, get emergency help, and start CPR while you keep warming them.