Car Camping in Winter Without a Heater
A parked car holds almost no heat, so sleep as you would in a tent: a pad with R-value 5 or more, a 0F bag, covered windows, one cracked. Never run the engine for warmth.
You can sleep in a car in winter without any heater, and you should plan to, because the car is not going to help you. A parked car is a thin metal and glass box; in one University of Oregon measurement an unmodified compact car that was 43F inside at 10 pm fell about 4.5 degrees an hour and was within 7 degrees of the 19F outside air by 2 am. So you stay warm exactly the way a winter tent camper does: a sleeping pad with an R-value of 5 or higher under you, a sleeping bag whose ISO limit rating is 0F (comfort rating around 14F) for nights in the teens, something covering the glass, dry clothes, and one window cracked so your breath does not end up as frost on the ceiling. Never run the engine or a fuel-burning heater for warmth in a closed car. That is not a cautious suggestion; it is how people die in parked vehicles every winter.
Why the engine and fuel heaters are off the table
The CDC counts more than 400 unintentional carbon monoxide deaths a year in the United States, and parked, idling vehicles are a recurring cause. In the 1996 East Coast blizzard, CDC documented 25 poisonings and 2 deaths from people sitting in idling cars whose tailpipes were packed with snow. CDC’s advice is blunt: do not sit in a parked vehicle with the engine running unless a window is open, clear snow from the exhaust before starting the engine, and never rely on an open tailgate, because exhaust gets pulled into the cabin. Falling asleep removes the one safeguard, which is noticing symptoms (headache, nausea, dizziness, confusion). An exhaust leak you do not know about does the same thing with no snow at all.
Portable propane heaters are the other mistake. The Mr. Heater Portable Buddy manual, in capitals, says “NEVER OPERATE THE HEATER WHILE SLEEPING,” requires a 9 square inch vent opening, warns the unit “may give off excessive carbon monoxide” without adequate combustion air, and lists the enclosures it is approved for (cabins, tents, truck caps, vans); passenger cars and SUVs are not on that list. A car’s cabin is a few hundred cubic feet, a propane flame consumes the oxygen in it and adds a lot of water vapor, and an oxygen depletion sensor is a shutdown of last resort, not a sleep aid. The same logic applies to candles, alcohol stoves, and charcoal. Carry a battery CO alarm anyway, placed near your head, because exhaust from the vehicle parked next to you is a real thing at trailheads and rest areas.
Insulation from below is the first dollar you spend
Lying on a folded seat or cargo floor, you lose heat downward by conduction into cold steel and foam, and a sleeping bag’s insulation is crushed flat under you where it cannot help. That is what the pad is for. The usual guidance is an R-value of 1 to 3 for summer, 3 to 5 for three seasons, and 5 or higher for winter, and R-values add when you stack pads, so a cheap closed-cell foam pad under a 3-season air pad gets you most of the way. A 4 inch self-inflating foam pad (R 7 to 8) solves it in one piece and also bridges the step between folded seatbacks and the cargo floor. An air mattress with no foam in it is the worst option in winter; the air inside circulates and carries your heat to the cold side. Pad widths that fit between the wheel wells of common SUVs are in the car camping mattress guide.
What a 0F bag actually means
Bag ratings under ISO 23537 come as three numbers: comfort (a standard woman sleeps relaxed), limit (a standard man sleeps eight hours curled up without waking), and extreme (survival, not sleep). The number in a bag’s name is almost always the limit. Sea to Summit’s Spark 0F, for example, lists a 0F limit and a 14F comfort rating. So a “0F bag” is a comfortable bag for the mid teens and a survivable one for single digits, and if your forecast says 5F you want the comfort number to cover it, not the limit. Synthetic bags are heavier and bulkier than down for the same rating, which matters in a backpack and does not matter at all in a car, which is why a $90 synthetic 0F bag is a sensible car-camping buy. Wear dry long underwear, a hat, and dry socks (the rating tests assume one base layer), and put the clothes you will wear tomorrow inside the bag with you.
Covering the glass
Auto glass is roughly R-1; it is the thinnest part of the box and a large share of its surface, and on a clear night it also radiates your heat to the sky. Reflectix (the foil bubble wrap sold in hardware stores) has an R-value of about 1.1 on its own, and its real contribution is reflecting radiant heat back and stopping air from circulating against the cold glass. Cut panels to each window using newspaper templates, back them with dark fabric if you want to look parked rather than occupied, and hold them with the window trim or magnets. Custom-cut insulated sets exist for popular SUVs, but a $25 roll and an evening with scissors does the same job. The doors and roof are insulated no better than the glass, so hang a blanket between the cabin and the cargo area to shrink the space you are heating with your body.
Condensation and the cracked window
A sleeping adult puts roughly a quarter liter of water into the air over a night through breath and skin, by the National Academies’ figures for respiratory and insensible water loss, and in a sealed car all of it lands on the coldest surfaces as droplets or frost. Crack a window on the lee side an inch, two windows on opposite sides if it is calm; the Oregon measurement found a cracked window cost about 7F of cabin temperature, which your bag covers easily, and a wet bag at dawn costs more. Rain guards (vent visors) let you keep the gap in snow or rain. Keep wet boots and jackets in a bag or in the footwell, not in your sleeping space, and wipe the glass in the morning before it melts onto your bedding.
Gear that is genuinely the answer
Sea to Summit Spark Down 0F
An 850-fill down bag with an ISO limit of 0F and comfort of 14F, 39.3 oz in the regular length, packing to about 10 liters. It is the pick when you also backpack or the car is small; in a vehicle its only downside is price. Check price on Amazon
Kelty Mistral 0
A synthetic 0F bag at 4 lb 6 oz (regular) that stuffs to 12 x 20 inches and costs about $90. Kelty does not publish ISO comfort and limit figures for it, so treat 0F as a limit and expect comfort in the mid teens; for a car it is the sensible first winter bag. Check price on Amazon
Exped MegaMat 10
Four inches of open-cell foam with an ASTM R-value of 8.1, in 72 x 25.6 in (4 lb 9 oz), 77.6 x 25.6 in, and 77.6 x 30.3 in sizes. The foam is why it is warm and why it needs no pump; it self-inflates, and a leak will not drop you onto the floor at 2 am. Check price on Amazon
Kidde COB10 Carbon Monoxide Alarm
A battery-only CO alarm with a sealed 10-year battery and an 85 dB horn, so there is nothing to wire and nothing to forget to charge. CPSC recommends battery-operated CO alarms outside sleeping areas at home; in a car, hang it at head height. Check price on Amazon
When not to do it
Hypothermia is a core temperature below 95F, and the early signs (shivering, clumsiness, sleepiness, confusion) are easy to miss in someone who is already asleep. Alcohol makes it worse by widening blood vessels and dulling the shiver response. If your gear is rated for the teens and the forecast says below zero, if your bedding got wet, or if you are sleeping alone and drinking, pay for a room. Also check the rules: many campgrounds and most trailheads allow sleeping in a vehicle at a paid site, rest areas vary by state, and private lots generally do not. Sedan owners have a smaller, harder-to-insulate space to work with, covered in car camping in a sedan; the sleeved SUV tents in SUV tents for the Ford Bronco family let you keep the hatch open all night behind mesh, which solves ventilation but not warmth.