How to camp in sub-zero cold

Sub-zero camping goes wrong in the planning, not on the mountain. The single most common mistake is packing for how cold it felt at 2 p.m. in direct sun, when the temperature that actually matters is the one at 4 a.m. after the sun has dropped behind a ridge. At Kaza, the winter base of the Spiti valley at roughly 3,650 m, a clear February night routinely falls to −20 °C and on the colder nights past −30 °C, before you add the katabatic wind that pours down off the ridgelines after dark and drives the felt temperature lower still.

Treat the overnight low as your design temperature and build the whole kit list backwards from it. If the realistic low is −25 °C, every link in the chain — sleeping bag rating, mattress insulation, heater output, fuel type, battery capacity — has to survive −25 °C with margin, because the system is only as warm as its weakest part. A −10 °C sleeping bag in −25 °C air is not a comfortable night, it is a cold-injury risk. Plan high, and a mild night is a pleasant surprise rather than a near miss.

Heat leaves a tent four ways — conduction into the ground, convection through gaps and thin fabric, radiation to the cold sky, and the moist warm air you breathe out — and ground conduction is the one beginners underestimate most. A cold floor will strip body heat faster than cold air ever does, which is why the layer under you matters more than the layer over you. In a rooftop tent the base itself helps: the CampTop 300Lux is built on a 300 kg aluminium honeycomb base with an integrated all-season thermal-control mattress that is engineered to stay warm in sub-zero conditions, and it ships with dual heater ports specifically so you can feed warm air in from a portable heater.

Manage the fabric and the openings deliberately. A double-skin or insulated shell holds far more heat than a thin single wall, and any unused opening is a heat leak — but here is the tension that catches people out: you cannot seal the tent completely, because trapped moisture turns into a frozen mess (Step 4). The goal is controlled ventilation, not a sealed box: keep a small high vent open for through-flow while closing the large panels you are not using. If you are still choosing a tent for cold work, our rooftop-tent range and the CampTop 300Lux are specced for exactly this — insulation, heater ports and ventilation designed together rather than bolted on.

Insulation only slows heat loss; in genuine sub-zero cold you also have to put heat back in, and the safe way to do that inside a tent is a diesel air heater with sealed combustion. 'Sealed combustion' is the load-bearing phrase: the unit burns fuel in a fully enclosed chamber and ducts the exhaust outside the tent, so only clean warm air enters the living space. The ThermaEvo AH5 is built on exactly this principle — a fully enclosed combustion system, a smart digital controller for precise temperature and timer control, and low-fuel and overheat protection so it can run unattended through the night without you waking to babysit it.

Altitude is the spec that separates a heater that works in the Himalaya from one that chokes. As you climb, the air thins and a heater tuned for sea level runs rich, sooting up and losing output exactly when you need it most — at 4,000 m on the coldest night of the trip. The ThermaEvo AH5 carries an altitude-compensation system that automatically re-calibrates the fuel-air mixture up to 5,000 m, which covers the entire driveable Spiti and Ladakh plateau. Size the heater to the space, mount it so the warm-air duct enters low and the intake draws clean air, and feed it from winter-grade diesel so the fuel itself does not gel in the line.

This step is non-negotiable and we will not soften it: never run an unvented flame inside a closed tent or vehicle. No petrol stove, no unsealed gas heater, no charcoal brazier, no improvised burner. Carbon monoxide is colourless, odourless and lethal, and a closed winter tent is the exact scenario in which it kills campers every season — people fall asleep warm and do not wake. A sealed-combustion diesel heater that vents outside (Step 3) is safe because its exhaust never enters the living space; an open flame inside a sealed tent is not, no matter how cold it is. Even with a properly vented heater, keep a vent cracked for fresh air as a second line of defence.

The same airflow that keeps you safe also keeps you dry. A sleeping adult breathes out a surprising volume of water vapour overnight, and in a sealed sub-zero tent that vapour freezes on the cold inner skin and then sheds down as a fine ice rain at first light, soaking your bag exactly when you least want it wet. The cure feels counter-intuitive — ventilate even though it is cold — but a small, constant through-flow carries the moisture out before it can freeze. Pair that with an all-season insulated mattress and keep your mouth outside the bag rather than breathing into the insulation, and you wake to a dry tent instead of a frozen one.

A heater fails, runs out of fuel, or simply gets turned off at 3 a.m. — so your sleeping system, not the heater, is what actually keeps you alive in the cold, and it has to work on its own. Build it in two halves. Under you, an insulated mattress with real cold-rated padding stops ground conduction; the CampTop 300Lux's integrated thermal-control mattress is designed for this, but on any setup the mat matters as much as the bag. Over you, a sleeping bag rated below your design temperature — if the night is −25 °C, a bag comfort-rated to −25 °C or colder, not a −10 °C three-season bag with a hopeful liner.

Loft is warmth: insulation works by trapping still air, so anything that crushes it — sleeping in bulky outer layers, an over-stuffed bag, damp down — destroys the very thing keeping you warm. Sleep in clean, dry base layers and a warm hat, never in the clothes you sweated through that day, because trapped moisture conducts heat straight out of you. Two field habits make the biggest difference at altitude: eat a fatty snack before bed so your body has fuel to burn through the night, and bring tomorrow's clothes and boot liners into the foot of the bag so they are warm and pliable at dawn instead of frozen solid. None of this is exotic; it is just done deliberately, every night.

The trip is not over when you survive the night — it is over when you drive home, and the cold that froze your tent also froze your fuel and flattened your battery. Below roughly −25 °C an untreated diesel engine simply will not start: the fuel waxes in the lines and the battery has nothing left to give after a night in the cold. Pouring hot water over a fuel filter at first light is a miserable, time-eating ritual, and in a remote camp with no recovery services it can strand you. A ThermaEvo WH5 engine pre-heating system is built for precisely this, rated to wake diesel vehicles in −35 °C to −40 °C and engineered with BS6/EURO6 DPF support so the cold start stays clean rather than dumping soot.

Protect the vehicle the way you protect yourself. Run winter-grade or additive-treated diesel and keep the tank above half so condensation has nowhere to freeze in the lines. Carry a second fully-charged battery or a lithium jump pack and keep it warm inside your sleeping bag overnight, where it holds its charge instead of draining in the cold. Park nose-out on hard ground so you are not digging out a frozen rut in the morning. A warm camp and a dead engine is still a failed trip; planning the start is as much a part of sub-zero camping as planning the sleep.