A parking heater costs four figures. A warm night at minus eight degrees costs, if you know how, about fifty francs' worth of gear and a little understanding of physics. We tried it out last January. Here is what works.
Rule 1: the cold comes from below
The biggest beginner's mistake is the thickest sleeping bag on the thinnest mat. Your body weight squashes every bit of down flat, so underneath you it is not the sleeping bag that insulates but only the mat. A good sleeping mat (check the R-value, from 4 upwards it is winter-proof) does more than any sleeping bag upgrade. The budget trick: a second, simple foam mat underneath. Layers beat thickness.
Rule 2: the windows are your weak spot
Glass provides practically no insulation. Thermal mats for the windscreen and side windows are the most efficient fifty francs you can spend in winter: they keep the warmth in and the ice out in the morning. For the rear windows, sleeping mats cut to size with suction cups will do.
Rule 3: the hot-water bottle trick
A hot-water bottle in the foot of your sleeping bag twenty minutes before you go to sleep, not on your tummy. Warm feet tell your body «all good», and your whole circulation relaxes. The community favourite: fill it straight from the water for your evening tea, which costs zero extra gas.
💡 Safety basics, not up for negotiation: never «heat» with a running gas stove or engine. Carbon monoxide is odourless and deadly. Always keep a window open a crack: two people breathe almost a litre of water into the air every night, and otherwise it ends up as ice on your ceiling.
Rule 4: moisture is the real enemy
Cold and dry makes for good sleep. Cold and damp makes for miserable sleep. So: open a window a finger's width (yes, really), put wet clothes up front in the cab, and air the whole vehicle out in the morning before the sun drives the moisture into the cushions.
Tested at minus eight degrees on a farm pitch in the Goms valley: with layered mats, window mats and a hot-water bottle, it stayed cosy in the sleeping bag all night. And the espresso in the morning, looking out at snow-covered four-thousand-metre peaks, was priceless anyway.
