The first winter I spent aboard in France, in a marina on the Atlantic coast, I made every mistake going. I sealed the boat up tight to keep the warmth in, ran a single fan heater off the pontoon, and by February there was black mould creeping up behind the chart table and water running down the inside of the hull every cold morning. The boat was warm. It was also slowly rotting. Six winters on, I am dry and comfortable, and the difference is almost entirely about managing moisture rather than chasing temperature.
This is the practical version of what I have learned, with the numbers that actually matter when you are paying for it.
Where the water comes from
Two people living aboard breathe out, sweat, cook and shower their way through a surprising amount of water vapour. Estimates vary, but a couple living in a small space can put two to four litres of moisture into the air every day. Add a kettle, a pressure cooker and wet oilskins and you see the scale of it. That vapour does not disappear. It finds the coldest surface it can reach and condenses there, which on a boat means the hull below the waterline, the underside of the deck, the inside of single-glazed windows and any uninsulated locker.
The cold surface that does the damage is the hull. Seawater around a French Atlantic marina sits at roughly 9 to 11 degrees through January and February, and the Mediterranean is not much warmer in the shallows, around 13 degrees. The steel or GRP an inch from that water is cold, your cabin air is warm and humid, and physics does the rest.
So heating alone is not the answer. A warm, sealed, humid boat is worse than a cold ventilated one. You need heat and air movement and insulation working together.
Ventilation first, because it is free
Before you spend a euro on heating, fix the airflow. I run two mushroom vents and a couple of Dorade boxes open all winter, plus I crack a hatch on the leeward side whenever the weather allows. A through-flow of air carries moisture out before it can settle. The counterintuitive part is that letting cold air in keeps the boat drier and, in the long run, cheaper to heat, because dry air heats faster and holds warmth better than damp air.
Small electric dehumidifiers earn their keep too. A compressor unit pulls litres out of the air on a damp day; I have watched mine collect close to two litres overnight. They draw around 200 to 300 watts, which on shore power is trivial. If you are wintering aboard in a marina you almost certainly have a shore connection, and how that whole setup works is worth reading up on in wintering aboard a French marina before your first cold snap.
Heating: what actually works on a boat
I have used four kinds of heat afloat. Here is my honest ranking for a French winter.
Diesel air heating (Webasto, Eberspacher and the Chinese clones) is what most serious liveaboards end up with. A unit blows warm dry air through ducting to the cabins and draws fuel from your tank. A typical 2kW unit burns roughly 0.1 to 0.25 litres of diesel an hour depending on output. With French marine diesel around 1.70 to 1.90 euros a litre in 2025, running it eight hours a day in the cold months costs me somewhere between 1.50 and 3.50 euros a day. The dryness is the real prize: the air comes out warm and with low humidity, which actively helps the condensation problem rather than adding to it.
Reverse-cycle air conditioning that also heats is common on larger boats with generous shore power. It is efficient down to a point but loses effectiveness once the water gets really cold, which is exactly when you need it most in January.
Solid-fuel stoves (a small bulkhead-mounted wood or coal burner) give a wonderful dry radiant heat and a flame to look at on a dark afternoon. The downsides are the space they eat, the fire risk and the chore of carrying fuel down the pontoon.
Electric fan heaters and oil-filled radiators are cheap to buy and run off shore power, but they add no dryness and a fan heater pumps out humid-feeling heat that condenses straight back onto the hull. Mine is now strictly a backup. Watch the marina supply too: many French pontoon bollards are fused at 6, 10 or 16 amps, and a 2kW heater plus a kettle plus a battery charger will trip a 6-amp post without ceremony.
Insulation, the unglamorous fix
The single best thing I did was insulate the hull behind the bunks and lockers. Closed-cell foam, cork sheet or even decent camping mat between the lining and the hull lifts the surface temperature enough that vapour no longer condenses there. I lined the worst lockers with closed-cell foam a few millimetres thick and the difference was immediate. Hatches and windows are the other cold bridge: simple acrylic secondary glazing or even bubble wrap cut to the window shape turns the wettest surface on the boat into one that stays dry.
Lift your mattresses off solid bunk bases too. Trapped air under a mattress against a cold hull is where mildew breeds. A plastic mesh underlay a centimetre or two thick lets air circulate and has saved more than one set of cushions for me.
A daily winter routine that keeps rot away
Mornings, I wipe down the worst window and hull surfaces, open the vents fully for half an hour while the diesel heater runs, and empty the dehumidifier. Cooking, I use a lid on every pan and run the galley extractor or open the companionway. Showering, I do it ashore in the marina block when I can, because a hot shower aboard dumps a litre of vapour into a sealed boat in five minutes. Wet gear goes into a heated locker or hangs over the engine after a run, never draped in the saloon.
It sounds like a lot. In practice it is ten minutes a day and it is the difference between a boat that smells fresh in spring and one that needs the headlining stripped out.
The money side
Budget realistically. Over a full French winter I reckon on the diesel for heating, the marina electricity (often metered separately and billed per kilowatt-hour, frequently in the 0.25 to 0.40 euro range), a dehumidifier or two and the odd replacement filter for the heater. It adds up, but it is a fraction of the cost of repairing water damage. If you are weighing whether a winter afloat makes financial sense at all, the broader picture is in the cost of living aboard in France for a year, and the paperwork side of staying put is covered in living aboard France as a foreigner.
What I would tell my first-winter self
Stop trying to make the boat hot. Make it dry. Ventilate even when it feels wrong to let cold air in, insulate the cold surfaces, choose a heat source that adds warmth without humidity, and do the small daily things that move moisture out. Get that right and a French winter aboard is genuinely lovely: quiet marinas, cheap berths, a warm dry cabin and the satisfaction of being one of the few boats with a light on in January. Get it wrong and you will spend the spring scrubbing mould and wondering why you bothered. The boat will be fine either way. It is you who has to live in it.
Sources: Meteo France and SHOM (winter sea temperatures), French marine fuel price surveys 2025, Webasto and Eberspacher technical data (diesel heater consumption).
