Walk into a closed-up boat in February and you can smell the problem before you see it. That damp, slightly sweet note of wet upholstery and cold GRP. Behind it: black spots on the headliner, rust blooms on chain plates, greening around portlight frames, and lockers that feel colder inside than the pontoon outside. Winter condensation is not a cosmetic issue. Left alone for four or five months, it degrades foam, wiring, plywood cores and electronics faster than a full season of sailing.

The physics is boring and unforgiving. Warm, moist air meets a cold surface, water drops out. Your boat in a marina is a large, cold, poorly ventilated cavity full of hygroscopic material. Here is how to keep it dry without babysitting it every weekend.

Why boats sweat more than houses

A house has cavity walls, insulation, and a heating system that runs on and off all winter. A boat has a single skin of GRP or aluminium in contact with water at roughly 6 to 12 °C, no thermal break, and internal air that starts every day near dew point. Every source of moisture inside, bilge water, damp fenders, wet oilies, a half-empty holding tank, even the wood in your galley, tries to evaporate. When that vapour hits the inside of the hull, the coachroof, or a stainless fitting, it condenses.

Three factors decide how bad it gets:

  • Air exchange. A sealed boat traps moisture. A ventilated boat lets it out.
  • Temperature differential. The colder the hull relative to the internal air, the more condensation.
  • Standing water sources. Bilge, tanks, damp cushions, wet ropes stowed below.

You can influence all three, and you should. Treating only one, cranking a dehumidifier while ignoring the bilge, for example, wastes energy and still leaves you with mould.

Ventilation first, always

Before you plug anything in, get air moving. A boat that breathes rarely grows mould, even without heat. A boat that is sealed grows mould even with a dehumidifier running, because you have created a warm, wet, unventilated box.

Practical steps for the winter lay-up:

  • Leave every internal locker, floorboard, cupboard and drawer open. Cabin sole hatches propped up. Fridge lid wedged open. Heads locker door off its latch. Air has to reach the cold surfaces behind them.
  • Fit at least two solar or dorade vents. One low intake, one high extract, ideally at opposite ends of the boat, so you get a slow convection loop rather than a dead pocket.
  • Take off the mainsail cover and sprayhood if they are damp. They rot in place and lock moisture against the coachroof.
  • Remove cushions from berths, stand them on edge, or better, take them home. Foam is a sponge for airborne moisture and holds it against the plywood underneath.

If you are one of those owners who keeps the boat in commission through winter, ventilation is even more important because you are opening and closing companionways with damp clothes on. That is a separate maintenance regime, and we cover it specifically in what to do when your boat stays afloat all year round.

Kill the moisture sources you can control

Most owners underestimate how much water is already inside the boat when they close it up. Before the first cold snap:

  1. Dry the bilge completely. Pump it, then sponge the last cupful out. A wet bilge will feed the internal atmosphere for months. If the bilge fills faster than you expect, find the leak now, not in April.
  2. Empty and rinse the fresh water tank, or fill it fully and dose it. A half-empty tank breathes warm damp air in and out through its vent every temperature cycle.
  3. Empty the holding tank and flush it. The gases coming off it are corrosive as well as unpleasant.
  4. Fuel tank: full, and treated. A full tank has almost no air space above the diesel, which means no condensation on the tank walls and no water forming at the bottom. Water in diesel is exactly how diesel bug takes hold over winter, so this is not optional if you want a clean fuel system in spring.
  5. Fridge and freezer: drained, wiped, and propped open.
  6. Wet lines and fenders off the boat. Store dry, or at least off the deck.

Do these six things and the rest of the fight gets much easier. Skip them and no dehumidifier in the world will keep up.

Heat, dehumidifiers and desiccants

Once the boat is ventilated and the standing water is gone, you have three tools left: gentle heat, active dehumidification, and passive desiccants. Each has a role.

Gentle heat means a low-wattage tube heater or a frost-protection heater, typically 60 to 120 W, set to hold the cabin a few degrees above outside ambient. The point is not comfort, it is to keep the internal surfaces warmer than the dew point of the internal air. Two low heaters, one forward, one aft, work better than a single stronger one, because you avoid a hot patch and cold corners. Never leave a fan heater running unattended. Check what your marina and insurer allow: many now require RCD protection, thermal cut-outs, and specific heater types.

Electric dehumidifiers pull water out of the air and dump it in a tank you have to empty, or through a drain hose into the sink. On a boat, choose a compressor model that can drain continuously, and route the hose to a through-hull that is above the waterline. Set humidity to around 55 to 60 %. Lower is a waste of electricity and dries out timber. Compressor models struggle below 10 °C, so pair them with a small amount of heat.

Desiccants (silica, calcium chloride tubs, hanging bags) are useful in dead spaces: chart table, engine bay, hanging locker, cutlery drawer, chain locker. They are not a whole-boat solution. Refresh or replace them every four to six weeks.

If you have lithium house batteries, be aware that they do not like being charged below 0 °C. Keeping the cabin above freezing is not just about condensation, it protects the pack. Worth reading up on how to look after lithium banks over winter before you leave the boat alone for three months.

The places mould actually starts

Owners often treat the saloon and forget the hotspots. Mould, in practice, tends to appear first in the same handful of places on almost every boat:

  • Behind and under cushions. The plywood berth base is cold, the foam is warm from the last time you sat on it. Take cushions off, or stand them vertically on slatted supports.
  • Inside the hanging locker. Damp oilies with a closed door: guaranteed. Empty it.
  • Chart table and instrument panel. Electronics generate a tiny amount of heat and cool down at night, cycling condensation across pins and connectors.
  • Chain locker. A wet anchor and 60 metres of chain will pour moisture into the forepeak all winter. Rinse the chain, let it dry ashore if you can, and leave the locker vented.
  • Under the engine. A cold engine block is a giant condensation surface. If you can, run the engine to temperature before winterising, then leave the engine bay open. Winterising the engine correctly matters here, and if you are on an outboard, the off-season storage steps are specific.
  • Textiles. Curtains, headliners, upholstery piping. This is where mould shows up cosmetically first. Cleaning and protecting them properly extends their life by years: see the routine for marine upholstery.

A quick mould inhibitor spray on suspect surfaces in November costs almost nothing and buys you a clean spring. Anything that already has visible growth: clean it with a diluted anti-mould product, do not just wipe it dry, because the spores stay.

Checking on the boat without being there

The hardest part of winter lay-up is not knowing what is happening on board. A heater trips on Christmas Eve. The shore power drops out for two days in January. The bilge pump cycles every twenty minutes because a stern gland is weeping. You find out in March.

The owners who come back to a dry boat in spring have one thing in common: they check. Either they visit weekly, or they have someone on the pontoon who does, or they have a way of seeing what the boat is doing remotely. Battery voltage, shore power state, bilge pump activity, internal temperature: these four numbers tell you almost everything about whether the boat is winning or losing the condensation fight this week.

This is where the Oria Box earns its keep in winter. It sits on the NMEA network, keeps an eye on battery state, shore power and bilge activity, and pushes an alert to the Oria app when something looks wrong. You still have to do the ventilation, the bilge, the fuel tank and the cushions in November. But you do not have to guess, in February, whether any of it is still working.

The question worth asking now, before the first frost: if the shore power failed tonight, when would you know?