Yachting Monthly reader Mike Watts considers how to keep condensation at bay on your boat over the cold winter months

Reducing condensation on your boat

When we went looking for our first yacht, many boats never made our short list, writes Mike Watts.

The reason?

They smelt damp, mouldy, unloved and uncared for. We certainly did not want our boat to end up like that, particularly as we like to sail outside the summer months.

A moist boat has any number of root causes.

Clearly it will always be operating in a wet environment, but there are many other causes of condensation beyond just that of the sea and spray.

Firstly, the air that surrounds us has moisture in it.

At 80% humidity at 20°C, then 1m³ of air will contain 13.8g of water and for now we will simply take 1g water as 1ml.

We each produce moisture, typically 400ml/day (17ml/hr) in our breath and 400ml/day in perspiration (sailing in the UK).

Burning 100g of butane for cooking produces 155ml of water.

So, the gas cooker may be producing around 200ml per day.

Using a pressure cooker can help reduce steam while cooking

Using a pressure cooker can help reduce steam while cooking

Cooking can produce a large amount of moisture.

Boiling a pan of pasta can easily produce 200-300ml of water vapour.

A typical small diesel heater has an output of up to 140m³/hr and if the air it is pulling in from the outside is at 10°C and 80% humidity then this air would contain 7.5ml of water/m³, so we could have over 1,000ml water entering the saloon per hour.

We all know how much heavier our sailing clothes are wet, so having them in the boat to dry off could easily introduce 100-200g (ml) of water per item.

Let us put those numbers into perspective.

If the internal volume of the yacht, above lockers, is 25 m³, then if the air has a humidity level of 80% at 20°C, then the air will contain around 345ml of water.

Two people breathing when sleeping could add a further 270ml.

By the time the temperature has dropped to 10°C in the morning, that same volume of air can only contain 235ml of water.

So, we have 615ml of water and yet the air can now only hold 235ml in the morning and so 380ml will have condensed out.

What can you do to control condensation onboard?

There are, in essence, three main strategies to reducing the problems of moisture on a yacht:

  • Minimise the amount of water vapour introduced to or present in the yacht
  • Minimise temperature variations
  • Keep surfaces above the relevant dewpoint temperature

Cooking is one easy area where we can minimise condensation.

When boiling water, be sure to do so in kettles or in pans with the lid on.

Using a pressure cooker is also a good option for stews and other recipes with a lot of water in them – remember to turn the heat down when it has reached pressure, so that it does not produce too much steam.

Rice can be cooked in a wide- neck thermos flask: simply heat one part rice and two parts water in a pan until it boils and then pour all the contents into a preheated flask. Leave for 20-25 minutes.

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This not only reduces the amount of steam you produce, but frees up a cooker ring.

Try to consider faster cooking versions of some foods, to minimise the amount of time steam is being generated.

Fresh pasta may not last as long in the cupboards, but cooks much faster and can usually be found in vacuum packed bags (it tastes better too).

Many yachts have air vents or hatches above the cooker but having them opened when it is windy is often not possible as the gas get blown out.

Having the heads door and companionway hatch open may sometimes be an option.

Close all the doors to the bedrooms prior to cooking so that warm and moist air does not reach the colder ends of the boat.

A cockpit tent is key for controlling condensation

Whilst many people see a cockpit tent as a luxury item, to us it is a vital part of minimising condensation in a yacht.

The initial advantage is that no matter which way the wind (and the rain) blows you can have the top hatch open when cooking so the cooking moisture goes out of the saloon.

Secondly, wet clothes and towels can be dried in the tent.

A common weather pattern in the UK is to have a clear sunny morning that steadily clouds up.

A cockpit tend can help keep condensation out of the boat

Leaving fabric items to dry covered on deck keeps the moisture out. Credit: Mike Watts

In the early morning warmth of the cockpit tent, items can have dried before you even get up.

You can also air and dry all the cushions and bedding.

When we are about to leave the boat, we aim to have all the bedding and mattresses in the cockpit tent for a period to air off, before packing the bedding into sealed vacuum bags.

If your yacht is on a mooring, this may be the best method of airing mattresses.

The entry hatch covers are now protected from direct wind currents and rain.

We improve the situation overnight by covering the horizontal and vertical hatches with sheets of cloth or PVC.

This reduces the variations of temperature inside the yacht and condensation on the hatches, and cuts out the early Scottish sunrise.

Aft cabins often have hatches opening into the cockpit and so these can now be left open inside the tent, even when it is raining.

In contrast, many forward cabins have poor ventilation and so closing the door for privacy at night actually worsens the condensation problem.


Putting insulation into the boat needs to be done with care to ensure you are not generating another problem.

We must be careful that warm air is not getting into a cold air space behind insulation resulting in condensation in areas we cannot see.

Moisture in the bedrooms and in the mattresses is a common problem.

Insulation needs care as you may make moisture problems worse

Insulation needs care as you may make moisture problems worse

Increasing the air flow under a mattress seems to us to be approaching the problem from the wrong perspective.

We sail in Scotland where the seawater temperature varies between 8oC and 14oC.

The area under the bed bases will be close to these temperatures.

We have stuck pieces of 25mm PIR insulation board on the underside of the bed bases and hatch covers.

The result; in the aft cabins there is now just one area of significant moisture, on the hatch cover of the batteries, where we had not been able to place any insulation.


There are basically three approaches to living on the yacht.

The ‘outdoor living approach’ means wearing warm clothes, keeping the hatches open so that the internal temperature of the yacht only varies by small amounts to that of the outside.

The ‘cool bedroom approach’ is to open the hatches and cool the yacht cabins down just before you retire for the night.

Finally, there is the ‘comfortable approach’.

We often have a mixed approach to heating if we are in a marina.

Dehumidifiers and electric fan heaters can help keep things dry down below, though a fan heater shouldn't be left on unattended

Dehumidifiers and electric fan heaters can help keep things dry down below, though a fan heater shouldn’t be left on unattended

We may run the diesel heater for an hour or so, as the warmth from the ducting heats much of the structure of the boat, thus reducing the number of cool surfaces on which condensation could occur.

We may then switch to a small electric fan heater.

An additional technique we adopt is to use a dehumidifier when the diesel heater is on and we have power available.

In fact, when it is not too cold, just running the dehumidifier can go a long way to keep the cabin warm.

When we have not been on power on anchor or on a mooring and it has been cool at night, we would aim to run the dehumidifier when we return to our marina at the end of the trip.

Preventing condensation when laying up

We live 250 miles from the boat and leave it ashore for 4-5 months over winter.

We make sure the inside of the yacht is as dry as possible before we leave it.

Whilst we take all the bedding home, we have to leave the mattresses and cushions, but these are always left propped up whenever we leave the yacht.

On the day before we leave the yacht we will run a heater and dehumidifier in each of the cabins to really dry out the mattresses.

We do not stay on the yacht when it is on its cradle and so we leave a heater and dehumidifier running overnight.

The next day, we aim to do very little in the boat and no cooking.

Keeping the boat free from condensation and keeping yourself dry over the winter can be a big ask

Keeping the boat free from condensation and keeping yourself dry over the winter can be a big ask. Credit: Alamy

The yacht is shut up so that there is just the gap between the vertical and horizontal companionway covers that is open to the air.

We leave a greenhouse heater on timer for around 6hrs overnight and a couple of the dehumidifying gel packs near the electronics.

To date, the boat has not felt stale when we have returned to it and the moisture traps have been around half full.

Whilst this approach works for us, yachts left in different environments may need a different approach.

A yacht left in the south east of England could well be subject to much higher variations in temperature over a 24-hour period, so the yacht will tend to ‘breathe’ more.

In areas where mist or fog occurs regularly, then the air moving into the yacht at night may already be supersaturated.

So, under these conditions, sealing a yacht and using a decent desiccant humidifier may be the best solution.

This could also be the most desirable solution in tropical areas where the warm moist air is so conducive to the formation of mould.

Relying on just ventilating the yacht is not the ideal solution and if that is the only option then any soft furnishings, books and anything else absorbent, needs to be taken off the yacht.

So, if you want your yacht to remain smelling fresh, you and your crew need to follow certain strategies throughout the year.

Controlling condensation: what’s hot, what’s not

The product you will need for a dry boat


A dehumidifier on board can help keep condensation at bay

Set you dehumidifier to drain into the sink, but don’t forget to leave the seacock open


The traditional form of dehumidifier uses a compressor to create a cold surface inside a box.

The warm moist air is drawn over this surface and as it makes contact, the moisture condenses into liquid water – in exactly the same way as it might on a cold beer glass on a summer’s day or on a cold mirror in a warm shower room.

This moisture is then collected in a reservoir and the dry air is pushed back out into the room.


A more modern method desiccant dehumidifier uses a slowly rotating wheel of desiccant (usually Zeolite or Silica) to absorb the water.

The dry air is then free to exit the machine.

Meanwhile, at the other side of the rotating wheel, the desiccant is regenerated by a small heater, which draws the moisture out of the material and ejects it either into a reservoir or via a pipe.



A diesel heater

These days, the top-selling brands use a remote diesel burner to heat fresh air via a heat exchanger and uses a fan to circulate that hot air around the boat.

This has the advantage of running off the same tank as your main engine and uses a fuel which does not ignite easily.


Propane heater

Propane or Butane heaters also blow hot air around the boat, but run from the boat’s cooking gas supply.

Some can also be hooked up to 230V power to run off the shore supply if available.

Tube heaters

For use on shore power and to keep a steady temperature on the boat this reduces condensation and protects against frost damage.

Tube heaters range from 40W, so draw the same power as a single lightbulb and can be set on a thermostat

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