Condensation in the cabin

[Dr. Peter]

Went sailing for five days in Autumn and each morning we had condensation. The first morning was bad because we were on the trailer, it was raining hard and we did not have the boom tent up and we were closed up tight.

The last night was much better. We were under the boom tent and allowed plenty of airflow through the cabin.

The thing is everything gets wet especially under matresses.

I have slept in boats with proper lining of the cabin roof and walls and there is no problem but this would be a big or expensive project for me.

What are the options? What have you done to reduce condensation in the cabin generally and especially under the matresses.

Peter

[CatBuilder]

The same thing that worked for condensation for the whole boat is what you need to do for under the mattress - ventilation.

Condensation is formed when hot, moist air encounters something that is below the dewpoint temperature. This causes the moisture of the hot air to condense onto the cooler object.

So, there are really only a few ways to take care of condensation:

1) Reduce moisture. This could mean a dehumidifier or stopping breathing, but both sound impractical on your boat. Avoid cooking pasta, boiling water or taking hot showers in the boat. No steam.

2) Keep everything warm. This is the insulation idea you had been thinking about. Would work for areas that are insulated, but would not work under the bunk or in other areas that have cold things (like metal or fiberglass in contact with cold water) that are in warm, hot air.

3) Ventilation. This is your main hope. Make sure you have adequate vents in the boat, in general. The boom tent was an example of good ventilation. Putting in dorade vents or the little solar automatic vents would be a good idea. The more the merrier, until your condensation problems stop. For under the mattress, it's the same idea. Put something under the mattress to keep it up off the cooler base it sits on.

Use something like this, or a cheaper item that works the same way, lifting the mattress up off the surface to allow more ventilation.

[Poida]

Hi Doc

One of the problems with forums such as this is, I don't know what you know, so, I may try and explain things you already know, so please do not take offence if I seem to state the bloody obvious.

Warm air holds more moisture than cool air so your cabin heats up during the day with a bit of sunlight. Victoria not a lot, but enough to warm up the cabin.

Warm air is also lighter than cool air so the warmest air (that holds the most moisture) in the cabin will be under the cabin roof.

When the rain hits the outside of the roof it will immediately cool the air just under the roof. You haven't said what material the cabin is made of as some materials are worse than others due to the different insulating qualities.

As the air under the roof cools it cannot hold the moisture it once did and the water condenses out of the air as droplets.

The air under the roof now becomes the coolest air in the cabin and sinks to the bottom of the cabin (a convection current) = moisture under the matress.

The best plan of attack is insulate the underside of the top of the cabin roof.
A thin layer of polystyrene or if that is not asthetically pleasing sandwich some poly between the roof and some ply.

All the best
Poida

[viking north]

One of the worst culprits is alchcol burning items, stove, heater, especially if the cabin is closed to ventalation.---Geo.

[Dr. Peter]

It sounds to me like a combination of these ideas will work. Has anyone done anything clever with a boom tent? I just use a quality plastic one. I was thinking could this be lined? Would it help?
Peter

[Pericles]

Bubble wrap.

[Dr. Peter]

Bubble wrap?

[Poida]

Pericles appears to be a man of very few words.

I would assume he means laying the bubble wrap directly on to the cabin as it has an insulating air pocket in it.

Not a bad idea, as roll of it would be cheap.

[CDK]

Originally Posted by Dr. Peter

The thing is everything gets wet especially under matresses.

A joke about incontinence would be the obvious thing here, but it is (A) your body evaporating approx. 600 ml of water above (B) a cold surface and (C) near 100% rel. humidity in the air (dew point).

There is nothing you can do about A, for B you can use self-adhesive foam, used in recreational vehicles and C. can be avoided by forced ventilation if the outside air is less humid.
As an alternative you could buy a dehumidifier which is just a plastic container with a perforated lid, filled with hygroscopic salt.

[Pericles]

I shall let you all into a litle secret. On an old 18 footer, I replaced the foam mattesses with long and capacious heavy cotton bags filled with polystyrene beads and placed them on canvas pipe berths instead the standard plywood berths that were then rebuilt for use as as easily accessed storage. Snuggling into my sleeping bag on the beards enabled me to lie on an even keel, as if gimballed in place and feel very secure with the boat heeled.

Air flow beneath the pipe berth and carrying the bead bags on deck to air to and lounge upon, kept mildew at bay. If conditions are really awful below, the waterproof bubble wrap keeps the bedding drier and adds extra layers of warmth.

Although polystyrene beads resist water absorbtion, the canvas bags I used could also be replaced with waterproof material.

http://www.beanbagrefill.com/

P

Addendum. Pipe berth can also be constructed with fishing net rather than canvas.

[hoytedow]

In the age of sail, salt dehumidifiers were developed to protect cargo. The old-fashioned salt dehumidifier consisted of a bag of salt suspended over a collection basin which was periodically emptied and the salt itself would have to be periodically replaced as it dissolved into the condensation.

[Pericles]

Which is why washing clothes in seawater seems to leave them forever damp as the salt is hygroscopic and attracts water. Diesel fuel is also hygroscopic.

[Submarine Tom]

In a word: Ventilation.

-Tom

[viking north]

Ventilation does help but as long as warm moist air is generated within the structure and it has a cold surface to condense on, paddle or no paddle you're still up the creek. In an uninsulated structure there's nothing like a small woodstove to keep things dry. ---Geo.

[CatBuilder]

I agree fully with Geo, even though I'm thinking the OP's boat isn't really large enough for a wood stove.

I have had these on my last couple boats, including my last catamaran:



The 26,000 Btu's the Little Cod puts out will heat a 45' monohull or 35' catamaran. Fantastic little stove. I'd know. I've bought two of them! ha ha ha

But what the wood stove does in addition to providing dry heat is provide incredible ventilation. Every bit of smoke you see coming out that chimney is air that is being drawn out of your boat. That air is constantly being replaced by fresh, outdoor air at the same rate it is going up the chimney. This makes for a wonderfully warm and dry boat, due to heat and high volume forced ventilation. I've lived aboard in Maine, full time over the winter for a few years with this stove as the only source of heat.

Here is a video of my winter home:

http://www.youtube.com/watch?v=S21gnXpaVc8