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Advice on the preventing condensation inside the boat

Oct 28, 2013
Pierson 303 Lake Lanier
Need some advice on the best way to prevent condensation inside the boat, which mainly occurs during the winter. Already have one Marinco solar exhaust fan. How effective are space heaters?
Apr 8, 2010
Ericson Yachts Olson 34 Portland OR
Prevention would call for some solid insulation on the hull below the waterline. That is, BTW, one of the few advantages of a cored hull.
Solution is, next to get the condensed-out moisture back into the air, and then out of the boat. If the air is dry enough it will take up the moisture (which, after all, is where the moisture came from in the first place).
I find that having air exchange with the outside plus having a way for inside air to circulate against the hull surface does a good job. We have two hatches with 'vent' positions, and two aeolian ventilators (Beckson Vent-O-Mate). And..... at least every other year we wipe down the inside hull with a bleach/soap solution, and dry with cotton rag towels.
We take the cushions home every fall, and have all the access plates on seats and berths open, too.
Or "cheat" and have a dehumidifier running all the time. They work by drying out the air so much that there's little moisture to condense out. You do have to empty the collection tank (about 2 gallons give or take) every week.

Note that all a heater does is allow the warmed air to hold more moisture, but the water is still there....
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Jan 20, 2005
Nauticat 321 pilothouse 32 Erie PA
for decades, we've we stored several 30-33 ft boats on the hard on the frigid shores of lake erie from oct to early may. we never had mold or mildew. boats were covered w a custom boattop winter cover; all portholes, doors and hatches were closed tight. we found that putting 3 or 4 open 1/2 quart containers of Damp Rid desiccant prevented mold or mildew. 1 in v berth, 1 in galley, 1 in aft cabin. damp rid prevented any moisture damage. of course we first removed all soft goods like towels, curtains etc .. we stored the dinette and berth cushions vertically to maximize evaporation of moisture and air circulation. it was not necessary to change out the desiccant containers mid season but it would be good if you could do so.( put the desiccant containers on cookie trays or in metal pans in case the container overflows. )
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Jan 19, 2010
Hunter 26 Charleston
Note that all a heater does is allow the warmed air to hold more moisture, but the water is still there....
I don't think that is the entire story on heated air. Heating the air lowers the relative humidity allowing condensed water to evaporate more easily. What you said makes sense if you are not exchanging any air.
May 24, 2004
CC 30 South Florida
Space Heaters cause thousands of fires each year. If you have access to shorepower, get yourself an electric dehumidifier, they are safer and will run more efficiently. Oh, and run a hose to bilge where the condensation can be discharged.
Dec 25, 2000
Hunter Passage 42 Shelter Bay, WA
No live aboard here, so this is what I do. Our boat has three of these, one for each cabin. In addition a marine grade space heater keeps the boat from freezing during those cold snaps. Keeping a hatch cracked lets moist air escape. This has kept our boat mold free and pretty dry.

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Jul 25, 2013
Macgregor 26S near Vancouver, BC
I run a dehumidifier full time, and light bulb, in my boat all winter long. It works great. But that's a boat on a trailer. I don't know how well it would work with the boat sitting on water.
Jan 7, 2014
Beneteau 45F5 51551 Port Jefferson
I tried the dehumidifier last season but they will not work if the air temp is below 65 degrees. I leave all cabinets and draws ajar. I have a canvas winter cover, this season I left the companionway slat open to let a little more air to circulate. That does not work well though when you have a sudden warm spell. The warm humid air condenses on the cold surfaces. But the boat breathes well and it dries quickly when the cold air returns and the temperatures even out.
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Oct 22, 2014
CAL 35 Cruiser Portland OR, moored EVERETT WA
The ideas shared are all spot on. One source of moisture are leaks. Be sure to fix all leaks to limit the water from entering the boat. Keep the bilge as dry as possible. Ventilation is your helper. More often the air if cold outside the boat is drier then inside (we humans put a lot of moisture into the spaces we occupy).

If you are finding the bed gets moist where you sleep, add a ventilation pad under the mattress/cushions. Something like a “Hypervent pad” can improve your sleep experience.
Aire-Flow Hypervent Mattress Condensation, Mold, & Mildew


Apr 3, 2019
Macgregor 26M Mobile AL
If you are on lake Lanier, your issues are likely the ambient temperature/humidity swings, as noted by @TimFromLI. You can heat the boat up to about 75 degrees (maintain all surfaces well above highest seasonal ambient dew point), or seal it up tight and run a dehumidifier with about 50 pint per day capacity. You will need to connect a drain hose to the unit to drain the condensate outside of the boat. In your area, the unit reservoir would need to be emptied multiple times per day until you get the boat dry, and possibly for the entire season (depending on infiltration) - making the reservoir impractical. Mechanical dehumidification will only work if you can seal the boat to largely prevent ambient air from getting in. You may have to run a heater in addition, but I would try the dehumidifier alone first. Due to inefficiencies, these units put off a small net heating capacity.

If your boat is in the water, the inner hull surfaces may be essentially at the water temperature. So, you will need to reduce the cabin air dew point to be below the water temperature to prevent condensation on the hull (assuming no core). This will give you some idea for dehumidifier specs. Get one that can maintain a dew point at or below the water temperature.

As @jssailem states above, the liquid water in the boat will negate efforts to prevent condensation as this water is continuously evaporating into the bilge air, keeping it saturated. This will work it's way into the cabin.

If you were in a more northern climate, or west in a drier climate, heat and ventilation would take care of it. Where you are, mechanical dehumidification, and making the cabin as air tight as you can, will likely be the only way to take care of it. Obviously, you should shut down your propane system, remove flammable liquids, take measures to prevent flammable vapor build-up, prior to sealing up the boat. It doesn't have to be totally airtight, but any cracks wider than about 1/16" should be corrected. Open vents, louvered openings, etc. could render the dehumidifier ineffective.

Good luck.
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Dec 31, 2016
Beneteau Oceanis 351 Charlottetown
I've used decadent moisture absorber the last 4 years placed in red solo cups, works great, in the spring never had mold or any moisture problems at all.
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Oct 28, 2013
Pierson 303 Lake Lanier
Many thanks for your thoughtful responses!
Thinking next winter I will combine a small space heater and a dehumidifier set up to drain into the bilge.
The bilge stays relatively dry by virtue of being controlled by a float switch.
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Jan 24, 2017
Hunter 34 Red Bank NJ
space heater bad idea plus some yards discourage electical cords to be plugged in during winter lay up.

1st dry out bilge and sump areas example showers

2nd moisture bags hang in forward, main, and aft cabins.

3rd get a five gallon bucket, brick, colander, aprproxemetly 10lbs of calcium "note the same calcium used in water softener or swimming pools"

Place the brick in the bucket, place the colander on top of the brick, fill the colander with the calcium pellets. The calcium will draw the moisture from the air and it will be deposited into the bottom the bucket.
Note the best place to put the bucket is in the main cabin.

4th place three alluminium trays filled with aprproxemetly 1 inch of scented cat litter. In all cabins
This will also help absorb moisture and boat odors.

I've been doing this for 30 plus years and never had any moisture issues.

If possible if you can rig up a solar powered fan to help air circulation within the cabin also helps.


Apr 3, 2019
Macgregor 26M Mobile AL
Need some advice on the best way to prevent condensation inside the boat, which mainly occurs during the winter. Already have one Marinco solar exhaust fan. How effective are space heaters?
I'm sorry if I missed it, but is the boat in the water over winter, or on the hard?


Jan 24, 2010
Catalina 310 Ocean City, NJ
I also use desiccant.. i pick up several bags of damp-rid and hang them in all the compartments..

They usually last the entire winter.