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Old 12-09-2018, 10:01 AM   #1
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Boat Condensation

Has anyone with a fiberglass boat figured out a good way to prevent the condensation from forming on the inside of the boat in cold weather?
Thanks JD
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Old 12-09-2018, 10:28 AM   #2
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We have two small Caframo heaters set to turn on when temp is down to ~mid forties, and a fairly large dehumidifier, going in our boat in Poulsbo WA through the winter months. When underway summers in BC and SE Alaska in cool weather, a Heatercraft "bus heater" or an Espar diesel heater do the job.
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Old 12-09-2018, 10:36 AM   #3
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We use several Davis Air Dryrs (sic) and a couple of space heaters set quite low. I not only want to keep moisture at bay, but want to keep the boat from getting too cold. This is our first "winter" in the water and, so far, so good.

We picked up the big Air Dryers online for much below retail...its worth looking around if you go that route.
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Old 12-09-2018, 11:48 AM   #4
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It is the reason dehumidifiers are manufactured and sold.
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Old 12-09-2018, 11:54 AM   #5
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Heat and air flow. Helps to minimize those activities that add moisture, like boiling spaghetti water.
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Old 12-09-2018, 12:20 PM   #6
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We use a de-humidifier, but before that...are there any sources of moisture inside the boat that can be adddressed? Wet bilge?



A (relatively) warm, wet bilge can present quite a large water surface area, and all of that is evaporating into your air. Then, that humidity goes looking for the nearest cold surface where it can condense, in this case it will be the inside surfaces of your fiberglass hull that are exposed to cold outside air.
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Old 12-09-2018, 12:29 PM   #7
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The Davis Air-Dryrs are just little electric heaters, they do not actually remove moisture from the boat, rather they raise the dewpoint of the air so that the air can hold more moisture. This approach won't work for Pieyed47 because as soon as that moisture-laden air touches a cold surface it will condense again.
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Old 12-09-2018, 02:36 PM   #8
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The Davis Air-Dryrs are just little electric heaters, they do not actually remove moisture from the boat, rather they raise the dewpoint of the air so that the air can hold more moisture. This approach won't work for Pieyed47 because as soon as that moisture-laden air touches a cold surface it will condense again.
Riveguy describes my problem better than I did. ( Pieyed47 because as soon as that moisture-laden air touches a cold surface it will condense again.) has anyone solved this problem? Thanks JD
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Old 12-09-2018, 02:44 PM   #9
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Riveguy describes my problem better than I did. ( Pieyed47 because as soon as that moisture-laden air touches a cold surface it will condense again.) has anyone solved this problem? Thanks JD
Limit the production of water; as in, don't live on the boat in the winter, dry the bilge, and limit rain/snow entry.

Run dehumidifiers. But, they want mid 40's or greater temperature. They do add heat as well as removing water.

Insulate the hull. Inside or out. That can take a lot of effort.

Ventilate a lot. But, that usually means the inside of the boat is at ambient temperature. Might be a problem!
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Old 12-09-2018, 02:53 PM   #10
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Sounds like I might be better off with a small dehumidifier.
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Old 12-09-2018, 03:23 PM   #11
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Winter liveaboard, comfortably in Toronto since 1994 (some winters in Bahamas)...... Ventilation, insulation.
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Old 12-09-2018, 07:58 PM   #12
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#1) I have never had my girl in Snow.

That said, I do get condensation inside the boat. I was having a real problem with mildew on my sheets along the hull liner. Finally I found a solution.



These were found inside one of those seat cushions that used to be legal in our dinghies. I have found the same stuff (plastic-y foam that bends and is not brittle) atop my solar panels. It is 1/4" thick

I have lined my hull inside all lockers. I also use it under and up the sides where my mattress touched the hull liner. Now, zero mildew through a few years. This stuff works at least in mild weather.

The coldest I've been aboard at anchor was 15 degrees during a cold snap in Georgia. Otherwise, into the 20's in north Florida. Both were pretty miserable -- and I heated with propane at that time (using a catalytic heater)

Anyway, good luck Cap'n JD
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Old 12-09-2018, 09:19 PM   #13
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Consider a good size dehumidifier...like a Danby or similar that has a 30 pint capacity ideally with the feature that lets you run a drain hose into your sink....about $200 and worth every penny.

No mold or condensation, dry bedding and all around comfort.

Also, get a hygrometer (cheap) to monitor humidity levels. research appropriate healthy humidity levels on the web.

Don’t bother with the cheap one pint units. You want the real deal.

Good luck!
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Old 12-09-2018, 10:44 PM   #14
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I run a dehumidifier on mine. Domestic one from the Lowe's home store. About $200, it works wonders. Pees in the aircon sump.
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Old 12-09-2018, 10:49 PM   #15
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Has anyone with a fiberglass boat figured out a good way to prevent the condensation from forming on the inside of the boat in cold weather?
Thanks JD
Buy a real household style dehumidifier.

Set it at 55% and you will never again get condensation.

I have two on my boat and love them. The boat is dry. It feels dry. after sealing up theengine compartment fpr the winter I put one there as well, and thedifference is remarkable!
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Old 12-10-2018, 01:31 AM   #16
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I've run a dehumidifier on the boat in winter storage 24/7 in a wet climate (PNW). No moisture, no mold, everything smells great in the spring.

But I just wonder why - since this problem is pervasive in both the NW and the SE and even most of the E - doesn't someone solve it? Which is why I started this thread. I'm closing in on doing this.
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Old 12-10-2018, 06:42 AM   #17
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No matter what you do , when you dump a gallon of pasta cooking water in the sink, the windows will fog.

The real question is how do you setup so the dripping water doesn't do harm , and condensation not freeze under your mattress?


Heat and fresh air.
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Old 12-10-2018, 09:48 AM   #18
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Old 12-10-2018, 12:13 PM   #19
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You have to pick between the "ventilate thoroughly and heat" and "dehumidify" options. You can't do both: if you are using a dehumidifier, the boat has to be sealed pretty tightly or you'll be trying to dehumidify the greater cruising area. Which works best depends on the boat, the owner, and the local weather. In sunny and low humidity California ventilation works great. In the SE you are running your air conditioner 24/7 anyway so there isn't a problem. In the humid NE and NW, ventilation works to an extent, if you don't mind the interior of the boat being at 50 degrees. Heating works, but not very well - you end up with 72 deg air at 70% humidity. Hard to keep all the exterior cold surfaces from condensing, and the air feels a bit like a tropical jungle without the breeze.

A dehumidifier is the only thing I've found that works well in a cool humid climate. You can cook a big pot of pasta, it will fog the windows near the galley for a short while, then it all gets taken out again. Reducing the humidity from 70% to 55% makes it much more comfortable as well.
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Old 12-10-2018, 02:32 PM   #20
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A small dehumidifier works well in an enclosed area. Have had one 24/7 on my smaller boat and makes a HUGE difference. A lot of AC units have a dehumidifying option.

The Trawler just keeps moving and condensation is not an issue.
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