Fighting Winter Condensation

Davidasailor26 · Mar 13, 2026

Rapid heating won’t cause the condensation by itself. The condensation is caused by warm moist air hitting a colder surface. The hatch frames are probably carrying the cold from outside since they’re mostly metal, and your tools are also probably still colder than the cabin air. The extra moisture is probably coming from your respiration when you’re working, and the byproduct of propane combustion. If you can manage to insulate the hatches well enough to keep the frames warmer that could help, but it might be difficult. If you can manage with just the electric heater without propane combustion that might be better. Other than that the best you might be able to do is just get better air circulation so the warmed air can get to the colder metal and carry away some of the moisture.

dlochner · Mar 13, 2026

As @Davidasailor26 says, the problem is too much moisture in the cabin. The source of that moisture is the propane heater. Burning propane in a heater or stove will generate a lot of moisture. Stop using the propane heater and provide a little ventilation in the cabin while working to remove the moisture that you are producing while breathing. Dress warmer and get some hand warmers for your gloves.

thinwater · Mar 13, 2026

Project_Mayhem said:
We're still dipping down to 20-30 degrees in Southern Lake Michigan and I'm still in the cabin working away. I have an electric and a small propane (with CO detector) to make things tolerable. I've noticed drops of water around the plexiglass bomar hatches which I strongly believe is condensation. The water is on the frames of those hatches as well as the frame of a portlight. None of them have a known leak. Another supporting piece of information is the moisture that's visible on my tools in my toolbox. I believe the rapid heating of the cabin (roughly 14.5k BTU between the two heaters) is causing the condensation. I'm thinking some insulation on the outside of the hatches might work but what kind of insulation can withstand the elements?

Desiccant gel isn't doing much and the yard wont let us keep power plugged in while I'm away

Propane = moisture. Also breathing, but that is hard to quit. Try to use only the electric heater. Open some ventilation. Insulate where that makes sense. (double glazing windows is actually pretty easy).
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Better, for your climate, install a proper vented heater without delay. Or you could try this.

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dlochner · Mar 13, 2026

Project_Mayhem said:
The propane heater is a must unfortunately. Even two electric ones cant keep up. It usually takes 2.5 hours to heat up the boat with my current setup.

@thinwater your article that mentions installing plexiglass gave me an idea. I think I'll try taping some plastic over the hatches from the inside before I fire up the heaters next time. An added bonus is that I wont lose any natural light. The Bomars in the article have a much more elegant method of installing screens. My Lewmar portlights has screens that just barely fit with gaps all round. They're also subject to 100% UV exposure since they are on the outside of the boat.

How warm are trying to get the boat? It is just as cold here in Central New York as it is in Chicago. Dressed properly I often work in the boat when the temperature is in the high 30's or low 40s. Is the boat covered? A good fitted cover goes a long way in keeping heat in the boat.

JBP-PA · Mar 13, 2026

How about switching to a portable diesel heater? They vent to the outside.

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JBP-PA · Mar 13, 2026

You could also run a real dehumidifier. They actually add a bit of heat too.

jssailem · Mar 13, 2026

TO combat the moisture, you might try a small dehumidifier in the boat. Not perfect, but it can draw out some of the moisture in the heated space. Attention is needed, as in my experience, the units will freeze up and be unproductive as the temperature inside the cabin gets below 55º.

dlochner · Mar 13, 2026

Project_Mayhem said:
Due to a miscommunication with the yard, the boat has it's mast sitting on the deck so It's only covered in dirt and snow this year. I do dress in layers and even have heated gloves for the really cold days but my productivity drops significantly. The heated gloves weren't cheap but I have zero regrets about buying them. I built a frame and tarped the boat over the course of two days. Before I had the heated gloves I had to warm my hands up every 15-30 minutes. After that I was able to work for a few hours at a time.

The propane heater is a must unfortunately. Even two electric ones cant keep up. It usually takes 2.5 hours to heat up the boat with my current setup.

@thinwater your article that mentions installing plexiglass gave me an idea. I think I'll try taping some plastic over the hatches from the inside before I fire up the heaters next time. An added bonus is that I wont lose any natural light. The Bomars in the article have a much more elegant method of installing screens. My Lewmar portlights has screens that just barely fit with gaps all round. They're also subject to 100% UV exposure since they are on the outside of the boat.

Carpeting isnt a bad idea for the floor. The bilge and floor are always freezing because of the engine compartment vents. I have a 2x4' heated mat as well but at only 120w, it takes a long time before it helps.

I might toss some tiny fans in the drawers of my tool box. It's not a perfect solution but it should help a little bit. Desiccant gel packets didnt make any noticeable difference. I tried leaving the drawers open but the entire thing eventually tipped over from the weight. I suppose some clips could hold it partway open

McMaster sells corrosion inhibiting paper lining the tool box draws will help eliminate rusted tools.

Carpet on the sole and foam gardener's knee pads will reduce heat loss from your body.

Light the work area with halogen work lamps. In addition to being bright, they throw off a lot of heat. This will help warm the area you are working in.

Close off areas of the boat you are not working in by closing doors or sealing off with plastic drop cloths. The electric heaters will have less volume to heat and should be more effective.

...3TacksontheWater · Mar 13, 2026

As long as you utilize combustion to produce heat, you will increase moisture. There is no way around it. It's late in the season so spring is just around the corner. I encourage you to consider items such as Under Armor's Cold Gear both the base layer (which is available in 3 levels depending on your exertion levels) as well as the Cold Gear outer layers for comfort during winter projects. It may be a bit late in the season but, at least you'll be ready for next winter's projects.

George

Stu Jackson · Mar 13, 2026

I believe it is foolish to be trying to work on your boat when the outside temperatures are, as you said in your OP, 20 - 30F. It could be unhealthy.

My boat saw her first snow after I moved to British Columbia in 2016 after living in San Francisco from 1978.

Once the surface temperature of the deck is lower than the inside temperature there will be condensation without enough heat to warm the deck surface temperature. This is just basic psychometrics (Laws of Physics related to HVAC work). If your boat is floating, it's worse, because the water is warmer than the deck surface. It will actually rain inside the boat if it is cold and humid outside in these conditions without dry heat inside. If on the hard, as many have said, propane heating and your breathing just adds moisture leading to even more condensation.

Take a break and wait til it warms up a bit. You deserve a break today!

Foswick · Mar 13, 2026

20-30 F isn't that cold. A single 1500W space heater (pretty standard for garages and such) should warm it up to 50-60 F. Maybe a bit more if things are closed up tight. Maybe a bit less if it is windy. But, that's warm enough if you're working on projects.

I have my heater set at 750W and the thermostat set to 60 F. It only runs continuously when ambient temperature drops to about 20 F. But, since my cabin temperature never falls below 58 F, I don't have a lengthy warmup.

My hunch is that the problem is not so much that the heater you have can't maintain a comfortable working temperature, but that you let the temperature drop, so you heavy a heavy lift to get the interior thermal mass up to a point that it isn't actively fighting your heater.

Of course, more insulation will help. I would be reluctant to add carpet until you get the moisture under control.

thinwater · Mar 13, 2026

Project_Mayhem said:
The propane heater is a must unfortunately. Even two electric ones cant keep up. It usually takes 2.5 hours to heat up the boat with my current setup.

@thinwater your article that mentions installing plexiglass gave me an idea. I think I'll try taping some plastic over the hatches from the inside before I fire up the heaters next time. An added bonus is that I wont lose any natural light. The Bomars in the article have a much more elegant method of installing screens. My Lewmar portlights has screens that just barely fit with gaps all round. They're also subject to 100% UV exposure since they are on the outside of the boat.

External canvas (Sunbrella) covers block light, but they can be light colored. They help the hatches last longer, so all of my hatches and windows have removable covers. However, they also provide considerable insulation, including the frames.

Bubble wrap is also a fast, cheap insulation expedient. What with Amazon shopping, we all have too much. Cut to shape, spray the window with water, and slap it on. It will stick and stay. Really. Ugly, but warm.

thinwater · Mar 13, 2026

A well-known contractor/mechanic trick is NOT warm gloves. Instead, wear insulated Carharts, layers on LEGs and top, or a snowmobile-type suit. Also a hat, with maybe a thin balaclava under it. More than you need. This is not a tough guy thing. The principle is that if you are very warm, nearly overheating, your hands will radiate heat. Always overdress for work that requires bare hands and not a lot of movement. Not a common thought process for office workers.

Near freezing with no wind (inside a boat) should not be a problem and should not require gloves if you are dressed properly. I think you are underdressing without realizing it.

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I'm not a fan of working on boats in the cold. Paints and adhesives don't like it no matter what I do. I prefer to get the boat in the water early, enjoy it, and then, with everything together, pull it out in warm weather and knock it out FAST. Pick some windless stretch with terrible sailing. Also, most tasks can be done at the dock in the summer during odd times and rainy days.

LLoyd B · Mar 14, 2026

I think Stu may have got it right today maybe you just need to give it another week and see how things go. After all not everybody's the same and it appears to me like you may have picked up a case of spring fever and you were infected little bit earlier than usual.

jssailem · Mar 14, 2026

Kids here are playing Soccer, my wife thinks we should go walking at Silver Falls State Park. With all the rain, they should be really putting on a display.

Talk about spring fever. :facepalm:

thinwater · Mar 14, 2026

Project_Mayhem said:
I took a three or four week break in the middle of the winter due to it being down right too cold and frustration over a couple of projects that I kept running into issues with. Other than that, I've been working through the cold. I typically take off my cold weather gear after a couple of hours. Around hour four or so, I crack the hatches to get some fresh air and keep the temps from getting too high.

It's pretty windy off the lake and the area around the companionway leaks a lot so that's likely why a 1500w electric heater takes forever to get the boat up to temp. The bilge and engine compartment are also subject to cold air through the engine vent hoses. I've been hesitant to block them as corrosion might build up on the wiring harness. I've read that Westerbeke (and know that Catalina), didn't use the best materials. I have a bunch of desiccant gel beads around the engine.

I'm limited to a single 20A circuit that may be shared with one of my neighbors. In the colder months, I'm usually the only person in this part of the yard. My 12awg extension cords do get warm at the plugs with the 1200w heater. I'm guessing it's due to wear and corrosion as they are left outside six months out of the year. Until I replace the ends, I dont think it's wise to add another medium to high load device such as a dehumidifier. I've also had the yard's breaker pop after a few hours when drawing a consistent 2000w from two heaters. How warm would the dehumidifier need to be to keep the lines from freezing? It could easily take a couple of hours or more for it to get up to room temp.

That diesel heater is tempting, especially at that price. How hot is the exhaust that vents to the outside? I see it has a heat exchanger so does that mean there's less carbon monoxide risk?

Amazon bubble envelopes crossed my mind. They're built pretty tough. I am definitely going to try putting some plastic up on the inside of the hatch. This technique has been around for decades for houses with older windows and it works pretty well. Plus, if there is some condensation or leaks, they can be dealt with.

Throw a large beach towel over the companionway to reduce drafts. Easy. Often, you can secure it with the winches. This worked so well that I later built a two-layer (inside and outside) version with a window that replaces the drop boards. Very warm. I can typically heat-up my 24-foot boat in 15 minutes or so. If your house was 20 degrees, furniture, walls, and all, and you turned on the heat it would take several hours to warm up.

Fighting Winter Condensation

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SBO Weather and Forecasting Forum Jim & John

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SBO Weather and Forecasting Forum Jim & John

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