updated 9/11/16

Condensation  and Ventilation

How to be warm, dry and comfortable living aboard a small boat in cold weather.

After thirty years in Hawaii we had a lot to learn about being comfortable in cooler weather. In the tropics it is easy. No condensation to worry about. Just protect the boat from the sun and rain with an awning. Open all the hatches and let the trade winds do the rest. As we settle in for our second winter in the sub-arctic rain forest of Southeast Alaska the challenges are somewhat different and we are applying lessons gleaned from many sources along the way. The learning curve was steep that first winter with snow on the decks in Friday Harbor, WA. Here is what we have learned about keeping the boat warm and dry through the winter or while cruising the high latitudes:


Heating is the obvious priority but it is imperative to heat intelligently and make every effort to limit the amount of water vapor released by combustion. Yes. Fire makes water, or more accurately, releases the water in the fuel. Some fuels contain more water than others. Unfortunately, the most popular fuel in modern boats is the worst culprit. Propane releases more water vapor during combustion than any other popularly available fuel. Propane, from a moisture content standpoint, is the worst choice for heating or cooking on a boat. It is, however the cleanest and easiest to use, especially for the cook.


When shore power is available, electric heat is probably best as no water vapor is generated. We use a 5200 BTU electric heater with variable speed fan that works very well to both heat and circulate the air. We also have a 5000 BTU Origo alcohol heater for use when shore power is not available. Either will keep us comfortably warm with outside temperatures down to about 20f. We are also looking into solid fuel and diesel heaters small enough for the Vega. Heater design, as well as fuel type, is worth mentioning. Heaters that bring in fresh air for combustion and send the water vapor produced by that combustion out through a chimney are best, especially when combined with a low moisture content fuel. Emerson White reminded me in an email: “Heating with an open propane or alcohol burner is alright, but it releases lots of water vapor and other exhaust gasses into the cabin. A closed and vented stove like a Dickinson (very popular up here in AK) can heat your cabin without consuming O2, releasing CO2, CO or H2O into the air that you intend to breath.”, an excellent point.


Warm air can hold more water vapor than cold air can. That is why your breath does not condense in the now 70f warm cabin. But two adults, just through respiration, produce an astonishing amount of water vapor (2 to 2.8 pints of water per day per adult). Add the steam from making tea and cooking plus the water vapor released by the combustion of your cooking and heating fuel and pretty soon it is condensing on every hard surface and dripping from the overhead. Your bunk is soaked in the morning from condensation under the mattress and the clothes in the lockers are wet through. You insulate where possible and use products like Dri Deck to keep stuff in the lockers away from the hull but it is still wet in the boat; so wet that you notice the automatic bilge pump is running more often. What is required is to evacuate all the warm, moist air before the water vapor condenses out of it and drips from that bolt head over your bunk into your ear. When it gets cold outside we must resist the urge to light the propane burners and seal up the boat “To keep the cold out”. This tactic just keeps the water in and results in a wet, miserable cabin.


To stay warm and dry we have to heat the air and get rid of the water. What to do?


The following are notes I took before I gave away my copy of "A Warm, Dry Boat", now out of print. I suggest you get a copy of if you can find it. Also four-inch and one-inch hole saws.


The ideal ventilation system would bring cold fresh air in at one end of the boat, through the heater into the living spaces and lockers, then take the warm, moisture laden air out through the opposite end.


Place intake vents as far as possible from exhaust vents


Active exhaust. Passive intake.


Minimum flow = 15 CFM(900 CF per hour) per crewman


Typical 30 footer = 1000 cubic feet of cabin space


4" Nicro Day/Night vent flows 1000 CF per hour (when the fan is running)


Use a low water vapor heat source.


Insulate, where possible.


Another note from Emerson on insulation: “With regards to insulation, you want to be sure to control vapor flow through that insulation. Water is drawn out of the air as it cools, and if the air can pass through the insulation the temperature falls as it migrated out towards the hull, drawing water out into the insulation. This leads to biological issues inside of the insulation, and reduces its effectiveness.”


Use Dri Deck and Hyper Vent to create air spaces against hard surfaces where possible, especially under mattresses and cushions.


Key Lessons Learned:


Above all, ventilate. A good rule of thumb is one four inch active exhaust vent and one four inch passive intake vent per crew member. A four inch vent will flow nearly twice as much air as a three inch vent so use the larger one where possible. The solar vents by Nicro seem a good idea but after 7 years we have to say that they do not work well in the high latitudes. There is just not enough sun when you need it most. 12 volt active vents are what you need but there are none commercially available from the usual marine chandleries at this writing. We have adapted 12V computer type fans for use with Nicro, Beckson and Vetus vents with excellent results.


Put vents in your lockers. These can be three or four inch holes with a chrome louver or other trim piece or just one inch round holes or half inch slots between lockers. Just make sure you have adequate air flow throughout the boat. You will probably need to add both active and passive intake and exhaust vents as well as circulating fans to any boat if you plan to live aboard in the high latitudes. Computer fans are cheap, quiet, use little amperage and are designed to run constantly.   Two or three for your active exhausts and a couple more for internal circulation should be sufficient for most boats.


Use non-absorbent insulation (Lealea has mylar/bubble wrap type insulation)


A properly insulated and ventilated boat should not need a dehumidifier (But it can't hurt.) I think that the best use of a dehumidifier is when the boat is unoccupied. A small electric heater with a low speed fan - no collection bucket, would be my choice. When the boat is occupied it should not be necessary.



On Lealea, the passive intake vents are three-inch Vetus mushroom vents on the after deck behind the coaming and in the cockpit well. The active exhaust vents are located in the forepeak. They consist of a four-inch Vetus mushroom with a 12 volt computer fan right forward in the chain locker, a four-inch Nicro solar Day-Night vent in the forward hatch and a three-inch Beckson “Bowl vent” over the head compartment with a Vetus 12 volt exhaust fan.


The heater is at the bottom of the companionway ladder. This arrangement brings cool air into the aftermost part of the boat, through the cockpit lockers and through vents into the main cabin. There it is picked up by the electric heater at ankle level near the bottom of the companion way and drawn out through the active vents forward, in the sleeping cabin.


Still tweaking the system but the principles clearly work. We still have work to do but the boat is comfortable and dry. The Nicro Solar vent, while it worked fine all summer simply does not work at all after the equinox at this latitude, nor did it work well at 48 North. We hope solar and battery technology improves because this is a great idea and works well in the tropics and during the long days of the Alaska summer. While we have an additional three-inch vent, the total flow through the two vents is less than we would like. We will probably add another four-inch exhaust vent in the form of a Vetus Mushroom with a computer fan and another passive intake mushroom aft. Another thing we need to do is improve ventilation in some of the lockers. Just a matter of drilling holes.