Dealing with Condensation when Cruising in Cold Climates

Tips for managing condensation on the boat when cruising in cold-water climates. The Dashews talk about what’s working for them, plus they’ve surveyed several other high-latitude cruisers.

We’ve been cruising in really cold seawater – as low as 37F (3C) and averaging about 41F (5C). As long as one has good heat, the cold water is not a problem, except for condensation and in some cases, mold and mildew. We are particularly sensitive to the latter as part of Wind Horse‘s crew is allergic to this stuff.

Condensation is caused by a cold surface coming into contact with air which carries more moisture than can remain in vapor state at the surface temperature. The moisture comes from our breathing process (each person emits about two quarts/liters per day); from cooking (the use of propane as a cooking fuel generates large quantities of moisture – pasta and hot drinks are major contributors); and from bathing.

Since all of these activities go on to some degree, what can you do to avoid condensation problems?

Step one comes in pre-departure preparation, in the form of insulation. Both fiberglass and metal boats are going to sweat in really cold water. So both require some form of insulation. Our preferred material these days is to use a non-absorbent product like Armaflex. On Wind Horse this is 1/2" (12.6mm) thick on the hull and 1/4" (4.3mm) on the framing members. Below the waterline, in 41F (5C) water we do not sweat. At the colder temperatures there is just a hint of moisture. This same material would work well for fiberglass hulls.

This insulation covers tank tops, hull sides, and deck. Cabin soles are cored with one inch (24mm) structural foam, providing a second layer of insulation.

Some boats use sprayed foam, or fiberglass insulation, both of which are less costly to install, but not as long lived as the Armaflex in humid environments.

Because we are relatively paranoid about mold and mildew, we go to great lengths to get moisture out of the boat. We do this by running a dehumidifier in the "basement" and by using the air conditioning for brief periods (typically when bathing and cooking) to pull moisture out of the air.

As a result, even on rainy or foggy days, we keep humidity in the salon at 50% or below. This keeps the basement area dry. The sleeping areas have a two inch (50mm) strip at the tank margins, forming a shallow "V", which is uninsulated, and this does sweat a small amount. By small we mean about a quart/liter over five weeks (we just dried these areas out).

If you have a hot air heater, and use a mixture of inside and outside air in the heat exchanger, this will pull out moisture. We have experimented with a vent at the high point of the salon (inside the entry way) and this helps as well (hot, moist air rises).

Insulating heater hoses or ducting, and the hot side of the pressure water system will reduce the temperature under the furniture so less moisture can be carried in these spaces which are adjacent to the cold hull.

To get a further fix on this issue we e-mailed three friends who are experienced cold weather cruisers. Jim and Jean Foley have spent time in Tasmania, Tierra del Fuego, and northern Greenland aboard their 67-foot Kelly Archer-built aluminum cutter. Jim’s comment was "Onora is very well insulated and has an open flame diesel heater so we did not have much condensation. We also learned to open the ports at night to let in the cold dry air, and the pasta was drained in the head with a hatch open to keep the humidity down in the cabin."

Evans Starzinger and Beth Evans have also spent much time in the higher latitudes aboard their 47-foot aluminum Hawk. Evans points out "I do occasionally use my wet/dry vacuum and vacuum our bilges, primarily to get the hair and dirt that falls down there but it also sucks out any moisture that might be down there."

Evans also points out that if it is cold enough, then mold and mildew are not going to grow.

John Harries and Phyllis Nichols, who have also cruised extensively in cold water aboard a 57-foot aluminum ketch points out that their uninsulated bilges are always wet and that they make a couple of gallons (8 liters) of condensation a day. They also find a small amount of mold in their bilges.


As we have mentioned before, we can have visibility problems with all our glass, especially when cooking. This inside fogging, shown on a cold morning in Mary’s Harbour, Labrador, is cleared up with three minutes of air conditioner time.

This is our third season of cruising with Wind Horse in cold water and the approach we use seems to be working well. In our context running the air conditioning gear and dehumidifier underway adds a tiny additional load to the engines. At anchor, where power consumption is more of an issue, we are finding ourselves running the genset for an hour or so every day, primarily to keep the boat dry (otherwise, we would wait until we moved to charge the batteries with the engines). For sailboats, who do not want to run the engine or genset at anchor, some combination of care with moisture producing activities, keeping the boat cooler (so there is less humidity in the air), an open flame heater and/or hot air heat exchanger with an outside air source, will keep humidity levels tolerable.

Posted by Steve Dashew  (July 5, 2008)

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