Practical advice on reducing overnight condensation please

thinwater said:
Lots of partial understanding and misinformation.

Ventilation Alone. When the outside temperature is pleasant, ventilation is generally enough. You can't get condensation if the humidity is the same inside and out. Well, that is mostly true, something summer sailors don't understand. If the water is cold, or if there is snow on the deck, the hull can be considerably colder than the air, and thus the more you ventilate, the MORE condensation you get. I'm not saying that is what the OP is talking about, but early season sailors know that cold water really draws condensation, if the hull is uninsulated. Summer sailors will not notice this.

Dew Point. You will have condensation anytime the hull or window temperature is below the dew point. That means that adding heat, if parts are uninsulated, will not always stop insulation. As one poster pointed out correctly (and was falsely criticized), heat without insulation can make condensation worse, because the hatch temperature has not changed much, but the air is holding more water. Thus, unless you are going to keep the air very dry, there is no substitute for insulation, both hull (if not cored) and windows. Won't single-glazed windows in your house sweat and frost? Of course they will.

Dehumidifiers. Consider the tiny thermo electric units (Eva Dry etc). They can be run on solar. I've had one for 7 years. I run it on a timer. Keeps the boat very dry, about 55% RH.

Heat. Ventilation is great unless you mind freezing. Drawing combustion air from the cabin sounds good, but it's damn wasteful; perhaps that works if it is cool, but not if it is cold (-10C is common here in the winter). My heater draws from outside, and the RH stays at 50-60%. The trick is to minimize generation (there is a fan in the shower and I cook many things using retained heat).

You need to understand RH (relative humidity) and dew points. All surfaces must be above the dew point, which ventilation alone will not always accomplish, not if people are adding any moisture at all, since the RH outside will be right at 100% during the wee hours.
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Hi, would you have a link to the Eva Dry you mention?

Many thanks
 
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