I'm in a climate that is cool and humid year round. Controlling cabin
humidity in my case is merely a matter of keeping the cabin
temperature at 15 degrees F above ambient to achieve 60% maximum humidity. A 20 degree F ratio will achieve 50% humidity.
If I used a heater and set the temperature at say 70 degrees F, that setting would only be effective when the ambient temperature was below 55 degrees (at 100% ambient humidity). And on cool drier days, I would waste money
by unnecessarily heating
the cabin. And as the ambient temperature dropped at night, I would waste more energy by maintaining the same temperature in the cabin.
So I instead installed a Honeywell humidistat (cost $35) that turns on a simple oil-filled heater (cost $40) with the thermostat set to turn it off at 75 degrees F. The humidistat is set at 60% humidity. I save a lot of money
on power this way because at night, when both the temperature and dew point drop, I can allow the cabin temperature to drop to follow the dew point down and don't waste energy trying to maintain a constant temperature in the cabin (I don't care about comfort when I'm not there). So this approach is a "constant humidity" rather than "constant temperature" approach. During the days, when it's sometimes dry, the heater doesn't run at all.
It's important to use a heater that does not have a fancy electronic temperature controller because the controller will lose its setting when the humidistat switches off its power. Also, the humidistat is only rated at 7.5 amps (900 watts at 120 volts), so a heater drawing more than that will overload the humidistat.
I use an oil-filled heater because its surface doesn't get hot enough to ignite anything. Its surface tops out at 250 degrees measured by my IR thermometer.
Of course, I use the same heater when I'm on board and set it for comfort instead of controlling humidity when I'm there.
Attached is telemetry from my boat's cabin taken over a week. You can see the "sawtooth" pattern in the temperature, which is the humidistat switching on and off. You can also see how the ratio between the dew point and temperature remain constant (except for some "free" heating
on a dry day), while the temperature is allowed to vary with ambient. The ambient humidity during this week was 100% for about 8 hours each day.
I believe this is the cheapest and most energy-efficient approach, at least for my cold/wet conditions. It is cheaper to add heat (provided you have some ventilation) - than it is to run a dehumidifier's compressor
to extract the water
I don't like the idea of sealing the cabin from outdoor airflow and running a dehumidifier because it just encourages mold
spoors to stick around and grow.