Although I don't live aboard, I do have a few suggestions from my experience in solving moisture problems in my boat.
First, as Jess pointed out, the key issue is the dew point of the air, which is another way of measuring the humidity. Any surface of the boat that is below the dew point, condensation will occur. Note that dew point is an especially useful way of measuring humidity, because it is independent of the air temperature - all that really matters is whether the temperature of cold surfaces is above or below the dew point.
One problem with small, enclosed volumes like a small boat is that ordinary respiration can fill the air with humidity. If you go to bed with the boat closed up tight (like you would do on a cold evening), much of the water you drink will be exhaled into the boat. The humidity will be there even if the boat is heated. The key is to minimize the cold surfaces that the moist air comes into contact with. If you've ever waken up in a camping tent in November or December, you've likely seen the kind of condensation that can occur.
Jess's pictures show that she is doing a good job at preventing this with her insulation on the sides of the boat. Once other place where I experienced condensation was in the storage under the settees, especially under the waterline. A couple of years ago I posted about how I solved this problem with bubble wrap
, which insulated well enough that the air never came into contact with a surface below the dew point. This area is critical, because often life jackets are stored there, and for a liveaboard there may be clothes stored there. These things would be susceptible to mildew, especially under the waterline. I like the Reflectix stuff that Jess is using above her settees, and have been meaning to replace my cheap bubble wrap with it. (One more thing to go onto my list for this winter.)
A dehumidifier can be a great help for a boat that is on shore power. On a 25 footer space is at a premium, so a full sized dehumidifier might be impractical. But a couple years ago I posted a thread about a tiny Peltier-type dehumidifier
that I found at Home Depot. It's still going strong, draws about 60 watts and sits on my galley counter, velcro'ed to the shelf behind it (to make it heel-proof). This year I attached a hose to the drain pan that I run to my galley sink so I don't ever have to pour it out.
As for the issue of dampness in the bilge, I think that could depend on whether Jess's O25 is inboard or outboard. If it's outboard, there's a good chance she can keep her bilge 100% dry, which will reduce the problem significantly. If she has an inboard with seepage through the packing gland, she will have a wet bilge, in which case carpet or foam over top would mean 100% humidity all the time in the bilge, which could be problematic. For this I have a question, not an answer. I have always wondered whether adding a small amount of bleach into the bilgewater could knock out any mildew. Maybe a capful every week would need to be added, since continued seepage through the packing gland and cycling of the bilge pump would dilute out the bleach. The risk could be that the bleach could prematurely age the bilge pump and hoses, but I would think that there might be a low level of bleach that could keep the water disinfected. What do you liveaboards think of this?