We looked at a thousand options: pricey HRVs (heat recovery ventilators), less-than-robust solar vent fans, bulky (read ugly) exhaust fans...and we didn't decide. And then we sort of forget to remember to figure out how we were going to deal with moisture in our tiny, well-sealed house.
Fast forward to winter. We cook with moisture-producing alcohol. We run water to wash our dishes. We take moisture-producing showers. We have moisture-producing bodies. You get the idea. As a result, we have enough moisture in the air that condensation forms along the bottom of many of our windows on these sub-freezing days. (Today, with the outside temp at 12 degrees, ice crystals formed on the shaded windows.)
Leave too much moisture in a house too long and you'll find mold. Seal up a house too tight without moving air and you build up carbon dioxide in the air. Obviously, we don't want either of those.
Long term, we plan to find an exhaust fan for the bathroom to deal with both moisture and any odors. Having an intake vent on the other end of the house would bring a little fresh air near our bed. It would also bring in cold on days like today. That's where having an HRV comes in. Essentially the warm air going out heats up the cold air coming in. The smallest, most efficient HRV we've found is the Lunos e2—but it comes with a high price tag ($1000+) and it requires thicker walls than we have. We have to keep looking for a ventilation system to manage our air quality.
In the meantime, we crack windows when we cook on our alcohol stove and after we shower. Anytime the temperature gets high enough, we open enough windows and doors to move air through the house. Plus, we have a carbon monoxide detector to monitor air quality and a thermometer/hygrometer that tells us how much water vapor is in our air.
To tackle the moisture issue, we started with a low-tech silica moisture absorber, specifically the Eva-Dry E-500 that had been recommended by another tiny house dweller. As far as we can tell, it has no significant impact on the humidity of our house. Yes, it has absorbed some moisture (the beads have turned from blue to pink, so it's time to plug it in and "renew" its absorbency), but it's designed to work in a small, enclosed space—not a 250-square-foot house. So, we bought a "petite" dehumidifier, also by Eva Dry, that pulls water out of the air and deposits it into a tank. This works; we can see the water collect in the tank after we turn the dehumidifier on and we can watch the humidity reading go down on our thermo hygrometer. Plus, the machine is relatively quiet and doesn't draw much electricity.
Before we plugged in the dehumidifier, our humidity readings were regularly around 55%. Too moist. Right now, with the dehumidifier running, we're at 46%. Ideally, with the weather this cold outside and the house between 65 and 70 degrees, we would be at 35%, according to what I've read.
Keeping the house warm in this weather, brings us to another purchase: our DeLonghi radiant heater. Actually, it's plural now; we bought a second heater, so we could position them on both ends of the house to keep things reasonably warm (around 64 degrees). We like the DeLonghi because it's quiet and efficient; once we get the house at a base temperature, we can leave the heaters on the energy efficient setting that draws lower amps.
When the weather dips in the evening, we fire up our Kimberly wood stove. With the stove burning, temps easily reach 75 degrees inside the house. We can feed the fire less and tamp it down to stay closer to 70, but lately we haven't minded the extra heat. (One important air quality note: the Kimberly stove draws air from outside—via a pipe that runs down through the floor—so that oxygen in the house isn't depleted.) The wood stove offers an added benefit, as well: it helps with our moisture issues by drying the air.