Of course it depends on what you're growing. Mine is too humid at night. Frost or dew forms on the inside of the greenhouse at night. When the frost melts in morning it drips here and there. By mid morning on a sunny day temperature rises and humidity plummets.

My climate is much drier than yours and warmer, 60/30F in winter. Dew points mostly 10-30F but can be 0-50F. We have weekly swings in temperature from about 15-20 to 65-70 F.

Well, my initial plan would probably involve heating to about 48-50 at night, maybe kicking it up to 60 during the day as a backup for days the sun doesn't warm it enough on its own.

It seems there's a balancing act between having enough ventilation to keep humidity and CO2 in check, but not so ventilated you're paying to heat your entire yard.

Couple more questions:

1. Regarding ventilation fans/shutters - do yours kick on in winter during cold but sunny days? How do you properly ventilate without too much cold air hitting them? My thought was could I use a hybrid system of natural venting plus a fan that kicks on at a higher temp? I know that for the shutters you can get a shutter motor to open the shutters a few degrees before the fan. However, can you do the same with the shutters on the fan itself? My thought was something like this:

You have the fan high up on one side of the GH and the shutters/vents low down on the other side. At 68 degrees or so, the shutters on the low side AND the shutters that cover the fan itself up high can open up, just to let air naturally flow in a bit without too much of a cold air shock. Mostly at this point to keep CO2 and moisture levels in check. Then, if it still continues to heat up despite the small amount of ventilation that occurs, the fan can kick on if it hits 80 or so.

2. Does such a fan exist that has a seperate shutter motor and control from the fan itself?

Do you already have a greenhouse or are you thinking of how you want to design a future one? If it is the latter, consider using a Subterranean Heating and Cool Storage system. They take the hot, moist air of the greenhouse and send it underground through tubes, using a slow moving fan. Because of the forced dewpoint, the air condenses underground, sending its heat into a storage medium the tubes are buried in. The air comes out the other side cooler and drier. At night, warm air is drawn from the heat sink to heat the air. The next day, the sun comes out and heats the space and the heat sink gets recharged. However, you can bank enough heat in the summer months that you can heat your structure in the winter with the heat you collected in the summer. And, it naturally dehumidifies the air in the winter which is really handy! If you plant your plants in the storage medium (usually its soil but can be stone), the moist heat at the root level will be great for your plants.

To get any real control of humidity you'd need more ventillation than you'd get by opening a couple shutters. I control mine by running the exhaust fans as much as possible. With the sunshine we get even in winter, humidity isn't an issue except at night and on a few cloudy days.

I've heard of people running exhaust fans even at night or on cloudy days to lower humidity. This causes the heater to run more.

Having a heater that vents outside lowers humidity. If the heater vents inside humidity goes up as water vapor is a product of combustion.

question:

Does the subterranean heat and cool system do anything about CO2 levels?

One reason you need to ventilate in winter is to ensure that CO2 levels don't plummet. Does the soil actually contain enough CO2 to bring CO2 levels back up with the air comes back out?

No soil isn't likely to provide much CO2. A compost pile would be better. Not sure how big is needed. It would also provide some heat.