Heating greenhouse with compost - moisture control

Hello everyone!
I've had a lucky strike end of last year and managed to lease an apartment that came with a hectare of land and a few greenhouses. Jealous, yet?
It's very run down, we will have to do a lot of renovations and the budget is small, so we're looking for clever solutions.
We're currently using one of the greenhouses and while it heats up beautifully when it's sunny, it stays pretty chilly in cloudy weather and doesn't retain the heat very much during the night. A waterbutt to store heat will eventually come. In December we tried to heat up the greenhouse by putting a huge compost pile of free woodchips in there. The pile heated up nicely and at least kept the greenhouse from getting frost inside. However, it also brought a lot of moisture and a lot of plants suddenly showed signs of mildew.
We weren't sure if the little heat the pile brought balanced out the downside of getting so much moisture in, and eventually just left the pile be to cool down again.
It did, the plants recovered and since we're having a very mild winter, I didn't think about it again.
The other day the Permaculture magazine arrived and I read an article about someone heating their greenhouse with a compost pile and even using the pile as a hotbed. I was very happy that it apparently worked for that person, but I didn't find any clue as to how.
Does anybody know how to heat a greenhouse with a compost pile without having your plants die from mildew? Does it depend on what I'm composting? Please help!