Just thought I'd post these photos and information about my attempt to heat the greenhouse with compost. Here are some pictures on my Flickr page: photo of hotbed This shows the 18"x36" hotbed with the hoops. On the left I put trays and flats for overnight warmth. The right has the lettuce planted in the six inches of triple mix on top of the hot compost. At night the hotbed is covered with plastic to form a mini hoophouse. photo of compost portion of hotbed. I left one side of the hotbed uncovered, so I could use it as a source for bottom heat as well as the heat source for the mini hoophouse-within-the-greenhouse. compost in bin in greenhouse. I had some extra feedstock and a big metal tub, so I made a composter. It was filled half full with the soil removed for the hotbed trench, soaked thoroughly (wet soil is better thermal mass than dry) and topped it with 8" of wet compost feedstock. The temperature is about 50c but it goes even higher with more moisture and regular stirring. This bin is useful as a base for trays of seeds or seedlings. Using a clear dome creates a greenhouse-within-a-greenhouse for added insulation. I could build a mini hoophouse over this tub, too, but the tray domes work just fine. So far so good. Plants on the hotbed and bin have come through minus 7 C nights with no problem. The rest of the greenhouse frosts up but everything over the hotbeds is safe and sound.