Sorry to be posting on this older thread but I've been testing out my greenhouse compost hot beds for the past few days and things are going pretty well. My greenhouse is 10'x12' with 6mil clear plastic. We're in Cda zone 6a (US zond 5a). Dirt floor. I wanted to make hotbeds in the greenhouse to help with overall temperature (to reduce the need for electric heat on really cold nights) but also to get an early crop of lettuce, and to provide bottom heat for seed germination (I'm running out of room indoors). 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 compostables, so I made this bin out of a big metal container. I filled it half full with the soil removed for the hotbed trench, wet it 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. 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 just fine. I worry that the lettuce seedlings will be too warm at the root level. But they're growing quickly, so who knows. The compost underneath will slowly cool (it is impossible to stir it) so eventually I won't need to worry about cooking the lettuce.

Ken: Your brother is right. I have a fully insulated floor both under and around the perimiter of my freestanding GH, that holds a constant 72 deg. temp in the worst deep, No. IL freezes with very little impact on our gas bill and I am convinced that all that base insulation made a difference. However, a fully insulated floor requires drains and unless you plan them, you might have to stick with perimiter insulation alone but with a triple wall, attached to a warm house, you have a heat advantage already. It would mean excavating around the outside perimeter, both down and a bit out to affect the reduction of heat loss you are seeking. You would need to dig a narrow but deep trench down to your frost line immediately outside the exterior, then lay another piece from the top of that foam "wall" to form a "flange" out about 2 ft., sloping it away from the perimiter, because it's pretty dense and waterproof and you want water to run away, not back down under your foundation. The material of choice for this type of application is 2" thick Type 150 insulating foam: the pink stuff. The Type 250 might be an even better choice. It's denser and the difference in cost for 4 X 8 sheets isn't all that great. For a structure your size, I'm guessing that you would need about 7 sheets, five for the deep walls and two more plus leftovers from the trenchs, to be cut in half lengthwise, for the 2 ft. wide flanges. Then, once the foam parts are all in place, use expanding foam like Great Stuff to fill any small gaps. BTW: That deep trench is an ideal excuse for renting a Ditch Witch.

I have no first-hand knowledge of the bubble-wraps; I hope someone who has had a chance to compare these will have some advice for you. In the meantime, the Charlie's Greenhouse catalog claims their bubble insulation is superior to bubble packing material and seems to run about 25 cents/square foot compared to your 20-cent/square foot material above. (Of course, there's also shipping.) You can check their website, www.charleysgreenhouse.com for details. There are several other suppliers, Farmtek is one, can't think of the others just now. BTW, my solar pool cover is a clear one, so doesn't interfere terribly with light transmission. In addition to flowering plants, I grew lettuces/mixed greens and herbs through winter without auxiliary lighting, even the cherry tomato produced lightly. I realize, however, that you are considerably farther north, so have correspondingly shorter daylight hours. Good luck this winter! I trust you'll have a very enjoyable time. Di

gibbsgirl, You definitely need to look into "Four Season Harvest" by Eliot Coleman. Also, you can look at "Winter Harvest Handbook" by Eliot Coleman. Basically, he says this: #1 - the goal is economically and biologically feasible, if you try to extend the harvest, rather than extend the growing. #2 - the goal also can be accomplished simply if you are choosing the right plants. You need cold hardy veggies. In general, these are the plants that do not FRUIT! Anything that you eat the leaves or root of are much better options than fruiting plants. Next, certain types of plants are far more cold-tolerant than others. If you time it correctly, and grow these types in late summer - mid-fall, they can get sufficient growth before the sunlight drops off drastically. From that point on, the plants will go semi-dormant (hibernate, as he calls it). The plants can handle freezing temps, but don't harvest, unless the temp is above freezing. A few of the veggies he mentions: Carrots, spinach, arugula, endive/escarole, beets, turnip greens, raddichio, kale and mache. Mache is the hardiest plant he says, and can form a nice base for a winter salad. As you may have noticed most of these are leafy greens. I guess mostly what you will get is salad fixings, with a few others. I wish I could speak from experience with these, but I am in the very infancy of this myself. I have just learned about these books, and wasn't able to plant early enough to have the plants for this year. The key is to HARVEST throughout the calendar, not to GROW throughout the calendar. Choosing correctly, as you seem to understand by your opening question, is the key.