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.

I found that PVC pipe over rebar worked pretty swell. My house was ~14' wide, maybe 5'-6" tall at the top of the hoop. I ran one pipe down the center of the roof as a purlin, and used 1x2 along each long wall for added support. 16-18 feet long or therabouts. Plain ol' plastic - well, it'll work if you secure it really, really, really well. No kidding - anything loose and it will flap it's way off, probably on the day you really need the protection. The greenhouse plastic costs lots more, but the stuff I got from Northern Greenhouse Sales went on, even mediocre tightness, and stayed on. It's the kind of material you do once and replace maybe 5-6 years later. I did the cheap stuff 3-4 times (same season) before I figured it was time to use the real thing. Venting is important. Cold days in the winter when temps were 35-40 and the sun shining - got me to 70+ inside during the daytime. When summer hits, temps go ballistic - you gotta have enough vents! I had a doorway at both ends I could leave open a bit for max cooling. Good luck whichever way you go. You can help insure the structure with some clothesline secured over the hoop house (like the low row covers) with a bungee on one end, so the elastic ensures the rope stays snug to the plastic - helps keep the flapping to a minimum.. :)

Hex, I like the soil warming idea. Turn your benches into deep flats by covering them with 6 inches of potting mix, with heating cables built in. Why not build little cloches (mini-hoophouses) right on the greenhouse benches? Cover them with transparent bubble wrap or some other material with good insulating properties. This will keep the warm air close to the plants, and will keep the humidity high. Now the need to heat the whole greenhouse is greatly reduced. Problem: since you will not be inside the cloche, the greenhouse will be chilly for you to work in. Also, this could get challenging if you are trying to grow big stuff, like lemon trees. Good point about the warm air collecting at the top of the greenhouse, where it either leaks out or loses its heat to the glazing. Concrete wall options: 1. Place insulation on the outside of the wall. Now the wall acts as a thermal heat sink, but it loses a lot of heat to the ground. 2. Place insulation on the inside of the wall. Now heat from the inside can't get into the wall, so the wall can't conduct heat to the outside air OR to the ground. But the wall is useless as a heat sink. Problem: the insulation will have warm, moist air on one side, and cold concrete on the other side. If the insulation is permeable to air, the warm moist air will get to the cold concrete and give a big condensation problem. If you want to avoid this, use insulation with a vapor barrier installed towards the INSIDE of the greenhouse. Now the vapor barrier is essentially at the inside air temperature, and there is no reason for condensation to occur. If you install the vapor barrier towards the concrete, it will be nearly at outside temperature, and moisture will condense on the surface of the vapor barrier. 3. I'm skeptical about the use of thermal heat sinks in extremely cold climates, but if you really want to try it, you need to solve the problem of heat leaking into the ground. I think a good way is this: build a separate thermal heat sink inside the greenhouse, maybe parallel to the concrete block wall. Design the heat sink with good insulation between it and the ground. This is practical, since the heat sink is no longer a load-bearing part of the structure. If possible, fill the heat sink with water, which has good thermal storage properties. Ideally, the south side of the heat sink would be painted black, and would be exposed to sunlight. Don't expect the heat sink to keep the greenhouse warm all night. It will probably help for a couple of hours. If you use the cloche method, the heat sink gets a lot less important. And the moist soil on the benches will also act as a heat sink.

The specs would be the same as standard hoophouses, only one a mini-scale. I will probably be constructing it next weekend. Here is what my plans are: Interior house: 3/4" PVC cut down 2 2.5 feet lengths, and a 4 foot length, connected with elbow (1/2 elbows, cemented INSIDE the 3/4" PVC) for the supports 4 foot long 1x2's will be connected to the bottom of the PVC on the sides, but the PVC will not be in the ground at all, just resting on the surface. (allowing easier access to the plants!) The hoophouse plastic will be connected in 4 foot sections half hoophouses, that meet in the middle and connected with large binder clamps). The plastic on the sides will be stapled to the 1x2 at the bottom of the sides, and the end of each half-hoophouse will be clamped closed at night, and opened in the day, according to the temp and sunlight forecast. Exterior hoophouse: (this is where it gets trickier) 3 foot rebar driven into the ground every 2 feet, on both sides. (driven into the ground 2 feet, leaving 1 foot exposed, for the PVC to go over). I figure to use 3/4 PVC for this as well, and just bend a 10 foot length, from one side of the rebar to the other. Again, they will be made into "half-hoophouses" 4 feet long. The end PVCs of each 4 foot half-hoophouse will be connected to the plastic with binder clamps. These will be secured to ensure the plastic fits tightly over the PVC ribs. The middle area of the half-hoophouses will be connected together (somehow...suggestions are welcomed). One final piece, is to connect the ribs to themselves, with a 1x2 on the end, and in the middle, to keep the shape intact, when removed. This connection can be made at the bottom of the end of the mini-hoophouse, but it will be made at the top of the interior hoophouse for the open end of the half-hoophouse, to allow the interior house to be connected together. (this is the part that is the most complicated, as I see it) The ends will again be gathered together and clamped closed, and opened as necessary according to the forecast. Entry into the hoophouse for watering, harvesting, etc. will be trickier than a full hoophouse. First, half of the outer hoophouse will be removed (raised up off the rebar and set aside.) The interior hoophouse center section will be then uncoupled. Now, the mini-hoophouse can be tilted to allow access to one side. Then, tilted back to allow enty to the other side. And the exterior hoophouse will be replaced. Then the same happens for the other half. Exterior removed, and set aside. Then the interior house tilted to allow access. Before putting the exterior back on, the halfs will be reconnected together. Well, that's the plan anyway. Sorry I don't have more information. Does anyone think it is actually worth the effort of actually connecting the half-hoophouses? Rather than just having them separate, or set up that way, but just not connecting the interior halves, and only connecting the exterior? (this would allow the 1x2 to be connected at the bottom for the middle to help hold the form when removing the exterior house.)