I haven't a clue as to what percentage of heat may be lost through the floor. All I do know is that if I stand on the uninsulated floor in our otherwise very well insulated and heated detached garage in the deep cold of winter wearing socks and tennies- my feet get really cold. But I can go barefoot on the glazed quarry tile in the GH in the same freezing weather and be comfortable, which is good because I often trudge out there in the early hours wearing only sheepskin slippers, then take them off to prevent soaking them as I splash around, topping off tanks. It could be that the quarry tile is acting as a heat sink, absorbing warmth from the air, then sending it back up? Again, I have no idea. All I do know is that my GH has stayed warm all though one of our worst winters in many years, holding a steady 72 deg. 24/7, on the cheap for fuel. It could also be all of the insulation combined, but we just received a $185.00 gas bill for the entire month of January that in addition to the GH, heated a 2300 sq. ft house, a detatched 4 car garage and fueled a gas WH, dryer, and stovetop burners, and this is is the slow cooked soup season. I can live with that.

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.

Um....just a retired chemist stumbling across this thread. The Glauber's salt should be used in SOLID form, not as a solution. The salt itself is just sodium sulfate with 10 waters as part of the solid crystal structure. Upon heating, the crystal structure breaks down, liberating the water molecules to form a very concentrated solution of sodium sulfate. When the temperature drops, the salt tends to recrystallize, releasing heat. I don't know about the practicability of scaling the process up to a greenhouse wall scale (I've got a simple PVC free-stander, so it's out for me...), but the salt is certainly not "toxic" in the usual sense (remember, ingesting too much water will kill you!) and it would not attack PVC pipes. Just based on supposition and observation in my own greenhouse (which gets both too hot and too cold), I would be most concerned about the kinetics of the process, i.e., how quickly would unstirred salt be able to make the initial phase change to liquid. I would not expect this to have much of an effect on the high temps of the greenhouse. The other process, that is the recrystallizing, has a tendency to occur rather quickly once it gets going, meaning all of the heat is going to be released over a short period of time, although admittedly it will take it time to radiate away from sizable samples of the salt as might be stored in PVC pipes. The only other issue that might be a problem is the potential for the liquefied salt to remain in that state even when the temperature drops, barring some kind of agitation (forming a supersaturated solution). Probably this could be forestalled by not using pure-grade salt but rather a (cheaper) technical grade. Or, you could just add a little dirt to each pipe! Edit: looks like there is some reasonably current research into the practical problems associated with use of Glauber's salt for thermal storage: http://intraweb.stockton.edu/eyos/energy_studies/content/docs/FINAL_PAPERS/4B-4.pdf This post was edited by steve_in_los_osos on Mon, Dec 3, 12 at 11:23

From the Mantis web page "The Mantis ComposT-Twin dual-chamber composter holds almost 25 cubic feet of raw material" that's 10 bushels in each chamber!" 14 lbs X .29 is 4.06 lbs total Nitrogen that I added using just the lawn fertilizer. Compost density - Who knows? Assume it's 30% water, and the organic matter weighs almost nothing. The tumbler is now about half full, now that it settled about 50%. Water weighs 62.42 pounds/cubic foot. 62.42 X .3 X 25 X .5 is 234 lbs of compost, before the N was added. That means that I added 1.735% Nitrogen by weight. Or a 57.6:1 C:N ratio, ignoring any N from the Starbucks Coffee grounds I added much earlier. So from this simple math, my addition of the fertilizer does not seem to me to be out of line at all. In fact, had I done this calculation earlier, I ought to have added twice the fertilizer than I added! Thanks for indirectly suggesting that I do this simple math as a double-check on my "ongoing experiment". Jim Since the core temperature was only rising slowly, perhaps a degree F or two a day, today I added 4 lbs total of Urea. I've yet to do the math on that, but mentally I'm not out of line with this addition, since it's about twice as strong as the lawn fertilizer. Here is a link that might be useful: Mantis ComposT-Twin page