I would like to see wooden structure that you will need to build to hold a waterbed up right.

tom: As plantman noted, the sheer weight of the water in a waterbed mattress set in an upright position would require far more than a simple wood frame.

If you are interested in using a water wall as insulation, check out the bags and frames made for that purpose, sold by Farmtek. They are not that pricey. At least look at them in the Farmtek.com website to get some ideas about how you might replicate them as a DIY project.

Hello. I too thought of that, but decided there was no way to properly brace it and even then I thought it wouldn't be worth the risk of even a small leak. Have you already built your greenhouse?
I asked ?s about thermal mass in fall. I've been thinking about it all winter. I want to garden in the winter and grow tomatoes/lettuce/cucumbers/squash/peas, ect, but don't want to pay an arm and leg to heat (yes, its insane because I live in Buffalo)
I obviously have not built yet. The plan is a 25x10 greenhouse/playhouse for the kids. It would not be all glass, but look similar to a sun shed (saltbox type using polycarbonate on the south wall and sloped roof and part of the e and w walls, then insulated solid wall for the n). The thermal mass would actually be sand used in conjunction with a solar pool heater along the north wall.

This is my thought: Dig down about 3 feet. Line with plastic then ridged insulation, fill with sand a few inches, then snake a plastic hose around it. Fill the rest of the way with sand and build the greenhouse over it. The hose would be attached to a solar pool heater attached to the north wall. The theory is the water will heat up then circulate through the sand, slowly warming it. Over the spring/summer/fall, the sand heats and then as the temperature in the greenhouse lowers below the temp of the sand, the heat is released into the greenhouse. The sands heat is partly regenerated every sunny day. I'm hopeful the heat will hold long enough to make it through a winter (yes, as I said, i'm crazy -hahahaha).
Thoughts anyone ?

Thanks everyone for clearing my head of a mistake. I've been racking my brain for an inexpensive water container (s) for thermal mass and a dark water bed seemed so right. Dull black 55 gal. plastic drums make sense if I had access to them but shipping is crazy for larger items and they are expensive in themselves. Like Kyeriel above, mine is not all glazing. It has a solid north wall. When I look at the options, water is the optimum choice. So, that leaves me with containers. Where and what. sigh.
~tom

Have you thought about using small containers for your water storage, and then painting them flat black? I remember articles many years ago in Mother Earth News where they built a solar room/green space which used concrete blocks on the N wall, with vermiculite/sand filling the cavities, and the S side of the wall painted flat black. They then put water barrels, (I believe they used standard 55 gallon steel drums), also painted flat black, against the N wall. Since this was a solar room/green space meant for year round living they also installed a woodstove in the space to provide supplemental heat.

As to the idea of the hose in the sand bed, that is basically the same as a radiant floor heating system. My suggestion is to look into using a flexible tubing called PEX. It is designed for this, whereas garden hose is not rated for high temps. You can get high temp hose, but the cost on it is too high to use in an application like this. I would also recommend using a small pump to aid in circulation. You could probably get by with a simple one bought for under a hundred dollars from a supplier like Harbor Freight. You will want to set it up on a thermostat to run the system, I would recommend one that is rated for high humidity areas, as I don't think the residential ones would last very long.