I wouldn't use perlite. It floats and would get nasty quickly. Plus being white, you would see it getting nasty quicker. I would use clay pots with the open end up and put a wire grid over them. The egg crate would work but i think it is too brittle/fragile. Drop one pot on it and you'll be replacing it. Clay pots will wick up moisture, they're cheap and easier to clean, and readily available. With the open ends pointed up, any air flow over the large surface area will boost humidity better, I suspect. I would use something cheap but durable for the walls, paint them with white marine epoxy paint, and affix some plastic coated hardware cloth or other type of wire grid over the panels. That will provide endless spots to hang mounts which you will undoubtedly, eventually, acquire in large numbers. You can also use the wire to hang additional lights or fans, or to hang pots higher up so they're closer to the lights, if needed. It will allow you to use your vertical space. Ventilation will be very important, especially if you have very cool and humid conditions, as most basements would. Sounds like a fun project. Post pics when applicable, please. -MB

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.

boxcar, The foam is waterproof, so if you lay it under crushed stone with no place to drain; it won't. However, if you have the patience to fiddle with the crushed stone, grading and packing it firmly to the center or one end and can dig out a drywell, you could lay the foam over it, leaving the top of the drywell open from the foam, then more stone on top, and the water would drain to the drywell. But it would result in a much nicer and easier to keep clean floor, not to mention tracking the stuff, to just overlay the foam with concrete patio blocks or bricks. As long as the crushed stone is well and truly packed, the foam really will support the bricks or blocks, that will lay easily and could be butted tightly to avoid the need to fill between them. You could even broom mortar into any cracks, wet it and when it dried, would have a really solid floor. BTW, just in case you aren't familiar with drywells- They are just holes dug down to below frost line, lined in the old days with mortared brick or stones and now, plastic liners that can be bought for the purpose. Or, you can use a length of fairly wide plastic culvert pipe. You could also use the same heavy plastic channel drain as I did, except that instead of the section with the drain drop leading to a sump pit, it would go directly into a drywell. There are a number of options, but for any drywell, always- the deeper the better, then fill it with large stones to the top of the liner and either set a made for the purpose cap or channel drain drop section over it, or top it off with with 1/2 inch hardward cloth and lay a piece of window screening over that, to prevent anything other than water from seeping into it. I drain my GH with a channel drain that connects to a sump pit, fitted with a sump pump, that empties the pit to a 150 ft. long line, but I fitted out my GH out to serve as a tropical fish hatchery, so often need to dump hundreds of gallons of water at a time, and have a large utility sink out there too, with H & C water, so it's a different GH use and set-up than most. I also laid OSB over the foam, then topped that with 3/4 inch concrete board, but that was because I wanted a glazed quarry tile floor and it requires far more elaborate under pinnings than do patio blocks or bricks. The tile also lines the kneewall, but there too, my needs were different. All you really need for a GH in which you only grow plants, is a means to drain off excess water from the plants and avoid too much dampness that might cause fungus on them as well as the interior walls of the GH, although the ability to hose down a concrete block or brick floor with cold water is a nice extra, for keeping the GH clean and help cool it on hot days. It really all depends on HOW you want to use your GH, and WHEN in relation to your climate, and how much time, effort and money you are willing/able to spend to fit it out for your own maximum use and enjoyment. After all, a garden GH is really only for pleasure, because unless you are feeding an army of kids, for what it costs to build and maintain one, it's cheaper to shop for veggies at Whole Foods and buy started plants at any nursery. So by all means, put a lot of thought and effort into your foundation and if, after you have the GH you need to back off on the fittings until your wallet recoups; do so and in the end, you will have what you really want. We spent an entire season preparing the foundation for my GH and by the time we actually got to erecting it, all we had time for before winter set in was to install the heater, but as anxious as I was to use it, patience paid off. (Of course, we are also a pair of arthritic old poops, so everything takes us longer now.)

@gardengal48 (PNW Z8/9) The problem is not what to do when the bags of stone are at the final destination. The problem is how to safely transport the material in a car or truck from the wholesale supplier to the job site. In our area, you get to a wholesale supplier and they give you empty one cubic foot bags and a shovel. It is self-serve. Once the bag is full, how do you secure it so that when a 100-pound bag of stone falls over, you do not get loose stone everywhere in the car or truck?